This "sandbox" is a place where Code Golf users can get feedback on prospective challenges they wish to post to main. This is useful because writing a clear and fully specified challenge on your first try can be difficult, and there is a much better chance of your challenge being well received if you post it in the sandbox first.

Sandbox FAQ


To post to the sandbox, scroll to the bottom of this page and click "Answer This Question". Click "OK" when it asks if you really want to add another answer.

Write your challenge just as you would when actually posting it, though you can optionally add a title at the top. You may also add some notes about specific things you would like to clarify before posting it. Other users will help you improve your challenge by rating and discussing it.

When you think your challenge is ready for the public, go ahead and post it, and replace the post here with a link to the challenge and delete the sandbox post.


The purpose of the sandbox is to give and receive feedback on posts. If you want to, feel free to give feedback to any posts you see here. Important things to comment about can include:

  • Parts of the challenge you found unclear
  • Comments addressing specific points mentioned in the proposal
  • Problems that could make the challenge uninteresting or unfit for the site

You don't need any qualifications to review sandbox posts. The target audience of most of these challenges is code golfers like you, so anything you find unclear will probably be unclear to others.

If you think one of your posts requires more feedback, but it's been ignored, you can ask for feedback in The Nineteenth Byte. It's not only allowed, but highly recommended! Be patient and try not to nag people though, you might have to ask multiple times.

It is recommended to leave your posts in the sandbox for at least several days, and until it receives upvotes and any feedback has been addressed.


Search the sandbox / Browse your pending proposals

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To add an inline tag to a proposal, use shortcut link syntax with a prefix: [tag:king-of-the-hill]. To search for posts with a certain tag, include the name in quotes: "king-of-the-hill".

  • \$\begingroup\$ What if I posted on the sandbox a long time ago and get no response? \$\endgroup\$
    – None1
    Commented May 15 at 14:05
  • \$\begingroup\$ @None1 If you don't get feedback for a while you can ask in the nineteenth byte \$\endgroup\$
    – mousetail
    Commented May 29 at 13:27

4705 Answers 4705

13 14
16 17

The too-short urinal problem


Golf Transmission

As in, the 5 speed transmission from the original VW Golf GTI.

Print the transmission pattern in as few bytes as possible.

1  3  5
|  |  | 
|  |  |
2  4  R

  • 2
    \$\begingroup\$ this pun was too good to pass up \$\endgroup\$
    – pacman256
    Commented Nov 16, 2023 at 14:50
  • \$\begingroup\$ Would extra whitespace that doesn't affect display be allowed? Would returning an array of strings be OK? \$\endgroup\$
    – Someone
    Commented Dec 12, 2023 at 18:17
  • \$\begingroup\$ ok i forgot to change this but its been up for like 2 weeks \$\endgroup\$
    – pacman256
    Commented Dec 12, 2023 at 22:43

English Golf

Your challenge is to golf some English text by replacing some words with their contractions. Here is a list of the contractions you need to support:

Words Contraction
are not aren't
can not can't
could have could've
could not couldn't
dare not daren't
did not didn't
does not doesn't
do not don't
had not hadn't
has not hasn't
have not haven't
he had he'd
he is he's
he will he'll
I am I'm
I had I'd
I have I've
I will I'll
is not isn't
it is it's
it will it'll
let us let's
might have might've
might not mightn't
must have must've
must not mustn't
need not needn't
shall not shan't
she had she'd
she is she's
she will she'll
should have should've
should not shouldn't
that is that's
that will that'll
there had there'd
there is there's
they are they're
they had they'd
they have they've
they will they'll
was not wasn't
we are we're
we had we'd
we have we've
we will we'll
were not weren't
will not won't
would have would've
would not wouldn't
you are you're
you had you'd
you have you've
you will you'll

This is just a minimal list but you can support other common (please show your source) English contractions should you so wish. Most of them have one of am/are/is, had, have/has, not or will as the second word, except for "let us".

If there is a choice of contraction you can choose either or go for a double contraction e.g. "couldn't've".

The contraction should keep the case of the words, e.g. "Let us go!" becomes "Let's go!".

This is , so the shortest program or function that breaks no standard loopholes wins.

  • \$\begingroup\$ Maybe some example when pure suffix replacement would fail(maybe "will" as noun?) \$\endgroup\$
    – l4m2
    Commented Nov 26, 2023 at 13:02
  • \$\begingroup\$ @l4m2 Well this is an oversimplification anyway for instance in questions the words are in a different order. \$\endgroup\$
    – Neil
    Commented Nov 26, 2023 at 14:35

Derive an MIU string

The MU puzzle in Douglas Hofstadter's Gödel, Escher, Bach involves a simple formal system called "MIU". It works as follows:

  • MI is an MIU-string
  • If xI is an MIU-string, then so is xIU.
  • If Mx is an MIU-string, then so is Mxx.
  • If xIIIy is an MIU-string, then so is xUy.
  • If xUUy is an MIU-string, then so is xy.

Your task, given an MIU-string, is to output a derivation of that string starting with MI and ending with the input string.

You can assume that the input is in fact a valid MIU-string and not something illegal like MCMLXVIII or (spoiler) MU.

Instead of taking input as strings of MIU, you can choose to take input as arbitrary precision integers using the digits 3 for M, 1 for I and 0 for U. In that case your output would start with 31, which represents MI.



This is , so the shortest program or function that breaks no standard loopholes wins.

  • \$\begingroup\$ Related. Personally a bit surprised there doesn't seem to be a dupe. \$\endgroup\$ Commented Nov 23, 2023 at 6:03
  • \$\begingroup\$ @FryAmTheEggman Unless there's way to solve possibility using some other properties, they'd be too similar \$\endgroup\$
    – l4m2
    Commented Nov 30, 2023 at 7:25
  • \$\begingroup\$ @l4m2 Personally, I think the guarantee of reachability makes them sufficiently different. It is possible that brute force is still the golfiest solution, but even then, not having to handle the fail case seems like a significant factor. I could certainly be wrong, though. \$\endgroup\$ Commented Nov 30, 2023 at 18:20
  • \$\begingroup\$ @FryAmTheEggman Don't a trivial upperbound exist? \$\endgroup\$
    – l4m2
    Commented Dec 1, 2023 at 0:41

Cyclically Sort a List

Given a list of Integers greater than zero, Sort each of the unique values in ascending order, then repeat the process on the remaining duplicate values and append.


[1,5,2,2,8,3,5,2,9] ==> [1,2,3,5,8,9,2,5,2]


Construct the Constructability sequence


Approximately Detect Primes

Given a number from 1 up to 100, your submission should decide whether it is a prime or not. But it is allowed to make up to 5 mistakes, that is, 95 numbers must be classified correctly.

  • 2
    \$\begingroup\$ This is a nice idea, though I imagine somebody will come up with a very simple clever solution and everyone else will just port it \$\endgroup\$ Commented Dec 13, 2023 at 13:43
  • 2
    \$\begingroup\$ FYI the obvious solution seems to be n%2*n%3*n%5!=0 which fails 6 times \$\endgroup\$ Commented Dec 13, 2023 at 16:56
  • \$\begingroup\$ @noodleman Yes I do fear the first, but usually the first few solutions are the creative ones. I did test a whole bunch of the simple modulo solutions, including yours, but I got 8 fails. I must have gotten something wrong D: (My thinking was that with 2,3,5,7 you should be good enough, and I thought with this number you might also have other solutions.) \$\endgroup\$
    – flawr
    Commented Dec 13, 2023 at 21:57
  • \$\begingroup\$ Ok I did make a mistake, I wanted it to be between the (2,3,5) (6 fails) and (2,3,5,7) (4 fails) solution. I updated it accordingly. \$\endgroup\$
    – flawr
    Commented Dec 13, 2023 at 22:01
  • \$\begingroup\$ I think at 5 fails, in a lot of languages this isn’t going to be very different from a plain primality test. Maybe the scoring criteria could be more like length + # of failures? \$\endgroup\$ Commented Dec 14, 2023 at 3:34
  • \$\begingroup\$ (can’t edit after 5 minutes) length * (# of failures / 100)? Similar to codegolf.stackexchange.com/q/265055/108687, just a thought it might be ok as is \$\endgroup\$ Commented Dec 14, 2023 at 3:39
  • \$\begingroup\$ How about you take a different set of numbers to test approximately? OEIS is full of sequences which are neither build-ins of many languages nor follow a too simple formula nor show an obvious pattern. This could result in more creative answers, even late surprising answers. \$\endgroup\$
    – Philippos
    Commented Dec 14, 2023 at 7:22
  • 1
    \$\begingroup\$ There's a 9-byte Python expression I expect most non-golflangs will copy. It can almost be 8 bytes but that leaves 6 errors. \$\endgroup\$
    – xnor
    Commented Dec 15, 2023 at 23:50

Name the Dutch ancestor

  • 1
    \$\begingroup\$ No problems with the question; I like it a lot. But I just want to say the auto-translated names on the linked page are very fun! "for noble tribal old-great-grandparents", "common stem oldenchantedgrandparents" and such. \$\endgroup\$
    – Tbw
    Commented Dec 27, 2023 at 4:27

Can you decrypt me?


Cops, post obfuscated code that hides a number \$n\$ inside its code. If \$n\$ chars are changed, the program outputs \$n\$. Otherwise, it outputs a different number. Both programs may not error, and there can be multiple solutions.


Find any combination of chars to change and what they should change into. You may not crack a cop's answer with anything of the form loop through all possible changes, find one that works.



N is 1.

Robbers' post:



Cops, the user with the most uncracked posts wins. Robbers, the user with the most cracks wins.

  • \$\begingroup\$ How could you stop robbers writes a code that try to apply cops code on every possible inputs until find out one matching output? \$\endgroup\$
    – tsh
    Commented Mar 16, 2022 at 11:22
  • \$\begingroup\$ @tsh uh, can you clarify? \$\endgroup\$ Commented Mar 16, 2022 at 11:51
  • \$\begingroup\$ Does this mean robber just need to write a program that for (i in AllPossibleInputs) if (CopsCode(i) == CopsOutput) return i? \$\endgroup\$
    – tsh
    Commented Mar 16, 2022 at 12:28
  • 1
    \$\begingroup\$ @tsh, is it possible to enforce "don't do that?" \$\endgroup\$ Commented Mar 16, 2022 at 13:11
  • \$\begingroup\$ This is a good format for a CnR, but you need to specify scoring criteria. For the cops, "shortest code" (code-golf) is probably good enough. For robbers, something like "most cracked answer". You should probably add a rule to allow uncracked cops' answers to become "safe" after a certain amount of time (probably around 1-2 weeks) \$\endgroup\$
    – pxeger
    Commented Apr 12, 2022 at 9:01
  • \$\begingroup\$ @pxeger does it need more clarifying and details? (other then scoring criteria) \$\endgroup\$ Commented Apr 12, 2022 at 14:10
  • \$\begingroup\$ No, if you add those things, it will be ok \$\endgroup\$
    – pxeger
    Commented Apr 12, 2022 at 14:47

Classify a Coxeter group

In this challenge you will take as input a Coxeter matrix representing a Coxeter system. Your program will then classify it as one of the following:

  • Spherical
  • Euclidean
  • Compact
  • Paracompact
  • Hypercompact

Your program should output one of 5 distinct values depending on which category the input falls into.

This is . The goal is to minimize the size of your source code as measured in bytes.

Spherical (finite)

A Coxeter system is spherical iff it is finite.

Euclidean (affine)

A Coxeter system is Euclidean iff the determinant of its Schläfli matrix is 0.

TODO better definition of this, or explain what a Schläfli matrix is.


A Coxeter system is compact iff every strict minor of its Coxeter matrix gives a finite group (and it is not itself spherical or Euclidean). That is every way to remove one generator from the system gives a spherical group.


A Coxeter system is compact iff every strict minor of its Coxeter matrix gives a spherical or Euclidean group (and it is not sperical, Euclidean or compact). That is every way to remove one generator from the group gives a spherical or Euclidean group group.


If a Coxeter system is not any of the above then it is called hypercompact.


I had this idea, but haven't had enough time to write up a specification. I'll explain things better later, define a Coxeter system and give the enumerations of each class.



In this challenge "cops" will write a computer program and compile it. They will post the result of the compilation (for simplicity we will just call this the "binary"), along with a description of the compilation (what compiler was used, what flags were used etc.), but not the source.

"Robbers" will then try to find source code that compiles into the same result.

The goal of the cops is to make as short of an executable as possible without being solved by the robbers, and the goal of the robbers is to crack as many cops answers as possible.


There are a lot of specifics here to deal with.

  • First let's be clear that the robber does not have to find the exact program used by the cop, as long as the program the robber presents compiles to the binary with the given description.
  • I'm going to require that the cops program consist of only 1 file. The contents of this file will be referred to as "the source".
  • When describing the compilation process the cop should include everything variable that affects the binary other than the source. A non-exhaustive list of things that may affect the binary is:
    • The version of the compiler used.
    • The architecture of the machine used.
    • Flags provided to the compiler.
    • The name or any other metadata of the file containing the source.
    • The number of times the compiler is run. (Yes unfortunately there are compilers that are not idempotent.)
    • The exact time the compiler was run.
  • While we call it a binary for simplicity, the cop's program can be compiled to any target. You can even present something that might normally be considered an intermediate stage of compilation.
  • The binary can do anything non-malicious. Don't write something that would do damage to the system it is being run on, but other than that you may do anything or nothing at all. Your binary may error, and the compiler may error while producing it.

Vulnerable, cracked, safe and revealed

Cops' posts here can have 4 different states:

  1. Vulnerable
  2. Cracked
  3. Safe
  4. Revealed

All cops' posts start as vulnerable. If a robber finds and posts a solution to a vulnerable post they will receive 1 point and the post will become cracked. If a post lasts 10 days in vulnerable without being solved it automatically becomes safe. A cop with a safe post may choose to reveal A and B and their safe post will become revealed. A robber can still solve a safe post, if they do they receive a point and the post becomes revealed.

Only revealed posts are eligible for scoring. Robbers are only permitted to solve vulnerable and safe posts.


Weave two lists, cycling if necessary


Where are zeros? Self-describing sequence


Count Syllables

The goal of this challenge is to write a program that can count the syllables in a word as accurately as possible.


On STDIN, your program will receive a number X followed by X lines, each containing a single word. Simple enough. (Should there be a limit on the size of X?) The words will come from this list.



Your output should be to STDOUT and have X lines. On each line should be the number of syllables counted in that word.



To score you program, it will receive a long secret list of words to test. All programs will receive the same list of words. For each word, the number of syllables that your program got wrong will be added to the score of the program. If it output a 4 or a 2 when the word had 3 syllables, then one point will be added. If it said a 15 instead of a 3, then 12 points will be added to the score. The lower the score, the better.

For example, if for the above input your program output 3 2 2 2 (which would be produced by a program that counts strings of vowels), then the program would receive a score of 2.


Your program should not access any external files (such as the word list). Also, your program should be no more than 5,000 bytes long (is this a reasonable limit?).

The winner will be the person whose program has the lowest score, therefor the most accurate syllable counter. The deadline for submissions is [some time at least a month away].


I am open to all constructive criticism. Is 5,000 bytes a reasonable limit for the program size? How long should the official scoring test be? How long should the deadline be?

  • 12
    \$\begingroup\$ This has one major flaw: the output is subjective. How many syllables do these words have? Every; victory; hierarchy; desire; oil; hour; poem. The only real way I see to work around this is for you to produce a marked-up version of the word list. \$\endgroup\$ Commented May 29, 2012 at 20:40
  • \$\begingroup\$ I was really worried about that, and I don't see a way around it. \$\endgroup\$
    – PhiNotPi
    Commented May 29, 2012 at 20:42
  • 2
    \$\begingroup\$ I personally would love to see more language processing challenges. I agree with @PeterTaylor on the difficulty of some words. Perhaps taking a specific text(s) and identifying explicitly in the challenge which words will have how many syllables? \$\endgroup\$
    – Gaffi
    Commented Jun 8, 2012 at 3:34
  • 2
    \$\begingroup\$ @PeterTaylor ...Or maybe you could filter ambiguous words out of the reference list? \$\endgroup\$
    – user16991
    Commented Feb 8, 2015 at 1:19
  • 1
    \$\begingroup\$ What's the point of the first line of input? \$\endgroup\$
    – msh210
    Commented Apr 27, 2016 at 20:05
  • 3
    \$\begingroup\$ If you provide a reference list, A hyphenated reference list, and hide a secret list which may or may not include members of the reference list, this would be a reasonable challenge \$\endgroup\$ Commented Sep 17, 2016 at 0:05
  • \$\begingroup\$ Do you plan to post this? If not, I'd be happy to adopt it. (If you don't respond within two weeks, by community standards, I'm allowed to do so.) \$\endgroup\$
    – MD XF
    Commented Aug 18, 2017 at 3:20
  • \$\begingroup\$ The example of inaccurate program that would score 2 - did you mean to output 3 1 1 2 rather than 3 2 2 2? \$\endgroup\$
    – Heimdall
    Commented Nov 9, 2017 at 18:31
  • \$\begingroup\$ A reference list could be dynamic: potential contestants can ask for words of their choice to be added to the list. They won't know what's on the secret list but will try to make their programs as accurate as possible (according to your syllable count) so they should always be able to ask for specific words they are not sure about. Of course, you could make it in different language. In my language, Slovene, it's much clearer how many syllables words have. How about Solresol, haha! \$\endgroup\$
    – Heimdall
    Commented Nov 9, 2017 at 18:38
  • \$\begingroup\$ I am going to adopt this if you don''t respond \$\endgroup\$
    – user63187
    Commented Dec 20, 2017 at 16:48

Play Simple 2-Dimensional Minecraft

Recently I found this video of "HansLemurson" showing a computer that was built in minecraft, which runs minecraft. He is playing minecraft on a computer that was built in minecraft that is running on his computer. To be specific, it is a two dimensional version with an 8x8 grid of cells. There is gravity, block placement, and even jumping. It is worth noting that the computer is single purpose. The same person has built programmable computers, but making them single purpose allows the computer to be much smaller.


The minecraft world is an 8x8 grid (one horizontal and one vertical dimension). The grid is comprised of either Xs (representing blocks) or empty spaces. The player is an X that is blinking on and off about once every second.

There are two modes in the game, controlled by a toggle switch. The first mode is movement. This is controlled by a WASD-like button arrangement. If the player chooses to move left/right/down, the computer checks to see if the space immediately in that direction is empty. If so, then the player moves into that space.

If the player chooses to move up, then the computer checks that the block underneath the player is solid. If so, then the player moves upward two units. Notice that this can propel the player into a solid block. If this happens, the player is obscured by the solid block, but can still move to an empty block next to him. When the player is inside on a solid block, the game continues as if the block isn't there, although the block is still there once the player leaves it.

After each move, the player falls down one unit if there is empty space there. This simulates gravity. This is also why moving up moves up two units, so that the gravity makes a net movement of up one unit. Gravity does not cause the player to fall all of the way to the ground, just one unit.

The second mode is block placement. In this mode, the same exact WASD buttons are used. Instead of moving the player, they toggle the state of the block in that direction. If the player presses "left" and there is a block there, then the block is destroyed. If there is not a block there, then a block is placed. Again after this move, the player is again subject to gravity. The blocks are not subject to falling.

Toggling the toggle switch does not count as a move, and does not invoke gravity.

The game board is a torus, so all actions (movement, block creation) can wrap around the board. The board does not scroll with the player. The player moves, and the blocks stay in the same place.

The challenge

You challenge is to write the shortest program that simulates this game. Your program should display and update the map correctly (with Xs as blocks, and with the blinking player). It should accept input from a button that toggles the state and four buttons for movement and actions. This is code golf.

There are imaginary bonus points for adding more features (block types, game size, etc) to your game.


  • 4
    \$\begingroup\$ With more complicated challenges I find that it helps to do a reference implementation so that you have a very concrete idea of how much work is involved. Aside from that, I like it. \$\endgroup\$ Commented Jun 3, 2012 at 20:11
  • \$\begingroup\$ Is the blink rate selected to fit with the ANSI escape sequence? Either way I would explicitly allow that, because it's the obvious way to do it on compatible terminals. \$\endgroup\$ Commented Jun 5, 2012 at 7:14
  • \$\begingroup\$ The blink rate wasn't selected to be anything specific. I think that I will broaden the restriction. Maybe any blink rate between 3 blinks per second to 1 blink every 2 seconds. \$\endgroup\$
    – PhiNotPi
    Commented Jun 5, 2012 at 20:21
  • \$\begingroup\$ @programmer5000 No, for two main reasons: First, challenges can go extended periods of time in the sandbox before they are posted and/or adopted. In the past I've posted challenges after not touching them for 4 years. Second, deleting this answer will not reduce lag, as deleted answers are still present, simply not visible. Users with sufficient rep will see all 4040 answers in the sandbox, and you will too once you earn the "view deleted answers" privilege. \$\endgroup\$
    – PhiNotPi
    Commented Apr 13, 2017 at 18:15

Bad Voice Recognition Calculator


Let's say you've decided to operate your computer using voice recognition software, but unfortunately you did a horrible job researching the various products out there and chose a package that does not recognize numbers as numerals, only words. (i.e. "one" (spoken) == "one" (typed), not "1".) Rather than spend more money to get another option, you decide to make do. Now you want to use the computer's calculator, but this poses a problem, since your machine doesn't know how to add "one plus one".


Implement a basic calculator that will read in a string of the written-out equation, perform the correct calculations, then return the result in its text form. Your code should be as short as possible; this is code golf.


  • Input/output will be using your preferred method (STDIN, ARGV, etc.).
  • Your calculator must be able to handle input and output within the billions (non-inclusive) -1,000,000,000 < i < 1,000,000,000, but you may expand to more if you wish.
  • Decimal values and/or parts must be accepted (0 < i < 1) up to 3 places/digits.
    • When calculating answers, proper rounding must be used, so "three point one four one five nine two six" must be returned as "three point one four two".
  • Basic calculator functions required:
    • "Add"/"Plus"/"Sum"/"And" (+)
    • "Subtract"/"Minus"/"Remove" (-)
    • "Multiply"/"Times" (*)
    • "Divide"/"Divided"/"Divide by"/"Divided by" (/)
    • "Raise"/"Exponent"/"Power"/"To the power of" (^)
    • "<Base>Root"/"<Base>Radical" (√)
    • "Point"/"Decimal" (.)
    • "Pi" (π)
  • All strings in the list above must be accounted for in your code, capitalization does not matter.
  • Numbers may be presented as their full value ("one thousand") or by digit (one zero zero zero).
  • Negative numbers may be assigned using "Minus" or "Negative".
    • The string "Minus" bust be accounted for as an operator and identifier. (see example)
  • "And" is only acceptable as an operator, not as part of a named number.
    • "one hundred and one"
    • "one hundred one"
  • "a" or the absence of a number does not equate to any number; all numbers will be explicitly accounted for in the program input.
    • "a hundred" does not equate to "one hundred" and is not a valid input.
  • No more than 2 terms will be used.
    • "one plus one minus one" will not be implemented.
  • If an invalid input is supplied, your function/program should handle the error and exit gracefully with an error description.

Example I/O:

  • "one add one" --> "two"
  • "five thousand thirty four subtract ten thousand six hundred" --> "negative five thousand five hundred sixty six"
    • Alternatively: "five zero three four subtract one zero six zero zero"
  • "three root twenty seven" --> "three"
  • "ten minus minus ten" --> "twenty"
    • Alternatively: "ten subtract negative ten"

Sandbox Questions:

  1. Is this too basic/complicated? (I'm assuming some languages will handle this much more simply than the method I have in my head...)
  2. Does the title fit?
  3. Are there any constraints that should be added/lifted?
  4. Are any more examples needed for clarification?

Thanks for your input, guys!

  • \$\begingroup\$ Not everyone says numbers the same way. Does the parser have to treat the following as equivalent? "negative one hundred five", "minus one hundred five", "negative one hundred and five", "minus one hundred and five", "negative a hundred five", "negative a hundred and five", ...? \$\endgroup\$ Commented Jun 15, 2012 at 15:12
  • \$\begingroup\$ @PeterTaylor I had had a similar thought re: operators. ("plus" versus "add", etc.) I think it would be more interesting to account for all, but given the wide variety of possible inputs, it may generally be better to limit the options to a specifically defined set (which I have yet to define). \$\endgroup\$
    – Gaffi
    Commented Jun 15, 2012 at 15:18
  • \$\begingroup\$ @PeterTaylor I've added some of these details. Please let me know if there's anything unclear about them. \$\endgroup\$
    – Gaffi
    Commented Jun 15, 2012 at 16:10
  • \$\begingroup\$ I don't spot any ambiguities in the parser. There is still an ambiguity relating to decimals, though. What precision should be used? Also, I notice now that there's no winning condition. Is this intended to be code-golf? (Ugh - tons of strings which will have to be hard-coded in most languages. I expect Perl has a suitable parser already in CPAN, though...) \$\endgroup\$ Commented Jun 19, 2012 at 9:03
  • \$\begingroup\$ @PeterTaylor I don't know where I went... I've updated the spec. re: decimal places and objective. \$\endgroup\$
    – Gaffi
    Commented Jun 29, 2012 at 13:24
  • 1
    \$\begingroup\$ @PeterTaylor metacpan.org/pod/Lingua::EN::Words2Nums \$\endgroup\$
    – msh210
    Commented Apr 27, 2016 at 20:37

Chess move

The Challenge

Write a program that gets a string containing a chessmove and a chessboard as input, and then outputs the chessboard.


The chess move will have this format:

<from square><to square>[<promoted to>]



The chessboard format is not fixed, but there must be a 1 to 1 relation between the board and the string to represent the board. Also the format of the input must bet the same as the format of the output. Two suggestions of what it could look like:


rnbqkbnr pppppppp 00000000 00000000  00000000  00000000 PPPPPPPP RNBQKBNR

It is not required to store anything except the location of the pieces, and validity of moves can be assumed.


Base score is character count (assuming your program can move pieces for all moves)

Bonus multipliers:

  • If the program updates the promoted piece, divide by 2
  • If the program also moves the rook when castling, divide by 2
  • If the program also removes the pawn when capturing en passent, divide by 2

The moves, and castling & en passent in particular are explaned on Wikipedia.

So basically writing a 100 character solution for the base problem gives the same score as an 800 character solution with all bonus multipliers.


If you would choose to use one of the board formats above, your input would look like one of these strings:

e2e4 rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR

e2e4 rnbqkbnr pppppppp 00000000 00000000  00000000  00000000 PPPPPPPP RNBQKBNR

Your corresponding output string would then be one of these:


rnbqkbnr pppppppp 00000000 00000000  0000P000  00000000 PPPP0PPP RNBQKBNR
  • 1
    \$\begingroup\$ Before I get on to more specific criticisms: as presented, without the bonus this is too trivial to be interesting. I suggest removing some flexibility: require Fen notation for the board position and algebraic notation for the move, and making the current bonus options mandatory. On specifics: it's not clear why you talk about storage; and the board position notations you suggest don't include enough information to know whether en passant is possible. \$\endgroup\$ Commented Dec 22, 2013 at 23:56
  • \$\begingroup\$ @PeterTaylor I agree that compared to chess programs this may be trivial, but I would like to make it a golf challenge. Compared to the hot code golf questions this is quite elaborate already in its basic form. (For a good solution the board design may need to be changed drastically). It is true that there is no attention to the legality of moves (whether it is possible to capture en passent) but for a mere viewer this is not required so I am not too worried about this. So far the chess questions seem to get very few answers as they tend to be complex and I hope to offer relatively easy entry. \$\endgroup\$ Commented Dec 30, 2013 at 11:02
  • 2
    \$\begingroup\$ Your point about en passant is valid - you had said in the spec to not worry about legality. I'll try to convince you of my first point: without the bonus, this reduces to: a) parse first four characters into (col 1, row 1, col 2, row 2); b) take board as a 64-char string; c) board[8*row_2+col_2] := board[8*row_1+col_1]; board[8*row_1+col_1] := ' '; print board. This is trivial compared to any good golf question. (Note that the hot questions at the moment are neither golf questions nor good questions). \$\endgroup\$ Commented Dec 30, 2013 at 12:14
  • \$\begingroup\$ This sandbox post has had little activity in a while. Please improve / edit it or delete it to help us clean up the sandbox. Due to community guidelines, if you don't respond to this comment in 7 days I have permission to vote to delete this. \$\endgroup\$
    – user58826
    Commented Jun 9, 2017 at 15:40

Black Box

Your task is to analyze a given situation for the game Black Box. Given a sequence of guesses and answers, your program is to either print the solution or suggest the next move.

The game

The board consists of 8×8 cells, with edges labeled like this:

I'll probably create nice images here, particularly to make sure that the squares of the board are really square.

i        I
j        J
k        K
l        L
m        M
n        N
o        O
p        P

The player shoots rays into the interior of the box, where they might get deflected, reflected or absorbed. He is told the position where the ray leaves the black box again, and from that has to deduce the positions of 4 atoms inside the black box.

I'll have to include more of the game rules here, but for now see Wikipedia.

Input and output

Input is a sequence of line, each consisting of two characters. The first denotes the point where the ray of light enters the black box, the second the place where it comes out again. In the case of a reflection, both characters will be equal. In the case of a hit, the second character will be -.

If the input is enough to fully determine the locations of the atoms, then output should be four lines giving the coordinates of each atom. The lines should be two lower case characters each, the first giving the row and the second giving the column of the found solution. The atom positions must be printed in lexicographical order.

If the input is consistent with more than one set of atom positions, then the output should consist of a single line containing a single character, which is the location where the next ray should be shot. That location has to be chosen in such a way that it can help find the solution. This is the case unless all of the atom positions consistent with the input so far would produce the same output for this next ray as well.

Your output has to be terminated by a newline character.


Let's take the atom configuration the Wikipedia article uses as an example as well:

i        I
j        J
k O    O K
l        L
m        M
n   O    N
o        O
p      O P

If the input were


then the output should be


but if the input were only


then the output might be for example



This is code golf, so shortest answer wins. However, I'll only accept answers which are practical in so far as they compute their result in reasonable time. I'd say no more than five minutes on my system where I'll evaluate the answers, and I'll simply hope that correct solutions will be much faster and incorrect ones much slower, so that the speed of my system doesn't make a difference. A submission which gives a wrong answer for one of my test cases will be disqualified until it gets fixed. I will probably point out the problem in a comment to that post.


Create a program with "exact repetition" in its source code

The task is to create a program, with the following restrictions placed on the printable ASCII characters in the source code: choose some k > 0.

  • Every non-alphabetic character has to appear exactly k times.
  • Every alphabetic character has to appear at most k times.
    • This rule differs from the former in order to avoid boring dummy identifiers while still making it a challenge to choose good library functions to call.

Character set definitions used:

  • Non-alphabetic characters are !"#$%&'()*+,-./0123456789:;<=>?@[\]^_{|}~ and '`' (backtick).
  • Alphabetic characters are ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz.

Note that no restriction is placed on characters outside of the range of printable ASCII characters (including control codes, tabs, newlines, higher unicode codepoints, etc).

What the program does is up to you; be creative. Some general guidelines:

  • Programs that do something interesting might have better chances, although more impressive code structure (i.e. fewer comments) is also beneficial.
  • Stuffing excess characters in comments is boring, and should be avoided/is discouraged.
  • Dead/no-op code isn't terribly interesting either, but is probably unavoidable and at least has to conform to the language's grammar.

This is : whatever has the most upvotes at Feb 1, 2014 gets accepted as the winner.

Example answer (C)

/*$$@``*/_[]={9.};main() {printf("He%clo \

Prints "Hello world!" (adapted from an answer to another question). Probably wouldn't score a lot (since what it does isn't terribly interesting). Each of the non-alphabetic characters appear exactly twice, and no alphabetic character appears more than twice.

For meta: I want to post this, but I'm worrying that "do something interesting" might give too little guidance and the question won't receive many answers.. thoughts? Is it good as-is, or should I come up with some task that one should be required to implement (and possibly change the ruling to code-challenge, with length + 2^(characters-in-comments) as the score)?


This is my first try at writing a challenge. Please let me know how I can improve it.

Roman Calculator

Create a basic calculator for Roman numerals.


  • Supports +,-,*,/
  • Input and output should expect only one prefix per symbol (i.e. 3 can't be IIV because there is two I's before V)
  • Input and output should be left to right in order of value, starting with the largest (i.e. 19 = XIX not IXX, 10 is larger than 9)
  • Left to right, no operator precedence, as if you were using a hand calculator.
  • Supports whole positive numbers input/output between 1-4999 (no need for V̅)
  • No libraries that do roman numeral conversion for you

For you to decide

  • Case sensitivity
  • Spaces or no spaces on input
  • What happens if you get a decimal output. Truncate, no answer, error, etc..
  • What to do for output that you can't handle. Negatives or numbers to large to be printed.

Extra Credit

  • -20 - Handle up to 99999 or larger (numbers with a vinculum)

Sample input/output

XIX + LXXX                 (19+80)

XCIX + I / L * D + IV      (99+1/50*500+7)

The shortest code wins.

  • 1
    \$\begingroup\$ You might want to be explicit about which variants of Roman numerals need to be supported. For example, do I have to understand IV as 4, or can I require that it be written as IIII? And what about, say, writing 8 as IIX instead of VIII, 19 as IXX or XVIV instead of XIX, or 99 as IC instead of XCIX? (All these variants have, AFAIK, been used classically.) \$\endgroup\$ Commented Feb 9, 2014 at 22:36
  • \$\begingroup\$ @IlmariKaronen thanks. I modified the question to be slightly more specific about that. \$\endgroup\$
    – Danny
    Commented Feb 10, 2014 at 14:09
  • 1
    \$\begingroup\$ I think that using IV, IX, IC, XC, etc. should be alright, but only allow one prefix. Also, 19 should be written XIX, not IXX. One other thing, can we assume that the operators will be separated by a space, or no? \$\endgroup\$
    – user10766
    Commented Feb 12, 2014 at 0:32
  • 1
    \$\begingroup\$ Hello! This looks like a good but abandoned meta post, would you be willing to offer it for adoption? (If you want to, you can still post to main.) \$\endgroup\$
    – user58826
    Commented Jun 9, 2017 at 16:06
  • \$\begingroup\$ 1. I don't need to handle I/III but need to handle I/III+II/III? 2. For the extra can I output maybe [V] for 5000? \$\endgroup\$
    – l4m2
    Commented Apr 12, 2018 at 15:05
  • \$\begingroup\$ @programmer5000 it was posted to main awhile ago. codegolf.stackexchange.com/questions/20670/… \$\endgroup\$
    – Danny
    Commented Apr 26, 2018 at 11:58

Create a calendar

We all know HDD-space is precious and bandwidth is expensive, therefore it is best to reduce the size of your executables. Let's start with your calendar:

Your task is to build a calendar app in at most 512 bytes. The calendar must at least support the following features, but additional features may gain you additional upvotes:

  • It must be able to show the current month with the current day highlighted
  • The user must be able to find out the week day of each day


  • Maximum code length is 512 bytes (counted as UTF-8 without BOM)
  • You may subtract the bootstrapping code (i.e. int main(int argc, char **argv) in C or <?php in PHP) and imports from the final size to allow for more verbose languages to be in
  • You may use standard time / date functions of your programming language, as long as they don't allow you to output a ready to use calendar
  • No network access (I said bandwidth is expensive!)
  • Voters decide on the amount of features / look and feel / creativity

This needs a tag for the size restriction, any suggestions?

  • 1
    \$\begingroup\$ "bandwidth is expensive" <sup>[citation needed]</sup> \$\endgroup\$ Commented Mar 22, 2014 at 5:27
  • 1
    \$\begingroup\$ Seems rather close to Output: Calendar Month \$\endgroup\$ Commented Mar 22, 2014 at 5:33
  • \$\begingroup\$ Who decides what counts as bootstrapping code? It seems odd to arbitrarily exclude code like that, and the examples you gave can be golfed a lot: they're more or less equivalent to main(){ and <? respectively. \$\endgroup\$ Commented Mar 24, 2014 at 20:49
  • \$\begingroup\$ @WanderNauta Bootstrapping code is the code that's essentiell to get a working noop program. \$\endgroup\$
    – TimWolla
    Commented Mar 24, 2014 at 21:00
  • \$\begingroup\$ @TimWolla That definition won't fly. A zero-byte file is a working noop PHP script, for example. \$\endgroup\$ Commented Mar 24, 2014 at 21:01
  • 1
    \$\begingroup\$ @WanderNauta A zero byte file is a working noop in every language. \$\endgroup\$
    – TimWolla
    Commented Mar 24, 2014 at 22:12
  • \$\begingroup\$ So what's bootstrapping code then? :) \$\endgroup\$ Commented Mar 24, 2014 at 22:53
  • 1
    \$\begingroup\$ for the limit I'd say code-shuffleboard or restricted-source \$\endgroup\$
    – Einacio
    Commented Mar 26, 2014 at 15:57
  • \$\begingroup\$ This sandbox post has had little activity in a while. Please improve / edit it or delete it to help us clean up the sandbox. Due to community guidelines, if you don't respond to this comment in 7 days I have permission to vote to delete this. \$\endgroup\$
    – user58826
    Commented Jun 9, 2017 at 16:28

Hi, first time golf questioner, hopefully I'm doing it right!

Maths Trade Calculator

A maths trade (or "math" trade if you prefer) is a way of calculating complex trades of arbitrary items in a circle of participants where not all participants want all items.

X participants have an item they would like to trade. Each participant is assigned a unique number, and provides a list of (numbers identifying) the items they would willingly trade their item for. They may provide an empty list (i.e. they would rather not trade).


X lines, one for each participant, comprising a unique number identifying them, followed by a colon, then a comma-separated-list of numbers identifying other items that they would trade for. e.g.:


The numbers identifying the participants will not necessary be in order, nor will they necessarily be 1 to X. You may assume that they will be numeric.

This string can be in STDIN, or an argument to a function, or similar and can be followed by a new-line or not, whatever the coder prefers.


One or more trade loops in which all participants are making trades they're happy with. Each loop should be on a new line and comprise a participant number, followed by "->", followed by the participant they should give their item to, then another "->", and another participant number etc, until the loop is closed and the last participant number matches the first one. Another line is added with the number of completed trades. e.g.:


Participants for which no valid trade is possible are omitted.

Output can be via STDOUT, or returned as a string, or something else, with an optional final new-line.

Trade rules

  1. A participant may not be involved in more than one trade
  2. A participant may not receive an item that they didn't want
  3. All loops must be closed
  4. Maximum number of possible trades should be completed (i.e. no submitting a zero-trade output and claiming it's valid). If there are multiple permutations, pick whichever you prefer.

This is a code golf challenge, so shortest working code wins.

Some more example inputs and possible outputs




For instance, in this trade: 9 stated that he would accept 1's item in a trade, 10 stated that he would accept 9's item, 3 would accept 10's and 1 would accept 3's. In the second loop, 2 receives 7's item, 5 receives 2's, 6 receives 5 and 7 receives 6's. (Other outputs are possible from this input.)







1->9->1 is also valid in this case, but both cannot be completed. Either is acceptable.

Thanks for reading guys! Let me know if there are any improvements I can make.

  • \$\begingroup\$ "can be followed by a new-line or not, whatever the coder prefers." How flexible is this? For instance, can I use trailing commas, like 1:2,4,7, if it makes my code shorter? \$\endgroup\$ Commented May 2, 2014 at 17:28
  • 2
    \$\begingroup\$ Will the participants always be numbered 1 to n and their input lines provided in order? If so, state it. If not, include a test case which fails if an implementation decides to ignore everything before the : in each input line. \$\endgroup\$ Commented May 5, 2014 at 10:02
  • \$\begingroup\$ @m.buettner I would say a trailing comma is not acceptable, on the end of any line, or the end of the input/output. \$\endgroup\$
    – Johno
    Commented May 6, 2014 at 8:55
  • \$\begingroup\$ @PeterTaylor Good tip. I'll correct the question to state that you can't assume that the numbers will be 1 to n, in order. \$\endgroup\$
    – Johno
    Commented May 6, 2014 at 8:57
  • \$\begingroup\$ Hello! This looks like a good but abandoned meta post, would you be willing to offer it for adoption? (If you want to, you can still post to main.) Due to community guidelines, if you don't respond to this comment in 7 days I have permission to adopt this. \$\endgroup\$
    – user58826
    Commented Jun 9, 2017 at 16:39

Design and Solve a Maze

(this question on hold while the details are ironed out)

Your task is to play the roles of both characters in this scene from Inception. In it, Cobb gives Ariadne a challenge:

You have two minutes to design a maze that takes one minute to solve.

Some liberties will be taken on that description. Most importantly, this challenge is not time-based, rather scores are based on the effectiveness of your mazes and maze-solvers.

I apologize for the many edits to this challenge as we iterate towards an easy and fair format..

Part I: Maze format

All mazes are square. A cell in the maze is represented as a zero-indexed tuple row column.

Walls are represented by two binary strings: one for horizontal walls (which block movement between rows) and vertical walls (vice versa). On an NxN maze, there are Nx(N-1) possible walls of each type. Let's take a 3x3 example where the cells are labelled:

A   B | C
D | E   F
G   H | I

all possible vertical walls are: AB BC DE EF GH HI. Translated into a string, the walls shown are 011001 for vertical walls and 010010 for horizontal walls. Also, by "binary string" I mean "the characters '0' and '1'".

The full maze format is a string which contains, in this order:

  • width
  • start cell tuple
  • end cell tuple
  • horizontal walls
  • vertical walls

For example, this maze:

   0 1 2 3 4
0 | |  E|  _|
1 |  _|_|_  |
2 |_ _ _  | |
3 |  _ _  | |
4 |____S|___|

is formatted to this:

4 2
0 2

Part II: The Architect

The Architect program creates the maze. It must play by the rules and provide a valid maze (one where a solution exists, and the end is not on top of the start).

input via stdin: Two positive integers:

size [random seed]

Where size will be in [15, 50]. You are encouraged to make use of the random seed so that matches can be replayed, although it is not required.

output to stdout: A valid size x size (square) maze using the format described in Part I. "valid" means that a solution exists, and the start cell is not equal to the end cell.

The score of an Architect on a given maze is

   # steps taken to solve
max(dist(start,end),(# walls))

So architects are rewarded for complex mazes, but penalized for each wall built (this is a substitute for Ariadne's time restriction). The dist() function ensures that a maze with no walls does not get an infinite score. The outside borders of the maze do not contribute to the wall count.

Part III: The Solver

The Solver attempts to solve mazes generated by others' architects. There is a sort of fog-of-war: only walls adjacent to visited cells are included (all others are replaced with '?')

input via stdin: the same maze format, but with '?' where walls are unknown, an extra line for the current location, and a comma-separated list of valid choices from this location. (This is a big edit that is meant to make it simpler to write a maze-parsing function)

example (same as the above 5x5 maze after taking one step left)

4 2
0 2
4 1
4 0,4 2

Which corresponds something like this, where ? is fog:

   0 1 2 3 4
0 |????E????|
1 |?????????|
2 |?????????|
3 | ?_?_????|
4 |__C_S|_?_|

output to stdout: One of the tuples from the list of valid choices

Each Solver's score is the inverse of the Architect's score.

Part IV: King of the Hill

Architects and Solvers are given separate scores, so there could potentially be two winners.

Each pair of architects and solvers will have many chances to outwit each other. Scores will be averaged over all tests and opponents. Contrary to code golf conventions, highest average score wins!

I intend for this to be ongoing, but I can't guarantee continued testing forever! Let's say for now that a winner will be declared in one week.

Part V: Testing

I have written a Python testing kit which includes a Maze class for parsing and writing in the proper formats, as well as an example architect/solver pair: Daedalus and the Minotaur

Available on both Dropbox and GitHub

Part VI: Submitting

  • I maintain veto power over all submissions - cleverness is encouraged, but not if it breaks the competition or my computer! (If I can't tell what your code does, I will probably veto it)
  • Come up with a name for your Architect/Solver pair. Post your code along with instructions on how to run it.
  • \$\begingroup\$ I suppose input is via STDIN? You might want to mention that explicitly, because at least the architect could just as well take the input via command-line arguments. \$\endgroup\$ Commented May 15, 2014 at 15:34
  • \$\begingroup\$ updated. I have a driver/referee program which will handle I/O; I'll update it to use stdin/stdout since that will no doubt be the easiest standard. \$\endgroup\$
    – wrongu
    Commented May 15, 2014 at 16:04
  • \$\begingroup\$ @m.buettner before de-sandboxing this, would you be willing to try the test kit? \$\endgroup\$
    – wrongu
    Commented May 15, 2014 at 18:20
  • \$\begingroup\$ I'd love to, but I'm afraid I'm too busy this week. Try ask for help in the chatroom. \$\endgroup\$ Commented May 15, 2014 at 18:35
  • \$\begingroup\$ Possible architect issue: With this scoring method (steps/walls), you can get a minimum score of 3 by simply putting the start/finish right next to each other with a single wall between. It takes three steps to go around. Most actual mazes I've seen have too many walls to make a score of 3 likely, much less guaranteed. \$\endgroup\$
    – Geobits
    Commented May 16, 2014 at 13:51
  • \$\begingroup\$ Thats a problem. What if the dist function was shortest path? Then only mazes which cause detours could get a score > 1 \$\endgroup\$
    – wrongu
    Commented May 16, 2014 at 15:09
  • \$\begingroup\$ That would probably be better. That way it's scored on best vs actual. It would take away the incentive to figure out how to build hard mazes with few walls, though, which was interesting itself. \$\endgroup\$
    – Geobits
    Commented May 17, 2014 at 3:01
  • \$\begingroup\$ Hey rangu... not sure if you're still planning to do this thing, but overactor just said something in chat which reminded me of your challenge and might be a neat way to avoid the combined score: split this up into two code-challenges, one for maze generation and one for maze solving. Each code-challenge's benchmark set (to determine the scores) would be the outputs of the other challenge's participants. Then you could just pick a best solver and a best generator independently. \$\endgroup\$ Commented Aug 1, 2014 at 11:18

Author note: I was thinking about new genres today, and I had an idea. What if there could be a challenge that encourages people to write good code, instead of the code-golf gibberish we all know? Here's a challenge that attempts to do that. (This could even possibly be a , which would be great because it would bring in a greater high quality question volume to the site, but I'm terrible at coming up with names. Feel free to suggest something in the comments.)

Build your own image editor


For this challenge, you will create the best GUI image editor that can perform the most tasks that you possibly can... from scratch.

Tasks and scoring

Here are the features / tasks used to score your program. Each task is worth a certain amount of points, which is specified in brackets before the task description. For convenience, each task will also be prefixed by an ID string so that you can refer to them when describing your program.

  • [1 A] Brush tool: Simple, click and drag the mouse to draw freestyle doodles. Must draw a contiguous path.
    • [1 A1] Ability to change the brush size.
  • {TODO: etc., add more}


Your editor must conform to the following requirements:

  • Must accept input via the mouse. Tools (brush, flood fill, etc.) can be switched and configured with keyboard shortcuts, by clicking icons with the mouse, through a menu, or however you would like.
  • You may not use a single built-in function to accomplish one or more of the tasks. For example, if your language has a built-in image flood fill function, you may not use it and must build your flood fill from scratch.

Final score and voting

This is the syntax you should use to describe your score in your answer:

# {language}, {your score} score
<sup>(features implemented: {A, A1, ...})</sup>

    {your code here}

{description, comments, other notes, etc. here}

The amount of votes your post has (upvotes minus downvotes) will be multiplied by {TODO: figure out a good number} and added to your score. (Do not add this to the score in your post, since votes change constantly; I will add them manually.) Voters, please vote according to the following criteria:

  • elegance and readability of the source code
  • ease of use of the image editor and how powerful it is
  • remember to sort by "active" so that you're voting for new answers too, and not just the top voted ones!
  • 3
    \$\begingroup\$ Your questions should be reasonably scoped. If you can imagine an entire book that answers your question, you’re asking too much. A really good answer to this would run into millions of lines of code. \$\endgroup\$ Commented Jun 4, 2014 at 15:52
  • \$\begingroup\$ @PeterTaylor Yes, I was a little worried about that, but if it's not broad enough, it will be easy to just implement all the features. Any suggestions for fixing this? I was thinking of adding a "brevity" criterion in the voting section, but that doesn't seem like an ideal solution. \$\endgroup\$
    – Doorknob
    Commented Jun 4, 2014 at 16:00
  • 3
    \$\begingroup\$ To be honest, the site for good code is Code Review. They already have a monthly challenge, for which they post snippets for review. I don't see a need to copy them. \$\endgroup\$ Commented Jun 4, 2014 at 16:12
  • \$\begingroup\$ @PeterTaylor Wait, isn't Code Review for questions and answers, not challenges and contests? In any case, is there any reason for that to prevent us from posting challenges like we always have? \$\endgroup\$
    – Doorknob
    Commented Jun 4, 2014 at 16:17
  • \$\begingroup\$ meta.codereview.stackexchange.com/questions/tagged/… . Surprised you don't know about it, given how dedicated you are to spying on them ;) In general, if a question is on topic for multiple stacks then there's no obligation to do the sensible thing and post it on the one which it best fits, but you should expect people to ask why you're not doing the sensible thing. I think you're going to have to work hard to turn this into a question which fits this site, whereas it's already a good fit for CR's challenge programme. \$\endgroup\$ Commented Jun 4, 2014 at 16:31
  • \$\begingroup\$ @PeterTaylor Hmm, that's strange. Wouldn't that be more on-topic here? (And I only occasionally pop in to their chat/meta to see what they're up to. :-P) \$\endgroup\$
    – Doorknob
    Commented Jun 4, 2014 at 16:32


Note for Sandbox: I have not finished (or really started) the control program for this game, because I wanted to see if there was interest in it before I dedicated too much time to the project. that means that the rule are still up to be tweaked, so please leave a comment if you have a suggestion, and comment or vote if you are interested in seeing this happen.

Diplomacy is a complex strategy game, with a very entertaining combat system. This challenge will be to write a bot to compete in a simplified version of diplomacy combat.



Countries (bots) will begin the game with 10 health, representing their remaining will to fight. The goal is to eliminate all other Nations by attacking them until they have 0 health.

The game will consist of several rounds. On the first round, all bots will receive 2 numbers as command line arguments: The first will be the total number of countries fighting, and the second will be their number in the list. Each following round, bots will receive a command line arguments containing the actions taken by each player last round and a list of all bots and their remaining health separated by commas, like so

 1:A2,2:S3,3:A4,4:A3 1:10,2:7,3:7,4:1

Each bot must then output a desired action, which is one two commands

  1. Attack a player. This is done by printing the letter A followed by the number of the player you with to attack. For instance, A3
  2. Support a player. This will give the player you support a boosted attack.

Resolving combat

After player have sent in their moves, attack scores will be calculated thus:

  1. All players start with a strength of 1, and one point is added for every player supporting them. For instance, if the moves are 1:A3,2:S1,3:A2,4:S2 then bot 1 has strength 2, bot 2 has strength 2, bot three has strength 1, and bot 4 has strength 1.
  2. After strength has been calculated, bots will deal damage based on their strength. The formula for damage is (Attacker's strength + 1) - (Defender's Strength) In the above situation, player 3 would take 2 damage and player 2 would take 0 damage. Note that, unlike regular diplomacy, attacking a supporter does not cut support.
  3. All attack take place simultaneously and independently. This means that if player 1 and 2 both attack player 4, then they each deal 1 damage. If player 3 were to support player 4, then player 4 would take no damage.

Round Ends

After combat has been resolved, countries that have 0 health will no longer be able to attack or support. However, they still will be listed in the input with an health of 0. When a bot is eliminated, all remaining bots will receive a single point.

Ending the game

The game ends when either 100 turns have elapsed or only 2 or less players remain. At this point, the player with the highest remaining health is the winner and receives 1 point. In case of ties, all tied bots will revive 1 point. If all bots die on the same turn, this is not a tied victory, but mutually assured destruction, and all bots will receive 0 points.


The control program will run 100 rounds of the game. The winner will be the country with the most points at the end of 100 rounds.


You may write in any language I can reasonably compile. I will make an effort to compile odd languages, but make no promises as to my ability to do so. Please provide your source code, an explanation, and a command line command to run your program.

  • You are allowed to write to a file. In fact, you are encouraged to do so.

  • Because this is a game where cooperation is paramount, you are allowed to write bots that work together, with the following restriction:

    • Only two bots can be written by a single player to work together at a time.
  • Standard Loopholes apply. You are not allowed to change the way the control program runs. If you provide invalid input to the control program, the program will just skip your turn. However, you are allowed to spy on other countries files, and all bot programs will be in the same folder at runtime. This is war, after all!

  • I reserve the right to disqualify any country that takes more than about a second to run, or that tries a loophole not mention within. That being said, if it is sufficiently clever I will probably let it go.

I will have source code up soon for a sample country that will be competing, and will post the control program when I finish it.

  • \$\begingroup\$ "In case of ties, all tied bots will revive 1 point". Is that supposed to say "receive"? "If all bots die on the same turn, ... all bots will receive 0 points." If there are two bots left, each of which has received 1 point from the earlier death of a third bot, and the two bots destroy each other on the same turn, what's the final score for the round? I'm not sure whether it's 0-0-0 or 1-1-0. "You are allowed to write bots that work together": but how can they identify each other? Do they have to use their moves as a covert channel? \$\endgroup\$ Commented Aug 29, 2014 at 14:28
  • \$\begingroup\$ "Support a player. This will give the player you support a boosted attack." Or defence. Might be clearer to say "boost that player's strength for the turn". Should also state whether or not it's possible to support yourself. \$\endgroup\$ Commented Aug 29, 2014 at 14:29

Check GenericScript source code for compiler errors

Given the source code for a GenericScript program as input, parse the source code to check that it conforms to the syntax rules for the language. The syntax definition for GenericScript is below. If a part of the source code is found to be invalid, the program should output "Invalid syntax", otherwise it should output "Valid syntax".

Win Criteria

This is code golf. Shortest code wins.


Source code will be considered to be valid if it matches the rule for "Program" below.

Program             = Sequence
Sequence            = Statement [Sequence]
Statement           = SequenceBlock | Assignment | If | While | Output
SequenceBlock       = "{" Sequence "}"
Assignment          = Identifier "=" (String | Bool); 
If                  = "if(" Bool ")" Statement ["else" Statement]
While               = "while(" Bool ")" Statement
Output              = "print(" String ");"
Identifier          = {Any sequence of alphanumeric characters prefixed with "var" }
Bool                = StringEquals | Identifier
StringEquals        = String "==" String
String              = StringConstant | OperatorConcat | Input | Identifier
StringConstant      = "'"StringContent"'"
StringContent       = Character [StringContent]
Character           = {Any character except for "'"}
OperatorConcat      = String "&" String
Input               = "read()"

Whitespace is defined as any sequence of the ascii characters 9, 10, 13 and 32. Whitespace characters are allowed between tokens but are not required.


  1. The answer should be a complete program
  2. Standard input/output allowed
  3. Standard loopholes apply
  4. Universally testable answers only

Test Input

Valid syntax:

print('What is your name?');
varInput = read();
print('Hello ' & varInput);

Invalid syntax:

if(read() == 'DoTask1')
  print('Executing you'r command');

Text Adventure Game


Your goal is to develop a complete text-based adventure game with the shortest code possible. The player navigates in a dungeon composed of rooms. The game objectives are to find the treasure, slain the dragon and rescue the princess.


A room description is as follows:

You are in <description>.
You can go <exits>
You see <object>      (optional)
  • exits can be "north", "east", "west", "south".
  • description can be "a adjective cavern", "a adjective room", "a adjective corridor", "a adjective hall", "a cell", "the dragon's lair".
  • adjective can be "dark", "murky", "small", "large", "narrow", "gloomy", "huge", "strange", "tiny", "broad", "old".
  • object can be "the princess", "the dragon", "a troll", "a goblin", "a sword", "gold", "a key", "a trunk".

Exit list must be comma-separated and end with "and". If there is no object in the room, the last line is omitted.

Example of valid description:

You are in a murky room.
You can go north, east and south.
You see a goblin.

The game accepts the following commands (case is ignored) :

  • GO direction : direction can be NORTH, EAST, WEST, SOUTH
  • TAKE item : item can be SWORD, GOLD, KEY
  • KILL monster : monster can be DRAGON, TROLL, GOBLIN. The DRAGON and the TROLL can be killed only if the user has the SWORD. If he hasn't, he loses the game. The weak GOBLIN can be killed with bare hands. When a monster dies, he disappears from the room. When the GOBLIN dies, he drops a SWORD. When the TROLL dies, he drops a KEY.
  • KISS person : person can be PRINCESS, DRAGON, TROLL, GOBLIN. Kissing the princess validates one of the objective of the game, and the princess disappears from the room. Kissing a monster results in player death.
  • OPEN object : object can be TRUNK. If the player has the KEY, the TRUNK object disappears and is replaced with GOLD.

The player can perform an action on an object only if the object is in current room. A room can contain only one object ; a given object can be found in only one room. At the beginning of the game, only the following objects are placed in the map : PRINCESS, DRAGON, TROLL, GOBLIN, TRUNK. Other objects are not yet created.


  • If an action cannot be performed (e.g. GO NORTH where there is no exit to the north, or TAKE DRAGON, or DANCE GANGNAM STYLE), the message "Sorry, I can't do that" must be displayed.
  • If an action can be performed, the message "OK" and the current room description should be displayed.
  • You can read game commands from console or as a program parameter, as you wish.

The dungeon should have at least 30 rooms. The dungeon should not contains a series of more than 5 exits in the same direction. The exits between rooms must be consistent, e.g. if you go north from room #1 to room #2, there must a south exit in room #2 leading back to room #1. Every room name should be unique. There must be at least one room of each kind (hall, cavern, corridor...)

  • A hall has at least 3 exits.
  • A corridor can have only 2 exits.
  • The cell has only one exit.
  • There is only one dragon's lair and only one cell, containing respectively the dragon and the princess.

The game ends when the player has been killed, or when he has taken the gold, slain the dragon and kissed the princess.

  • If the player dies, the message "You have been killed by X !" is displayed, with X being the name of the monster.
  • If the player wins, the message "Well done adventurer ! you've conquered the dungeon." is displayed.

Player should not be able to win the game in less than 40 turns.


You are in a murky room.
You can go north, east and south.
You see a goblin.
You are in a murky room.
You can go north, east and south.
You see a sword.
You are in a murky room.
You can go north, east and south.
You are in a narrow corridor.
You can go south and east.  


The shortest code wins.

  • \$\begingroup\$ @Martin Thanks for your comments! I've updated the question. \$\endgroup\$
    – Arnaud
    Commented Aug 21, 2014 at 7:59
  • \$\begingroup\$ Provided the player ignores the troll and goblin (i.e. doesn't try to kiss or kill them), they don't do anything? \$\endgroup\$ Commented Aug 21, 2014 at 8:30
  • \$\begingroup\$ @Peter you're right. Maybe the player should kill (with bare hands) the goblin in order to get the sword, and then kill the troll (with the sword) to get the key. \$\endgroup\$
    – Arnaud
    Commented Aug 21, 2014 at 8:50
  • \$\begingroup\$ "The map must be spatially coherent" still doesn't disallow always going left without ending up in the same room twice, unless you specify that the rooms are all meant to be square (which is what I think you had in mind). Also, I still think that "at least" 30 rooms is unnecessary. Who would implement 8 additional rooms if they don't have to. It will definitely be shorter if I omit the two longest adjectives and just use all available combinations of the remaining ones (giving 30 unique rooms). So you can omit two adjectives and the "at least" right away, I'd say. \$\endgroup\$ Commented Aug 21, 2014 at 9:19
  • \$\begingroup\$ I think it's fine to keep "at least" there for flavour, same with additional adjectives. Also, someone might figure out a way to make the code shorter with a longer adjective (for that reason, having a few more adjectives might be nice) \$\endgroup\$
    – FireFly
    Commented Aug 21, 2014 at 9:38
  • \$\begingroup\$ @Martin I've added a criteria "Player should not be able to win in less than 30 turns", to force the golfer to implement more rooms. \$\endgroup\$
    – Arnaud
    Commented Aug 21, 2014 at 9:39
  • \$\begingroup\$ @SuperChafouin That doesn't force it though. I just need to place the goblin at the end, troll at the beginning, trunk at the end, so that you need to traverse the map 3 times. \$\endgroup\$ Commented Aug 21, 2014 at 9:51
  • \$\begingroup\$ @Martin It's also here to prevent the dungeon to be too straightforward to solve, e.g. if all the objects are in 5 adjacent rooms near the player start location. \$\endgroup\$
    – Arnaud
    Commented Aug 21, 2014 at 10:51
  • \$\begingroup\$ +1 for golfing in Inform 7. \$\endgroup\$
    – Lopsy
    Commented Sep 20, 2014 at 2:56
  • \$\begingroup\$ Hello! This looks like a good but abandoned meta post, would you be willing to offer it for adoption? (If you want to, you can still post to main.) Due to community guidelines, if you don't respond to this comment in 7 days I have permission to adopt this. \$\endgroup\$
    – user58826
    Commented Jun 9, 2017 at 16:58
  • \$\begingroup\$ @programmer5000 Yes no problem :-) \$\endgroup\$
    – Arnaud
    Commented Jun 10, 2017 at 1:07

Simulate a Quantum Circuit

Work-in-progress until I can make sure I know what I am doing and can finish the spec. or maybe

Quantum computers are the way of the future! Why wait, when you can simulate one now?

Your mission is to determine the output of a quantum circuit given its input and a diagram of logic gates.


You will simulate a single quantum register and apply a series of quantum logic gates to it. A quantum register is a group of qubits. The state of a register is described by a vector of 2^N complex numbers, where N is the number of qubits in the register.


Above is a representation of a 3-qubit register. Each letter (a b c etc.) represents a complex number. There is an addition restriction that:

|a|^2 + |b|^2 + |c|^2 + |d|^2 + |e|^2 + |f|^2 + |g|^2 + |h|^2 = 1

Quantum gates

Gates are represented by a 2^N x 2^N square unitary matrix, where N is the number of input qubits. All quantum gates have the same number of outputs and inputs, since they neither create nor destroy qubits, they modify them.

A common quantum gate is called the Hadamard gate and acts on a single qubit. The matrix [H] looks like this:

1/Sqrt(2)  1/Sqrt(2)
1/Sqrt(2)  -1/Sqrt(2)

If we let [R] represent the following 1-qubit register:


Then the application of the gate is represented by [H][R] and gives the following result:


It is still true that the sum of the squares of the absolute values is equal to 1.

(TODO: explain how to apply gates to larger registers)


Measurement collapses the state of the quantum register.

(Todo: Explain how measurement works)





The goal of this challenge is to implement an AI for the game of BS, also known as Bull Shit, Cheat, Bluff, and numerous other names.

The game is outlined in this wikipedia article.

The Rules of the Game

For the purposes of this challenge, the game will work like this:

  1. A standard 52-card deck is dealt out to the players
  2. The current rank is set to Ace
  3. The play order is randomized
  4. The player holding the Ace of Hearts goes first
  5. On each player's turn:
    1. The current player plays some number of cards
    2. The current player states how many of what rank they played
    3. Other players may declare 'BS'.
    4. If any player declares 'BS':
      1. All players are notified of which players declared 'BS'.
      2. The played cards are revealed to all players.
      3. If the played cards are inconsistent with the current player's statement:
        • The current player adds the played cards and all cards in the pile to their hand
      4. If the played cards are consistant with the current player's statement:
        • The last player to declare 'BS' that round adds the played cards and pile to their hand.
    5. If no player declares 'BS':
      1. The played cards are added to the pile, without revealing them.
      2. If the played cards were inconsistant with the current player's statement, the current player may declare 'Peanut Butter'.
    6. If the current player has no cards in their hand, the current player wins.
    7. The current rank is incremented. (If the current rank is King, it becomes Ace.)

The Messaging Protocol

Play will be conducted via messages passed to the standard input and received from the standard output of each program. Each message will be terminated with a single newline character.


Card ranks are represented as one of A, 2, 3, 4, 5, 6, 7, 8, 9, T, J, Q, or K. Card suits will be represented as one of S, C, H, or D. Cards are represented as the rank, followed immediately by a suit. For instance, the Ten of Clubs would be represented by TC, and the Three of Hearts would be represented by 3H.

A hand of cards will be represented as a space-delimited sequence of cards. For instance, a hand containing the Queen of Spades and the Six of Diamonds could be represented as QS 6D or 6D QS.

Player Identification

A player will be represented by their nickname, followed by a number from 0 to 32768, in parenthesis, formatted as an integer. This number is guaranteed to be unique within a particular game. A player's nickname must have at least one character, can have up to 32 characters, and may only include letters, numbers, and underscores. For instance, a player with nickname ExampleAI and ID number 16480 would be identified in the game as ExampleAI(16480).

When the game begins, each program will recieve a message containing their unique ID:

Unique ID: uniqueID

Each player will reply with their desired nickname:

Nickname: name

Names may contain only alphanumeric characters and underscores.

After all players have responded with their nickname, the standard play sequence begins.

Standard Play Sequence

When a player's turn begins, each player will receive a be given a list of the players and their card counts, in order of play:

Players: player[count], player[count], ... player[count]

Each player will be informed of the contents of their hands:

Hand: initial_hand

The current the player will then receive this message:

Your turn: current_rank 

The current player will reply with a space-separated list of of cards:

Play: list_of_cards

Once they have submitted their play, all players will receive the number of cards, formatted as an integer, along with the current rank:

Player player plays: nunber_of_cards x current_rank

Each other player may then declare BS on that play by sending any message up to 32 characters, containing the capital letters B and S, and otherwise only contains lowercase letters and spaces. So any of Bull Shit, Bananna Split or Bacon Sandwich would be acceptable.

During this period, the current player may declare Peanut Butter by sending any message up to 32 characters, as long as it contains the capital letters P and B, and otherwise only contains lowercase letters and spaces. So any of Peanut Butter, Pancake Batter or Polish Bacon would be acceptable.

In order to allow the game to move faster, if a player does not wish to declare either of these things, they must instead send:


After all players have responded, all players will receive a list of players who called BS, in the order they called it:

Called BS: player, player ... player

If no player called BS, this message will still be sent --- it just won't have any players listed. If any player did call BS, then all players will recieve:

Player player had played: list_of_cards

If they were bluffing, all players recieve:

Player player was bluffing.

And the current player receives:

Your bluff was called: list_of_cards_recieved

If they were not bluffing, all players recieve:

Player player was not bluffing.
Player last_player receives the pile.

The last player who called BS recieves this message:

You misjudged: list_of_cards_received

The list of cards received will contain, in reverse chronological order, the contents of each play since the last call. (Separate plays will not be delimited in the list.)

If no player declared BS, and the current player was bluffing and declared Peanut Butter, then all players recieve the message:

Player player was bluffing.

If the current player has no cards left in their hand, all players receive this message, and the game terminates:

Player player won!

Otherwise, the next player's turn begins.

Example Game

The following might be considered a typical (abbreviated) message transcript:

Unique ID: 16481
> Nickname: Alice
Players: Alice(16481)[18], Bob(16479)[17], Charlie(16480)[17]
Hand: 2D 7S AS TC 5S JS JC 3C 8H 9D 5D AH 7C 6C 4D KC KH KS
Your turn: A
> Play: AS 2D AH
Player Alice(16481) plays: 3 x A
> PB
Called BS:
Player Alice(16481) was bluffing.
Players: Bob(16479)[17], Charlie(16480)[17], Alice(16481)[15]
Hand: 7S TC 5S JS JC 3C 8H 9D 5D 7C 6C 4D KC KH KS
Player Bob(16479) plays: 2 x 2
> BS
Called BS: Alice(16481)
Player Bob(16479) had played: 2H 2C
Player Bob(16479) was not bluffing.
Player Alice(16481) takes the pile.
You misjudged: 2H 2C AS 2D AH
Players: Charlie(16480)[17], Alice(16481)[20], Bob(16479)[15]
Hand: 7S TC 5S JS JC 3C 8H 9D 5D 7C 6C 4D KC KH KS 2H 2C AS 2D AH
Players: Alice(16481)[3], Bob(16479)[41], Charlie(16480)[8]
Hand: KC KH KS
Your turn: K
> Play: KC KH KS
Called BS: Charlie(16480), Bob(16479)
Player Alice(16481) was not bluffing.
Player Bob(16479) receives the pile.
Player Alice(16481) won!

Your implementation may be written in any language, provided that you, upon request, provide a link to a suitable free-as-in-freedom compiler or interpreter that I can download and run at no cost. You also need to provide a UNIX command that can start your program.

Sandbox Questions

I want to gauge the community's interest in my problem before finalizing the spec and writing the control program.

I also need to get some idea of what sort of time-limiting scheme would be reasonable. In order to be able to to a lot of runs, I will need to be able to ensure that each AI doesn't take too much time to make its decisions, or prevent a stuck AI from holding up a game. I also need to be able to ensure that there is no motivation to deliberately stall a game. For example, if an AI determines that it is very unlikely to win, it might stall in order to prevent the game from finishing.

I would also like feedback on the messaging protocol:

  • Are there any additional messages that you think should be passed?
  • Would it be more convenient/clear if one or more of them were formatted differently?
    • Would it be better to use a different format for the plays message?
    • Would it be better to use different words to help distinguish the plays and played messages?
  • \$\begingroup\$ It looks like quite a tough challenge, but should be enjoyable! \$\endgroup\$ Commented Sep 4, 2014 at 16:54
  • \$\begingroup\$ @PopeyGilbert By the way, there was one thing I accidentally left out that I need feedback on. Specifically, time limits - to deal with intentional stalling, getting stuck, or taking too long to decide. \$\endgroup\$ Commented Sep 4, 2014 at 17:05
  • 1
    \$\begingroup\$ A question and a feedback. Does our program run as "stop and run" or must receive feedback continuously? And for feedback. I honestly think that the whole username things is kinda confusing. Maybe if you just use only unique id? (Like just simple 0,1,2,3 instead of username) \$\endgroup\$
    – Realdeo
    Commented Sep 5, 2014 at 8:33
  • \$\begingroup\$ Oh! More things! I also realize that suit doesn't really matter, right? (We only use suit for deciding who goes first), so fmpov, you can ditch the communication protocol for the suit. (No need for S,C,D,H) We can just use simple random from the computer. Question: What will happened if everybody make infinity loop of pass. For time limit, I prefer 1 second. If no response, make it auto pass. (KOTH chess time limit is 2 seconds. That's why 1 second is good enough) \$\endgroup\$
    – Realdeo
    Commented Sep 5, 2014 at 8:38
  • \$\begingroup\$ @Realdeo Card suit is also used to distinguish between separate instances of a card. If Alice plays 3x2 (2S 2D 2C), is not revealed, and Bob gets the pile later, and then Bob plays 3x2 (2S 2C 2H), and this is revealed, it is important for Alice that she knows all four Twoes have passed through Bob's hand. There are other ways that can be used as well. \$\endgroup\$ Commented Sep 5, 2014 at 13:29
  • \$\begingroup\$ @Realdeo I am not sure what you mean by "Does our program run as "stop and run" or must receive feedback continuously?". If you mean, "Does an AI program halt in the periods where no response is expected from it?" the answer is no. \$\endgroup\$ Commented Sep 5, 2014 at 13:32
  • \$\begingroup\$ @Realdeo If everybody makes an infinity loop of pass, then eventually someone will run out of cards, since you are required to play at least one card each turn. \$\endgroup\$ Commented Sep 5, 2014 at 13:33
  • \$\begingroup\$ @Realdeo Which is why making a automatic pass after a timeout not work when waiting for a player to decide their play. Perhaps a simple rule like 'if you take more than 1 second to decide what to play, four cards are selected at random from your hand'. \$\endgroup\$ Commented Sep 5, 2014 at 13:36
  • \$\begingroup\$ And if a player has less then 4 cards? I think in some AI website, like aigames.com, they're like forced to give up that hand? You really want to test your entry before put them in the arena(like vsing a bot dummy?) Either way, this is a good challenge =) \$\endgroup\$
    – Realdeo
    Commented Sep 5, 2014 at 13:40
  • \$\begingroup\$ @Realdeo Also note that you can actually play more than four cards in one play. A case where you might wish to do this is when: the next player is very close to winning and some other player is close to winning and you believe(all opponents believe(your next opponent will bluff) and the next opponent will not bluff and the next opponent believes(the other opponent close to winning will call BS against them)). A little convoluted, but could happen. \$\endgroup\$ Commented Sep 5, 2014 at 13:42
  • \$\begingroup\$ @Realdeo Just to explain what I mean, is that there are two people close to winning, each of which would like to dump a large stack of cards on top of the other. Because of this, they both let your obvious bluff slide because they believe that will let them dump a large stack on the other. \$\endgroup\$ Commented Sep 5, 2014 at 13:47
  • \$\begingroup\$ Don't worry I understand. This is a really famous high school game in my country. It just... a little bit too complex for CR. When I saw chess KOTH, I was kinda pessimist. This one? This may deserve it's own AI website. #seriously. I'm just trying to simplify this game =) \$\endgroup\$
    – Realdeo
    Commented Sep 5, 2014 at 13:50
  • \$\begingroup\$ Let us continue this discussion in chat. \$\endgroup\$ Commented Sep 5, 2014 at 13:50
  • \$\begingroup\$ RE the messaging service, I think the other players should be able to see how many cards the other players have. Also, card counting should be prohibited because that would make the game too easy. \$\endgroup\$
    – Beta Decay
    Commented Sep 10, 2014 at 17:31
  • \$\begingroup\$ @BetaDecay First off, according to the protocol, every player is informed of every other player's hand size at the beginning of every round. \$\endgroup\$ Commented Sep 10, 2014 at 18:27

Turn my keyboard into a piano

I'm sure we've all thought "man, wouldn't it be cool if my keyboard played musical notes as I program?". Of course, the answer to that question is a resounding no.

Regardless, it's what you're going to make.


Input will be given, in real-time, on the keyboard.

The keyboard mapping that you will use is given in the diagram below. This kind of layout is used by several music programs already.

A keyboard layout, with notes assigned to several of the keys.

You can see that the bottom row (Z, X, C, V, B... ., /) represents all of the white keys, and the black keys are added on the row above (S, D, G, H, J... L, ;). This is then repeated on the two rows above, except the notes are an octave higher.

The notes C5 to E5 are repeated both on the lower rows and the upper rows.

So, if the user were to input Q (or ,) on the keyboard, middle-C (C5) should play. Similarly, if they input B, G4 should play.

The diagram above is an edited form of an image found here.


The only output will be sound. The actual sound used is up to you (it could be a piano sample or the internal beeper), but it should output sound at the correct pitch.

The program should not terminate by itself - the user should be able to keep inputting notes until they get bored.


  • A diagram of the US keyboard layout (for comparison with the image above) can be found here.
  • A table of the frequencies of notes can be found here. The range of notes that you will be using are from C4 to E6, inclusive.

Rules and Disambiguation

  • This is , so the shortest correct implementation wins.
  • Input should be given in real-time (i.e. no pressing Enter between each inputted note).
    • There should be no greater than a 0.25 second delay between pressing a key and hearing the note.
  • Only programs that have the notes correctly mapped to the QWERTY keyboard will be accepted.
  • The only output should be sound. There should be nothing displayed (except for a mandatory console window or similar).
  • Polyphony (multiple notes playing at once) is not part of the specification.
  • The program should not terminate by itself - a user should be able to keep pressing keys and hearing notes until they decide to close it.
  • The waveform outputted is not important (it can be a beep or a piano sound or whatever you like); the pitch, however, should be accurate.
    • The file size of any sound files used will not be counted in the bytecount.
    • Please link any sound files you use with your answer.


  • There are probably some obvious things that I've forgot to explain - please point these out!
  • Also let me know if any of the wording is confusing.
  • I've assumed that the readers will at least know very basic music theory (e.g. that there are 12 semitones in an octave). Is this okay?
  • "Polyphony is not part of the specification" - polyphony is not the focus of the program, and therefore shouldn't be a consideration when submitting answers (i.e. if the shortest implementation means that one note will stop when a new note is played, then that's fine). Is this acceptable?
  • Should I exclude the use of any external libraries that are designed to play sound or designed to repeat a function at regular intervals? Should I count sound file size in the score? I'm worried that having either of these would limit the possible entries to those that can use the internal beep (so, C/C++, C#, Python, Java?...).
  • Another problem may be portability, or the lack thereof (e.g. C/C++ using the Windows API to access the Beep() function).

Really meta: answers/edits by me won't be done until the morning (approximately 9 hours from now).

  • \$\begingroup\$ Does sound need to continue until key_up, or is just a short "beep" okay no matter how long the key is pressed? \$\endgroup\$
    – Geobits
    Commented Sep 10, 2014 at 0:26
  • \$\begingroup\$ "Of course....resounding no.... regardless.." why shit on your own question? Change it to a slightly ironic "Of course we have! Well, that's what you're going to make" \$\endgroup\$ Commented Sep 15, 2014 at 21:37
  • \$\begingroup\$ 1,2 The wording is mostly OK Regarding @Geobits point, I think a short beep has to be acceptable. One must delve deep into API to know when a key is released (if the hardware tells at all.) Make the US keyboard requirement clearer. Take the linked image of a US keyboard, mark note names on it in color and include it in the question (I may do it for you.) Without having both on the same diagram, I found it hard to work out which notes on my Spanish keyboard end up in odd places. 3. Theory is very basic here, no need to explain Maybe link to en.wikipedia.org/wiki/Scientific_pitch_notation \$\endgroup\$ Commented Sep 16, 2014 at 23:20
  • \$\begingroup\$ Allow libraries for sound functions. Otherwise it'll definitely be won by a language with builtins. Portability is always an issue with sound, but on codegolf.stackexchange.com/a/25242/15599 the OP did get my Windows answer working on his Linux machine after many comments. Avoid polyphony. Computer & phone keyboard matrixes can't handle it properly. If you press 1,3,7,9 on a numeric keypad and release 1, the release can't be detected because there's still an electric path through the other keys. Real music keyboards have a diode on every key, or individual wires, to avoid this problem. \$\endgroup\$ Commented Sep 16, 2014 at 23:39
  • \$\begingroup\$ Hi, @steveverrill, thanks for your comments. I haven't really been very interested in PCCG recently so I haven't been replying/making changes. Thanks for the suggestions - I'll review the post sometime soon. \$\endgroup\$ Commented Sep 17, 2014 at 0:07
  • \$\begingroup\$ If this is code-golf, then I suggest that you choose a set sound to make it fair for everyone. \$\endgroup\$
    – Beta Decay
    Commented Jul 5, 2015 at 7:49
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