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

4725 Answers 4725

60 61
63 64

King of the Hill: Risk(k)

We all know these well spend evenings where family members and friends got to temporary enemies while playing Risk the board game. The rules are not so complicated, the world is divided in territories on 6 different continents. enter image description here The different players take control over those territories and afterwards play to conquer the neighbors, form alliances and betray each other until one player reaches world domination.

But wouldn't it be cool, if you just write a bot for you to play this game? here comes your chance.

This is a King of the Hill challenge to write the best bot to play Risk(k). Like in the original game (but not entirely) the world is divided into 6 continents with a total of 41 territories. Afterwards the bots will play this game with reduced rules in a round-based setting. Your bot can be written in or . Although any other language is possible which is capable of producing a dll with cdecl-functions or to implement interfaces. The controller can be found on Github.

enter image description here

A short classis-game explanation

For all of you, who aren't familiar with this game. Every player plays on his own against all other players. The game begins with the claiming of territories (by positioning a single unit there) which is done consecutively for every player until all territories are owned by a faction. Now the players get the chance of using all units they have left to enforce their territories. Now the game begins round-wise. Every player makes a Attack-Stage and Move-Stage. In the attack-stage the players can attack other territories. This is done with dices. When thrown (the attacker and defender), the dices on each side are ordered and the highest ones are compared to each other. Whoever has the higher dice destroys a unit of the opposite side. Equal dices are considered a tie and no unit dies. You can only attack with max. 3 units at the same time, even when you have more than 3 units on your attacking territory. Also a territory with one unit cannot attack to prevent territory loss. Likewise a territory with two units can only attack with one etc. . When a territory falls to zero units, the attacking territory has to send units over (at least on and max. all except one - in the controller, all units except one are send automatically). After the attack stage, one player gets the chance to move units in the Move-Stage. One can only do so, if the territories you want to move units between are connected by territories owned by you. If your are done with this, the next players turn begins. The game ends, when one player has conquered the whole world.

The game&rules:

  • All bots have to obey the rules. Cheating bots will be punished by the program or by me. Standard loopholes are forbidden.
  • Each bot must be in a dll file (name.dll) in the same directory, as the controller.
  • Each program with the c++ interface must end with name.cpp.dll
  • When the program is started each bot gets loaded into the program and is checked if everything works. The bots get instantiated only one time when the game starts.
  • All bots loaded are participating in the game.
  • The initialization stage begins:
    • Each bot gets asked through the interface about its name and color.
    • All bots get introduced to another with their names.
    • All bots get consecutively called to select one of the remaining free territories. This goes till no territory is free anymore.
    • All bots get to distribute the units left to all territories. [UnitCount = Ceil(82 / BotCount) ]
  • The game will stop for now till the user pushes the start-button.
    • The game will be round based where each bot will get consecutively called in four stages: Strategy-Stage, Enforcement-Stage, Attack-Stage, Move-Stage
    • The Strategy-Stage gives your bot the opportunity to plan a strategy for this round.
    • The Enforcement-Stage will give your bot the opportunity to distribute all new units to your territories. Units are gained to: Count of owned territories divided by 5 and floored, continent-bonus (owning a whole continent: North America: +5, South America: +2, Europe: +5, Africa: +3, Australia: +2, Asia: +7), capital-bonus (+1 for each owning of: East US, Brazil, North Europe, South Africa, East Australia, China) and +1 if you conquered a territory last round. You're guaranteed to get at least one new unit per round if your bot is still alive.
    • In the Attack-Stage you can order the program do make attacks on enemy territories which can lead to loss on your or the enemy side or you conquering a new territory. The attack will be called multiple times (max.: 100 times) as long as you make an attack every call.
    • In the Move-Stage you can move units around owned, connected territories. You can also again plan your strategy there for the next round.
    • Each round is time constricted for every bot with 200ms. Taking longer than this will lead to a punishment (suspension) for the next round.
  • The game ends when only one bot still lives or when two bots get into an infinite loop (which is surprisingly possible and not rare for the random-bots)

The controller&interface

How the interfaces work is explained in the template files and the example-bots.

For c#, .NET Programmer

Your bot has to be in a class which inherits and fully implements the IBotInterface. The class must be compiled into a class-library (dll). Afterwards you can just copy your dll into the game-directory and start the game.

EmptyBot is an empty template which implements the minimum required and adds a lot of useful helper-methods. It can be used as a template.

RandomBot is a bot which implements random behavior in all functions and stages. It acts as a full working example.

For C++, C, cdecl-function-able-languages

Your dll must implement eight functions with an external definition and cdecl-calling convention. A minimal implementation can be looked at at EmptyCPPBot.

Your dll can afterwards be copied into the game-directory which can be started normally afterwards. Your dll must have the following name structure: nameOfYourDll.cpp.dll to distinguish it from the managed dll's.

RandomCPPBot is a bot which implements random behavior in all functions and stages. It acts as a fully working example.

Contest Rules

  • Players may submit multiple bots and are free to edit them to the deadline.
  • A submission must be made as an answer on this thread. The source code can come in pieces, as full project, single file or whatever. And may be provided via download, push request or simple pasted code in the answer with code-tags. If not compile-able with Visual-Studio, i request fairly simple instructions how to compile it myself.
  • A submission must specify the name and color of the bot. Although not necessary, an explanation of the bot's strategy would be nice.
  • Bots are not allowed to use sources outside the dll/program (no files, no webrequest or similar things).
  • Bots must be compatible with the provided interface and work under windows. Custom interfaces are not explicitly forbidden as long as they don't generate a advantage or are not compatible with the main-interface/program.
  • The competition will be held in the provided controller (possible altered to automate the process) with all pairings possible to find the best 4 bots.
  • The last 4 bots are fighting each other in 10 games. The bot which has won the most, wins the tournament. In a tie situation, the bots will take a single match against each other (golden goal), until one wins.
  • Rules are can be changed when necessary which would be introduced on top of this thread.
  • The contest ends when a week long no new submissions got made or when I say, its time to end this.
  • And again, just to be sure: standard loopholes are forbidden.
  • \$\begingroup\$ How do you organize players into games, how do you score a single game, and how do you aggregate those scores? When are you instantiating each bot (only once for the tournament, once for each game, or once for each method call?) I'd recommend describing the rules of risk in full for those poor souls that don't know them. \$\endgroup\$ Commented Mar 30, 2017 at 21:45
  • \$\begingroup\$ I added a relative "short" description of the classical game. Also I added the answer to all your question into the thread but i'll also anwser it here: 1. All players in the game directory are participating. The tournament will provide automatic pairings if necessary. 2. You score a single game by letting your bot win it. 3. You can't. But you can restart infinitely often. 4. All bots are instantiated on the start of the program. But i could change that to the start of every game (so a reload also instantiate the bots new). \$\endgroup\$ Commented Mar 30, 2017 at 22:35
  • \$\begingroup\$ 1. What if there are lots of players (10 or even 20?). How would you automatically pair? 2. So the second to last survivor gets the same number of points as the first one to die? What happens if there's an infinite loop? 3. You have to come up with some way to combine the scores across multiple games. 4. Are you OK if submissions store data across games? How do submissions know when a new game starts? \$\endgroup\$ Commented Mar 30, 2017 at 22:46
  • \$\begingroup\$ Risk is a famously unbalanced game in which luck is very important. To balance that out you'll need to play hundreds of games per match-up. Each game involves thousands of player decisions. I would not be surprised if it ends up taking a week to run the tournament, and part-way through that week someone might submit a new bot. \$\endgroup\$ Commented Mar 31, 2017 at 12:31

Check for repeated repeated words


Your code should either read in a stream/file containing printable ASCII text or define a function that takes a string containing printable ASCII and output any repeated words (including their repetitions). If you read from a file then it can have a name of your choice. Output can be a single linefeed-separated string, or a list of strings (one per repeated word).

A word is defined by the regex [0-9A-Za-z'-]+, i.e. it's a run of letters, digits, apostrophes and/or hyphens.

A word is considered to be repeated if it occurs twice or more in succession, separated only by one or more spaces. Repetition is case sensitive: WORD, Word and word are all different.

Test Cases

Individual test cases are separated by an empty line. For each test case, the first line is the input, subsequent lines are the output. Note that the last test case does not contain any repeated words.

Hello how how are you?
how how

Hello my my friend. Is that that your pen pen pen?
my my
that that
pen pen pen

This is not. not a case of repeated? repeated words! Neither neither is this. 

  • \$\begingroup\$ Why do you disallow functions? \$\endgroup\$
    – mbomb007
    Commented Mar 31, 2017 at 16:08
  • \$\begingroup\$ @mbomb007 I don't :) \$\endgroup\$
    – user9206
    Commented Mar 31, 2017 at 16:13
  • \$\begingroup\$ "No other ways of getting input are allowed." Which means function arguments. \$\endgroup\$
    – mbomb007
    Commented Mar 31, 2017 at 16:16
  • \$\begingroup\$ @mbomb007 That text shouldn't have been there. Thanks. \$\endgroup\$
    – user9206
    Commented Mar 31, 2017 at 16:23
  • 3
    \$\begingroup\$ If underscores aren't a valid word character for this challenge, you'll want to add a test case that has underscores. I can see a lot of regex submissions being based on \w otherwise. \$\endgroup\$
    – DLosc
    Commented Mar 31, 2017 at 17:17
  • 1
    \$\begingroup\$ REgex would would win \$\endgroup\$
    – user63187
    Commented Mar 31, 2017 at 20:28

Island Golf #3: Random Generation

Given the dimensions of a rectangular patch of ocean and the desired number of land tiles, randomly generate a valid island of that many tiles within that rectangle.


Your input will be three positive integers:

  • w, the width of the grid
  • h, the height of the grid
  • n, the number of tiles to make the island

w and h will always be 3 or greater. n will always be 1 or greater. Also, n will be small enough to fit inside the grid allowing for a border of water tiles: specifically, n <= (w-2)*(h-2).


Your code must output a w by h rectangular grid consisting of two characters, representing land and water. (In the examples below, land is # and water is ., but you may substitute any two distinct characters you wish.) There must be exactly n land characters, in one contiguous block, representing an island. For example, an input of w=11, h=9, n=40 might result in the following output:



  • The land tiles must all be contiguous (i.e. there's only one island).
  • Land tiles can be connected horizontally or vertically, but not diagonally.
  • The water tiles must also be contiguous (i.e. there must not be any lakes).
  • The outer border of the grid must remain as water tiles.

Some illegal outputs:

.#.#.   Multiple islands

.#..    Diagonal connection

.####.  Contains a lake

.###    Border contains a land tile


Your solution may be a full program or a function. Any of the default input and output methods are acceptable. Any of these definitions of randomness are acceptable. Every valid island for a given input should occur with nonzero probability.

You may take the input numbers in whatever format and base is convenient for your language. You may take the three numbers in any order.

Your output may be a multiline string, a list of strings, or a 2D array/nested list of characters/single-character strings. Your output may (optionally) have a single trailing newline. As mentioned above, you may use any two distinct characters in place of #..

Please mention any unusual I/O methods/formats in your submission, so others will be able to test your code more easily.

Test cases

Given as width, height, size. Note that the first three test cases specify islands of maximal size for the given width and height.

3, 3, 1
9, 3, 7
5, 5, 9
5, 5, 1
10, 10, 60
80, 22, 1100

Validation program

Here is a validation program in Pip to test whether your output for a given input meets the spec. It expects the three parameters and an island in the format given in this question; for example:

5, 5, 9

If your code has a different output format, you'll need to convert it to this format before plugging it into the validation program.

Sandbox questions:

  • This seems like a pretty obvious random-generation challenge, but I didn't find a duplicate in a quick readthrough of previous challenges. Did I miss anything?
  • Should I add more stringent rules on what counts as random? Related reading
  • Should I allow output to be a 2D array (or nested list) of any two distinct values, not just chars and single-char strings? (E.g. integers 0 and 1, booleans false and true, etc.)
  • \$\begingroup\$ To the downvoter, if you see this: What's your reason for downvoting? If it's something I can improve about the challenge, I'd like to know so I can improve it. \$\endgroup\$
    – DLosc
    Commented Apr 6, 2017 at 1:28

Calculate Wind Chill

The Australian Apparent Temperature (aka, wind chill) in °C AT is given by this algorithm from the Australian Bureau of Meterology (wp, source):

enter image description here

AT = Ta + (0.33 * e) - (.7 * ws) - 4.0


Ta = Dry bulb temperature (°C)

e = Water vapour pressure (hPa)

ws = Wind speed (m/s) (at an elevation of 10 meters)

The water vapour pressure in hectoPascals e is given by this algorithm:

enter image description here

e = (rh / 100) * 6.105 * exp( ( 17.27 * Ta ) / ( 237.7 + Ta ) )


Ta = Dry bulb temperature (°C)

rh = Relative humidity [%]

exp represents the exponential function

The domain of:

  • Ta is -273.15°C to 2e7°C.

  • e is the real numbers

  • ws is 0 m/s to 2e7 m/s

  • rh is 0% to 100%

For inputs outside these domains, your code can do anything, including give the right answer.


Given a dry bulb temperature in °C, a wind speed in metres / second, and a relative humidity in %, your code should give the Apparent Temperature in °C, accurate to 0.1°C.

Assuming your platform or language can represent reals, for correct functions correct_func,

enter image description here

or in C, fabsl( correct_func(Ta, rH, ws) - expected ) < 0.1.

Test cases

value for Ta, rh, ws -> output

0   ->  -4.0
2   ->  -3.3529916671770903
4   ->  -2.6916697830145546
6   ->  -2.0132006039049877
8   ->  -1.3143308806029346
10  ->  -0.5913412821173161
12  ->  0.16000376605969002
14  ->  0.9445099696431676
16  ->  1.767610915646344
18  ->  2.6354298170017625
20  ->  3.5548453789325833
22  ->  4.5335619088440815
24  ->  5.580183785538598
26  ->  6.704294397098089
28  ->  7.916539649854112
30  ->  9.2287161435045

You can use a builtin function for the exponential function, ex, if you like.

This is , so the shortest code wins!


Factor Sort

This challenge involves sorting positive integers based on a lexicographical ordering of their prime factorizations.


lexicographical sorting, used in dictionaries, applies lexicographical order which extends alphabetical order to words:


When programming this sort, however, we typically don't extend alphabetical ordering per se, but rather we extend the order of integers used for an encoding. For example, the same sorting above through ASCII encoding is really:

97 97
97 97 97
97 97 114 111 110
97 98
97 98 97 119 100 111 110 101 100
97 98 99
97 98 101 114 100 101 101 110

It is this type of ordering that we're after here... lexicographical ordering by extension of numeric comparison as opposed to alphabetical order.

The Challenge

In this challenge, you will be sorting positive integers by their ordered prime factorizations (ordered in the sense that the primes are listed smallest to largest). To handle the special case number 1, we can simply say its prime factorization is an empty list, which lexicographically sorts prior to any other number's prime factorization. We'll call this type of sorting factor sorting.

For example, the numbers from 1 to 10, factor sorted, are: 1 2 4 8 6 10 3 9 5 7. To see why, here they are again with the ordered prime factorizations:

 1 []
 2 [2]
 4 [2 2]
 8 [2 2 2]
 6 [2 3]
10 [2 5]
 3 [3]
 9 [3 3]
 5 [5]
 7 [7]


Write a function or program that factor sorts a list of positive integers. Input and output can be anything reasonable, so long as the input is in the specified arbitrary order and the correct output order is apparent from the output.

Keep in mind that the output should be factor sorted numbers, not their prime factorization.

If it matters, numbers in the input will always be ≤ 7928, so:

  • The only primes in the prime factorization list are the first 1000 primes
  • Composites have factors no larger than 89 inclusive

This is code golf; shortest code in bytes wins.

Test cases

1 2 3 4 5 6 7 8 9 10
1 2 4 8 6 10 3 9 5 7

100 200 300 400 500 600 700 800 900
800 400 600 200 900 300 100 500 700

1472 4417 1425 1452 4480 200 339 2868 3835 4760
4480 1472 200 4760 1452 2868 1425 339 3835 4417

2 4 6 46 62 466 622 4666 6238
2 4 6 46 62 466 622 4666 6238

  • \$\begingroup\$ This is extremely trivial in golfing languages (e.g. in CJam it's {{mf}$}), so may attract negative attention and may not attract answers in any language in which it's an interesting challenge (if there are any). \$\endgroup\$ Commented Apr 3, 2017 at 11:27
  • \$\begingroup\$ I think asking to compare two numbers makes for a nice challenge than sorting them. Implementing sorting based some a key function has been overdone, and many languages just have a built-in for it. Also, I suspect there are strategies for comparison that do not compute the factor vector. \$\endgroup\$
    – xnor
    Commented Apr 3, 2017 at 22:27
  • \$\begingroup\$ @xnor "I suspect there are strategies ..." I'm confused about what you're trying to convey here. Granting your suspicion... suppose there's indeed another way to meet the specification. Why would that matter? \$\endgroup\$
    – H Walters
    Commented Apr 4, 2017 at 3:05
  • \$\begingroup\$ @HWalters I mean that a challenge to simply compare rather than sort might allow a wider variety of solution strategies, which makes it more interesting to golf. \$\endgroup\$
    – xnor
    Commented Apr 6, 2017 at 1:59
  • \$\begingroup\$ Is this in the OEIS? \$\endgroup\$
    – anna328p
    Commented Apr 19, 2017 at 6:12
  • \$\begingroup\$ @Mendeleev Not sure how to answer this. Strictly no, since it's not a sequence. If we factor sort all positive integers, then we get an infinite number of infinite "chains" of numbers, each chain of which is ordered. The first such chain is A000079, powers of two, so if you stretch you could say this is A000079. Powers of 3 excluding 1 is another chain after this; 2 times powers of 3 excluding 2 (i.e., 6, 18, 54, ...) is another chain between the two; etc. There's no "second chain" (chain immediately after A000079), and no immediate predecessor chain to powers of 3 excluding 1. \$\endgroup\$
    – H Walters
    Commented Apr 19, 2017 at 15:01

Dice Roller

Since I have played a lot of tabletop role playing games, I am looking for an easy to use dice roller. But also being lazy, I would like to have the shortest possible solution, making this code-golf.


You will receive as input a string, composed of 2 or 4 variables.

In the case of 2 parameters, both non-negative numbers, they represent the number of dice and the value of the dice rolled.


"2 6" means rolling 2 6-sided die (colloquially written 2d6).
"3 8" means rolling 3 8-sided die (3d8).

In the case where 4 parameters are given, the first 2 are still numbers, as in the case of 2 parameters. The third and forth parameters represent a dice modifier, and whether the modified applies to the dice individually or the sum of all rolled values, respectively. The third parameter is a (possibly) signed number (e.g.: 2, +1, -2, ...), and the forth is a character string, either 'ind' (if the modifier applies to the individual dice) or 'all' (applies to the total sum of the dice rolled).


"2 6 +1 ind" means rolling 2 6-sided die, and a modifier of +1 is applied to each individual die (can be written 2d(6+1)).
"3 8 -4 all" means rolling 3 8-sided die, and a modifier of -4 is applied to the sum of the roll (can be written (3d8)-4).

Dice Properties

  • Only the following dice are acceptable (second parameter): 2 3 4 6 8 10 12 20 100.

  • Regardless of modifier, a dice gives at least a value of 1 when an individual modifier is applied (so for example, "1 6 -10 ind" will return a value of 1, the minimum value allowed by this stipulation). There is no max value to an individual modifier.

  • When a modifier is applied to the sum of the dice rolled (4th parameter is "all"), the sum can be less that the number of dice rolled. For example, "2 6 -20 all" will give a sum range of -18 to -8, as the lowest value of 2d6 is 2 and highest value is 12, pre-modifier. There is no min or max value to a this modifier.


The output will be the values of the individual dice rolled, and the sum of all the dice.

Valid examples:

"2 6" => "3 6 : 9"
"3 8" => "1 8 5 : 14"
"2 6 -1 ind" => "1 2 : 3"
"2 6 -10 ind" => "1 1 : 2"
"3 8 +1 ind" => "2 9 6 : 17"
"2 6 10 all" => "3 6 : 19"
"2 6 -10 all" => "3 6 : -1"

Invalid examples (incorrect results):

"2 6" => "3 7 : 10" (7 is not a valid result on a 6-sided die)
"3 8" => "1 8 : 9" (not enough dice rolled)
"3 8" => "1 8 5 4 : 18" (too many dice rolled)
"2 6 -1 ind" => "1 6 : 7" (6 is not a valid result on a 6-sided die with a -1 modifier applied)
"2 6 -1 ind" => "0 5 : 5" (the die has to have a minimum value of 1 with individually applied modifier)
  • \$\begingroup\$ There's this. As far as I can tell, the only difference is the exact input format and your fourth parameter. I can't say whether or not people will close it as a duplicate (although there's a good chance they will, because all it does is optionally multiply the bonus by the number of dice), but it might generally be unpopular if the community feels it doesn't add anything interesting over an existing challenge. \$\endgroup\$ Commented Apr 4, 2017 at 18:51
  • \$\begingroup\$ @MartinEnder The forth parameter, limits on which dice can be rolled and the negative value modifiers. As for as I can see, the question you linked to doesn't have any consideration for XdY-Z, which I wanted to include as it introduces the minimum 1 constraint for the dice rolls. But that's why I put it in the sandbox first, to see what the community thinks :) \$\endgroup\$
    – AntonH
    Commented Apr 4, 2017 at 18:55
  • \$\begingroup\$ @MartinEnder Would it maybe work if I posted a link to that question, but also specified the differences between the questions? \$\endgroup\$
    – AntonH
    Commented Apr 4, 2017 at 18:57
  • 1
    \$\begingroup\$ I personally don't think that the max(1, roll) makes a sufficient difference. I know it's frustrating when you put a lot of effort into a challenge spec and it turns out to have already been done (the same or similarly), but if it's almost the same as an existing challenge, it's usually better to try something else. It's ultimately up to you, of course. \$\endgroup\$ Commented Apr 4, 2017 at 19:06
  • \$\begingroup\$ @MartinEnder It got stuck in my head and wouldn't leave, so I had to get it down :P I'll wait a bit to see if anyone else has anything else to add, but if it doesn't go anywhere, it's fine. \$\endgroup\$
    – AntonH
    Commented Apr 4, 2017 at 19:13
  • \$\begingroup\$ @MartinEnder I wouldn't close it as a dupe. They're similar, to be sure, but a big chunk of the linked challenge is input parsing and validation, which isn't present here. I also think that the fourth parameter (ind vs all, though I would suggest to let people use two distinct values rather than string-matching) coupled with the possibility of negative modifiers all combine to make this challenge distinct. \$\endgroup\$ Commented Apr 4, 2017 at 19:28
  • \$\begingroup\$ @AdmBorkBork Instead of "ind"/"all", would you suggest numerical (0/1) or letter ('i'/'a'), or something else? \$\endgroup\$
    – AntonH
    Commented Apr 4, 2017 at 19:33
  • 1
    \$\begingroup\$ Any of the above. If input parsing isn't the primary goal of your challenge (and it doesn't seem to be), don't limit folks on what inputs they and their chosen language can use best. \$\endgroup\$ Commented Apr 4, 2017 at 19:41

Find the Translation Table

Given two strings, find the translation table between the two, if the translation is not possible, output false. Probably most easily defined through examples:

Valid Cases

bat,sap = ["bt","sp"]

sense,12312 = ["se","12"]

rabid,snail = ["rabd","snal"]

Falsy Cases

banana,angular = false (not the same length, impossible).

animal,snails = false (different character patterns, not a translation).


  • Input may be as a 2 element array or as 2 separate inputs.
  • Output can be as an array, on separate lines or space delimited, but must be similar to how I have it shown.
  • False output may be 0, -1 or false. If your language uses something different, ask.

Sandbox Questions

Should I allow inputs of different length to be part of the translation? This will increase the complexity of the question by 100%. I'd suggest using - as the marker for a removal of a character in the translation to make it work. While it makes it more complicated, if this already exists I planned to use that to avoid a duplicate. Let me know what you think. I really like the extension idea though.

  • \$\begingroup\$ Would empty string "" be an acceptable falsy output? Or empty list []? \$\endgroup\$
    – mbomb007
    Commented Apr 6, 2017 at 21:50
  • 1
    \$\begingroup\$ Shouldn't the second valid test case be ["sen","123"]? \$\endgroup\$
    – Laikoni
    Commented Apr 6, 2017 at 22:46
  • 1
    \$\begingroup\$ 1. The examples suggest, but the spec does not state, that trivial translations should be excluded. Or would ["rabid","snail"] also be an acceptable output for rabid,snail? What about ["rabidz","snailz"]? 2. The comment "(different character patterns, not a translation)" seems misleading to me. E.g. given input abc,ddd I would expect that despite having different character patterns the output should be truthy: to wit ["abc","ddd"]. 3. Does the order of the elements in the translation tables matter? I.e. would ["tb","ps"] be acceptable output for the first example? \$\endgroup\$ Commented Apr 7, 2017 at 10:03

QR Code Editing: Minimum Change

? This is pretty non-trivial, and finding optimal solutions is probably computationally difficult.

QR codes are a way to represent URLs and other strings of text using a 2D image. What you might not know, however, is that a significant portion of a QR code is error-correction or unused space.

In this challenge, your program will be given a square of 1s and 0s representing a QR code. This QR code may not be functional, or it may be valid or point to some destination. Your program will also receive a string, representing the data that the new QR code should be encoded with. The goal of the program is edit the original QR code to contain the new content, but also to do it with as few pixel-flips as possible.

Here's an image describing some parts of the QR format:

enter image description here

QR code art generator, demonstrating how much of a QR code's space can be modified without changing the content: https://www.qrpixel.com/

Spec is only partially completed, I'll probably have to find some decent resources on QR code formats and error correction.

  • \$\begingroup\$ sound like it will make an interesting challenge once the spec is fleshed out. I assume you're going to include all the qr code info that we need? \$\endgroup\$
    – Liam
    Commented Apr 9, 2017 at 22:36

Pratt certificates

(bumping this proposal to see if there's any interest or comments. If so, speak now; if not, I'll delete)

Your task: write code that generates a Pratt certificate for a prime number, and write code that verifies an existing Pratt certificate.

What's a Pratt certificate?

A Pratt certificate for a prime number p is a proof, of a particular type, that p is indeed prime. Historically, it was used in situations where proving the primality of p required a computation that was slow due to factoring p-1, but verifying the certificate (once the initial computation generated it) was quite fast.

A Pratt certificate for p is a recursive structure consisting of three parts: the prime p itself; a "witness" integer g (which is actually a primitive root modulo p; see the next section for its properties); and Pratt certificates for all primes dividing p-1. The prime p=2 is special: a Pratt certificate for 2 is just 2 itself.

For example, here is a Pratt certificate for p=3911:

{3911, 13, {2, {5, 2, {2}}, {17, 3, {2}}, {23, 5, {2, {11, 2, {2, {5, 2, {2}}}}}}}}

The witness is 13, and the prime factors of 3911-1 are 2, 5, 17, and 23; each of those new primes itself has a Pratt certificate, which are respectively 2, {5, 2, {2}}, {17, 3, {2}} and {23, 5, {2, {11, 2, {2, {5, 2, {2}}}}}. In this last Pratt certificate, the prime factors of 23 are 2 and 11, so a Pratt certificate for 11 must be included, and so on.

How do we generate a Pratt certificate?

Given a prime p, a Pratt certificate can be generated by finding a primitive root g modulo p; factoring p-1 into primes (keeping only one copy of each prime factor); and recursively generating Pratt certificates for every prime factor of p-1.

How do we verify a Pratt certificate?

Given a prime p, a witness g, and the prime factors q1, q2, ... of p-1, a Pratt certificate is verified by checking:

  • that p-1 has no prime factors other than q1, q2, ...;
  • that the power g^(p-1) is congruent to 1 modulo p;
  • that none of the smaller powers g^((p-1)/q1), g^((p-1)/q2), ... are congruent to 1 modulo p; and
  • that each of the Pratt certificates of q1, q2, ... are themselves valid.

Scoring and technicalities

You must write two programs or functions (or one of each): one that takes a prime number as input and returns its Pratt certificate; and one that takes an input formatted like a Pratt certificate and returns a truthy or falsy value depending on whether it is an actual Pratt certificate.

  • You may choose any reasonable format for the Pratt certificate: nested lists (like the examples in this question), indented multiline strings (like the example on the Wikipedia page), or something similar that a human being could be trivially trained into parsing by eye. You may use any reasonable convention for the trivial Pratt certificate for 2.
  • However: whatever format you choose for the Pratt certificate, your certificate-generating code must output the same format that you take as input to your certificate-verifying code. Note that your certificate-verifying code must be capable of verifying any possible Pratt certificate (in your format) for p, not just the one your other program generates for p.
  • If you want, you may write a single program or function that accomplishes both tasks; in that case, your code can either determine which task is being asked of it implicitly from the input, or it can allow the user to instruct it which task to perform in some reasonable way.
  • Regardless of whether you use one or two programs, no calculation can be shared or saved between different runs of the code. The programs must work correctly, on any individual prime input and on any individual certificate-shaped input, if it is the first time that code is ever being run on that system.
  • You don't have to handle bogus input. You may always assume that the input to your first program is an actual prime number, and that your input to the second program syntactically matches your Pratt certificate format.
  • Built-ins that generate or verify Pratt certificates are not allowed. Other types of built-ins (for example, those that factor integers, raise integers to powers in modular arithmetic, find primitive roots) are acceptable.
  • This is , so shorter code (in bytes) is better. If two programs are used, the total number of bytes in both programs is the score; if one program is used, its number of bytes is the score.

Example Pratt certificates given prime inputs

(Note that there are many possible witnesses for any given prime, but the rest of the certificate is unique up to reordering.)

31 -> {31, 3, {2, {3, 2, {2}}, {5, 2, {2}}}}
127 -> {127, 3, {2, {3, 2, {2}}, {7, 3, {2, {3, 2, {2}}}}}}
229 -> {229, 6, {2, {3, 2, {2}}, {19, 2, {2, {3, 2, {2}}}}}}
1093 -> {1093, 5, {2, {3, 2, {2}}, {7, 3, {2, {3, 2, {2}}}}, {13, 2, {2, {3, 2, {2}}}}}}
65537 -> {65537, 3, {2}}

(All the above outputs are examples of truthy inputs for the Pratt-certificate checking code.)

Example falsy inputs for Pratt-certificate checking

{31, 2, {2, {3, 2, {2}}, {5, 2, {2}}}}
{31, 3, {2, {3, 2, {2}}}
{31, 3, {2, {3, 2, {2}}, {5, 2, {2, {3, 2, {2}}}}}}
{127, 2, {2, {3, 2, {2}}, {5, 2, {2}}}}
{85, 4, {6, 5, {5, 2, {2}}}, {14, 3, {13, 2, {2, {3, 2, {2}}}}}}

Median fractals

I define 'Median fractal' as this.

Median fractal L1 is a equilateral triangle.

for median fractal Ln, Draw Ln-1, then for each triangle, draw all 3 medians in the triangle.

You will be given an integer n, draw Ln.


Any suggestions, guys?

  • \$\begingroup\$ Would benefit from 1. A definition of the median of a vertex; 2. Diagrams of L_2 and L_3; 3. A better name. It's a variant on triangle centre fractals, but perhaps would be best called the vertex-median fractal. \$\endgroup\$ Commented Apr 15, 2017 at 12:12
  • \$\begingroup\$ I'm not sure what exactly "draw all 3 medians in the triangle" means. I think providing images of the first few fractals would help a lot to make the construction better understandable. \$\endgroup\$
    – Laikoni
    Commented Apr 17, 2017 at 7:42
  • \$\begingroup\$ Should add: graphical output questions should at the very least specify minimum sizes to avoid trivial answers which give a 1x1 pixel bitmap output - although I suspect answers will favour vector graphics formats. \$\endgroup\$ Commented Apr 18, 2017 at 16:24

These would be separate questions, and each would link to the other.

Cops: Make a bad password policy

Over the years, people have come up with some pretty bad password policies. Your challenge is to make such a policy, and to make a program that takes advantage of the weakness of this policy to brute force passwords written in it (a "crack"). For details on what constitutes a "crack", see the robber thread [link]. It should restrict the user to as small a selection of passwords as possible. Robbers will attempt to find cracks for your policy. If your answer is uncracked after 7 days, you make mark it safe by posting your crack. The "worst" uncracked policy (that is, the one that allows the fewest passwords) wins.

A "policy" is defined as a list of well-defined restrictions ("rules") on valid passwords. Here is an example:

  • Passwords must contain only digits, letters, and the characters '*&^'.
  • Passwords must be at most 8 characters long.
  • Passwords must not contain dictionary words (if you use this one, you must use a freely and programmatically accessible dictionary and tell us where to find it).


  • must contain no more than 10 rules,
  • may not involve encryption of any kind, and
  • must be clear and unambiguous

For example, these would be a bad rules:

  • The MD5 hash or zipfile of the password must not contain the letter "a". (Uses encryption of a sort)
  • Passwords may not contain special characters. (It's unclear what counts as a special character)

Your answer must contain both the policy and the number of passwords it allows.

Robbers: Crack the bad password policy

Over the years, people have come up with some pretty bad password policies. Your challenge is to crack such a policy. These can be found in the cops thread [link]. To crack a policy, write a program to brute force passwords that adhere to that policy. Your program must run in linear time on the number of possible passwords allowed by the policy. You may do this by enumerating or iterating over all possible passwords in some way, such as by calling a function (called, for example, guessPassword) for each possible password. One option is to simply hardcode the possible passwords, if there are only a few. Here is an example (JavaScript):

function* getPasswords() {
    yield* ['a', 'b', 'c'];

Or, using the second approach:

function bruteForce() {
    for (let pw of ['a', 'b', 'c']) {
        if (guessPassword(pw)) {
            return pw;

Whoever cracks the most policies wins.


  • Is the linear time approach good? My original thought was just "your program must halt in a reasonable time", but that seems too vague, and this allows a person to crack even policies with a lot of possible passwords (though those hopefully wouldn't win anyway).
  • I could also just say, "pick a password in your policy and robbers will try to crack it", maybe having the cops supply a hash of the password.
  • 1
    \$\begingroup\$ I'm worried that this sort of challenge needs far too many arbitrary-seeming restrictions on the policy to make it work. The restriction against prime numbers is already fairly arbitrary, for example. Additionally, "linear time" doesn't make sense here; there's a finite number of possible passwords (according to the victory condition), meaning that any program that enumerates them runs in O(1) (thus faster than linear) by definition. Also, I think it's fairly easy to encode an NP-complete problem into the challenge in an understandable way; many are pretty intuitive. \$\endgroup\$
    – user62131
    Commented Apr 17, 2017 at 9:30
  • \$\begingroup\$ @ais523 Okay, if I require cops to have valid cracks, a few of those restrictions go away, since we don't have to worry about people making policies that are impossible to crack. Also, I know the programs would technically all be constant time, but I'm not sure how else to specify what constitutes a "fast" crack. I could just say, "must run in a few minutes on my machine" or something. \$\endgroup\$ Commented Apr 17, 2017 at 16:12
  • \$\begingroup\$ "I'm not sure how else to specify what constitutes a "fast" crack" is IMO evidence that this idea is not workable. \$\endgroup\$ Commented Apr 17, 2017 at 17:57

shortest angular path

Let's say you have a robot and you measure the orientation of a wheel (e.g. for tracking the exact distance), but the sensor just returns an angle between -180° and 180°. Then you have the problem that even if the wheel just moved three degrees from 179° to 182° the sensor will tell you that the wheel made a jump from 179° to -178°. This makes analyzing and interpreting the data a little bit cumbersome. Given a list of subsequent measurements your goal is now "smoothing" them out, such that from each entry A to the next B, in the list ([...,A,B,...]) there is at most a 180° jump.


  • You can also assume other (single number) representations of angles, e.g. radians (0 to 2*pi) or gradian (0 to 400) or number of turns (0 to 1).
  • If it is more convenient, you can assume that the sensor will return data in [0°,360°] instead of [-180°,180°].
  • Which way you go if the jump is exactly 180° (or -180° or 540° etc.) is up to you.


[0 10 90 80 180 -130 -90 -120 -143 170 0] (Input)
[0 10 90 80 180 230 270 240 217 170 0]    (Output)

Get the Decimal! (Posted)

  • \$\begingroup\$ Rounding on the digit, or return the exact digit? (important distinction if people can generate out to x and then take the last digit, for example) \$\endgroup\$ Commented Apr 21, 2017 at 15:11
  • 1
    \$\begingroup\$ Please delete this now that it is posted. \$\endgroup\$
    – user58826
    Commented Apr 27, 2017 at 17:24

Smooth an array

A typical signal processing operation consists in smoothing a signal to reduce noise.

A very basic way to perform such smoothing, on an array of integers, is using the following formula:

enter image description here

where y'_t is the smoothed value at index t, y_t is the original value at index t, and where β is a smoothing parameter (in [0,1]). Note that y' = floor(y + 0.5) is the classic round half up operation.

For t = 0, there is no y'_(t-1), thus we set that y'_0 = y_0.

Applying that operation on a sequence of integers pictured below on the left, produces the sequence of integers pictured below on the right:

enter image description here


Given a list of integers and β, output that list of integers smoothed in that way.

All integers of the list will be guaranteed to be in the interval [1, sup) where sup is the maximum integer representable in the integer type you use in your language. All integers of the input must be taken, and all integers of the output must be printed, in the decimal base (unless your language does not support decimal numbers, in which case you may use the standard base your language uses).

The input list is guaranteed to have at least 2 elements. You may take that input list in any way or format that is sensible in your language.

β is guaranteed to be in [0,1].

Test cases

TODO add more

List                               β     Output
[1,2,7,3,11,13,26,5,18,4,3,2,1]    0.5   [1,2,5,4,8,11,19,12,15,10,7,5,3]


This is , so the shortest answer in bytes wins.


Output the Name of a Number

Idea from a recent question on math SE.

Write a program or a function that takes an integer from range 0 to 255 (inclusive) and outputs the name of its unsigned 8-bit binary representation. Input can be in any integer format you like, but the outputted name must match the number's unsigned 8-bit representation.

The name of the number is determined by the positions of its binary digits that are 1:

1 − John
10 − Watson
11 − Watson John
100 − Kevin
101 − Kevin John
110 − Kevin Watson
111 − Kevin Watson John

Let the names for the eight bits be Laura, William, James, Mary, Alice, Kevin, Watson, and John.

So now the name of 255 (11111111 in binary) is:

Laura William James Mary Alice Kevin Watson John

Zero doesn't have a name under this system so you may output an empty string or nothing at all. Trailing whitespace is allowed for all outputs.

Test cases:

<input (as decimal)>
<8-bit binary>

Alice Watson

James Alice Watson


Laura James Mary Alice Kevin John

James Mary Kevin Watson John

William James

James Kevin John


Laura William James Mary Alice Kevin Watson John

This is code-golf, show the shortest answer in bytes wins.


Optimize for the Test Cases

Given a set of test cases (pairs of input and output strings) as input, output a program in a programming language of your choice that consistently and deterministically produces the correct output for each input.


  • You may choose any programming language for the output programs, so long as it is a programming language by our definition, it existed prior to the creation of this challenge, and it is capable of taking arbitrary ASCII strings as input and producing arbitrary ASCII strings as output.
  • Your solution must use the same programming language for all inputs. The output programs do not need to be in the same language as the solution.
  • The output program must consistently and deterministically produce the correct output for each input. That means that, no matter how many times the program is run, it will always produce the correct output (barring any uncontrollable accidents like cosmic rays twiddling bits). PRNGs are allowed so long as a constant seed is provided (making the output consistent and deterministic).
  • The test cases' inputs are provided without quotes, but you may require that the input be quoted if it is necessary or convenient (as per our usual policy on quoted input). However, you must either always or never have quotes on the input - you can't have some with quotes and some without.
  • It does not matter what the output program does for inputs that are not part of the specific test case the program was created for.
  • You may optionally take the number of test cases as input, if that is convenient or necessary.
  • Every input in a set of test cases will be mapped to a unique output. Input values will not be repeated within a set of test cases (so [(3, 5), (3, 6)] wouldn't be a valid test case).


Your score will be equal to the sum of the byte counts of all generated programs for the set of scoring cases. Lowest score wins. This is a competition within output languages, not a competition between output languages, so don't be afraid to choose a language that is more verbose than others. I reserve the right to change the scoring cases should it be necessary.


A C++ program that outputs (poorly-golfed) Python functions:

#include <iostream>
#include <string>
#include <sstream>

using namespace std;

const string function_skeleton_start = "lambda s:{";
const string function_skeleton_end = "}[s]";

int main() {
    int N = 0;
    string in, out;
    stringstream program;
    program << function_skeleton_start;
    cin >> N;
    for(int i = 0; i < N; ++i) {
        getline(cin, in);
        getline(cin, out);
        program << "'" << in << "':'" << out << "',";
    program << function_skeleton_end;
    cout << program.str() << endl;
    return 0;

An example input:


The corresponding output:

lambda s:{'foo':'bar','bar':'foo',}[s]

Test Cases


  • \$\begingroup\$ I'd be very interested to see some of the answers this challenge would get +1 \$\endgroup\$
    – MD XF
    Commented May 10, 2017 at 1:21

Output "Hello, World!"... Even before it runs?

Please note that this challenge only applies to languages that have a build stage and a build log.

Output "Hello, World!" to the build log. The program may or may not be compiled successfully, the job is to just output "Hello, World!" somewhere in the build log.

Example (C/C++)

#pragma message("Hello, World!")
  • 1
    \$\begingroup\$ could it be more interesting as a rosetta-stone challenge? \$\endgroup\$ Commented May 13, 2017 at 9:32

Is it a number?

Given an input, output truthy if it is a valid floating point number, and falsy if it is not. The format that is used in this challenge is [-][<integer>][.][<integer>][e[-]<integer>], where square brackets specify optional values. At least one group in the first section ("mantissa") needs to exist.

Here are some examples of inputs:


5e3         # equal to 5000
-.5245e3    # equal to -524.5
155.0e-3    # equal to 0.155
-5e-1       # equal to -0.5
.           # equal to 0.0


feed me numbers
123 456


  • Standard loopholes apply
  • This is a , shortest answer wins. However, it will not be accepted.

  • \$\begingroup\$ For reference, -?\d*(\d+|\.\d*)(e-?\d+)? will match it if it's a number; it will match parts though so beware of that. \$\endgroup\$
    – user42649
    Commented May 17, 2017 at 4:15
  • \$\begingroup\$ How to handle empty input, and is -0 valid? \$\endgroup\$
    – Pavel
    Commented May 17, 2017 at 4:19
  • \$\begingroup\$ @Phoenix Empty input is invalid. -0 is valid for this challenge. \$\endgroup\$
    – anna328p
    Commented May 17, 2017 at 4:19
  • \$\begingroup\$ Please add those as test cases. \$\endgroup\$
    – Pavel
    Commented May 17, 2017 at 4:57
  • 1
    \$\begingroup\$ Also, -, e, .e4, -e4, -. \$\endgroup\$
    – Pavel
    Commented May 17, 2017 at 4:58
  • 1
    \$\begingroup\$ This has definitely been done before here \$\endgroup\$ Commented May 17, 2017 at 6:55
  • 1
    \$\begingroup\$ How is . equal to 0.0? Will the valid inputs be as integers, strings or either? \$\endgroup\$
    – Shaggy
    Commented May 17, 2017 at 12:33
  • \$\begingroup\$ @Shaggy This is meant to represent floating point literals. Valid inputs are strings. \$\endgroup\$
    – anna328p
    Commented May 17, 2017 at 21:12

Don't break the bridges!


You are a worker, who is in charge of managing a set of bridges, connecting a square grid of "nodes":

N - N - N
|   |   |
N - N - N
|   |   |
N - N - N

(the grid here is 3 by 3, but they can be larger).

Each of the bridges has a set capacity from 1 to 10, and each of the bridges has a number of cars over them, also from 1 to 10.

  • If a bridge has a higher capacity than the number of cars on that bridge, then it is considered "safe", and you can cross over it.
  • If a bridge's capacity and number of cars going over it are equal, then it is considered "stable". It won't collapse, but you can't cross over it.
  • If a bridge has a lower capacity than the number of cars on that bridge, then it is considered "collapsing", and you only have a limited amount of time to fix it.

When a bridge has n capacity and m cars, with n smaller than m, the time it takes to collapse is:

      m + n
ceil( ----- )
      m - n

You must take materials (and therefore reduce the bridge's capacity) from other bridges and arrive to those bridges on time to fix them! To get materials from a bridge, you must cross over it. For example, take this small arrangement:

A - B

The bridge between A and B (which we'll call AB) has 3 capacity, and let's say you're on A, and want to take 1 material. To take the material, simply cross from A to B.

Now, AB has 2 capacity, and you have 1 material on you. You may only cross over bridges that are "safe", though (or if you're fixing a bridge, which is explained in the next paragraph).

To fix a bridge, you must go over it, thereby depositing all materials needed to fix the bridge. For example, in the example above, if AB had 1 capacity and 2 cars currently on it, and you had 2 material on you, once you cross the bridge you will have 1 material, because that is all that's required to fix the bridge.

You must fully cross a broken bridge before the bridge collapses, otherwise it will break. Each crossing of a bridge takes 1 hour, and the time it takes for the bridge to collapse is shown in the formula above. For example:

C - D

In this example, if your starting node was A, and CD only had a "lifespan" of 2 hours, the bridge would collapse before you can get to it (crossing AB takes 1 hour, crossing BC takes another hour).


Your task is to make a program that calculates, given a list of bridges, which are represented themselves as lists of two elements (first element is capacity, second element is cars on the bridge), whether or not it's possible to fix all of the bridges. The bridges work from top-to-bottom, left-to-right - so an input of

[[3 2] [3 2] [2 5] [5 1]]

means that the actual grid looks like this:

 A --- B
 |  2  |
3|2   2|5
 |  5  |
 C --- D

So AB has a capacity of 3 and 2 cars, AC has a capacity of 3 and 2 cars, BD has a capacity of 2 and 5 cars, and CD has a capacity of 5 and 1 car.

Rules / Specs:

  • Your program must work for, at least, 10 * 10 grids.
  • Your program may accept the input as either a string with any delimiter, or a list of lists (see example I/O).
  • Your program must output the same value for true for all true values, and it must output the same value for false for all false values.
  • You can either submit a full program or a function.

Example I/O:

[[5 5] [5 5] [1 1] [3 3]] => true
[[2 5] [2 2] [3 3] [1 2]] => false
[[3 2] [3 2] [2 5] [5 1]] => true

NOTE, you can take the input like this as well:
[[3, 2], [3, 2], [2, 5], [5, 1]] (Python arrays)
3,2,3,2,2,5,5,1                  (Comma-separated string)
3 2 3 2 2 5 5 1                  (Space-separated string)
  • \$\begingroup\$ "You may only cross over bridges that are "safe", though. To fix a bridge, you must go over it". Huh? \$\endgroup\$ Commented May 20, 2017 at 10:53
  • \$\begingroup\$ @PeterTaylor Fixed. \$\endgroup\$
    – clismique
    Commented May 20, 2017 at 11:02
  • \$\begingroup\$ Will the input always have a square number of elements? \$\endgroup\$ Commented May 20, 2017 at 11:38
  • \$\begingroup\$ @ETHproductions The nodes are square, but the input won't always have a square number of elements, because the number of paths diverges from the number of nodes (4 for 2 * 2, 12 for 3 * 3, etc.). I'm all ears to better input formats, I understand this one would be hard to process. \$\endgroup\$
    – clismique
    Commented May 20, 2017 at 11:40
  • \$\begingroup\$ Ah, so it'll always be a number of the form 2*n*(n-1), or 4 times a triangular number. Not sure if there is any better input format... \$\endgroup\$ Commented May 20, 2017 at 12:33

How many times?

Inspired by Rotational symmetry of string.

Given a string, return its shortest substring that, when repeated a number of times, will produce the original string, as well as the number itself.

Test cases:

"AAAAAAAAAA" => "A", 10
"" => "", 0
"Don't repeat yourself. Don't repeat yourself. " => "Don't repeat yourself. ", 2


  • You may return the substring and the number in any format you'd like, as long as it's consistent and clear i.e. you can always separate the number correctly from the substring e.g. abc12 isn't allowed since you can't determine whether abc is to be repeated 12 times or abc1 is to be repeated 2 times. The substring must be returned verbatim, while the number can be returned in any generally allowed format per meta consensus.
  • The substring must be the shortest possible.
  • For the empty string, you must return "" and 0.


  • Is the challenge description golfed enough? :P
  • May I improve the return/output format?
  • \$\begingroup\$ Pleas can you change the abc2 should be repeated 1 time to abc1 should be repeated 2 times? It makes more sense. \$\endgroup\$ Commented May 20, 2017 at 21:16
  • \$\begingroup\$ @RandomUser Typo; should be fixed. \$\endgroup\$ Commented May 20, 2017 at 21:17
  • \$\begingroup\$ Related, supertask \$\endgroup\$ Commented May 21, 2017 at 7:02
  • \$\begingroup\$ What range of characters are possible within the string? Can newlines occur? \$\endgroup\$ Commented May 21, 2017 at 8:58
  • \$\begingroup\$ Also, why not just specify that the output format is "12\nabc"? Otherwise there may be arguments on everyone's answers about whether or not they meet the requirements. \$\endgroup\$ Commented May 21, 2017 at 8:59
  • \$\begingroup\$ @SteveBennett And that's what the second question in the "Sandbox" section is for. Please note that I do not intend to restrict the output format like this. \$\endgroup\$ Commented May 21, 2017 at 11:47

Build the Chain Quine

This is an puzzle.

Each person will write a program that is not a true Quine but does output its source when given the source of the last program as input. If anything else is input your program may do whatever you wish (undefined behavior) as long as it does not print the source code. The first program will be a true Quine.


  • Standard rules apply

  • You may not write a submission in a language that has already been used

  • You may not answer twice in a row


The goal is to have as many valid links in the chain as possible.


This is a little sparse because I am still in the brain-storming phase of development. I just wanted to write this down so I wont forget it and, of course, to get feedback. I am not even really set on a winning criterion yet. If you have any ideas/suggestions I am really excited to hear them (thats why I put it in the sandbox).

  • \$\begingroup\$ This is semi similar to my answer chaining quine. So I wish you good luck \$\endgroup\$
    – user63187
    Commented Feb 27, 2017 at 16:36
  • \$\begingroup\$ yeah sure thing! meta.codegolf.stackexchange.com/posts/11615 \$\endgroup\$
    – user63187
    Commented Feb 27, 2017 at 16:38
  • \$\begingroup\$ Mistake! meta.codegolf.stackexchange.com/questions/2140/… \$\endgroup\$
    – user63187
    Commented Feb 27, 2017 at 16:40
  • \$\begingroup\$ As with so many quine challenges, this falls afoul of universal quine constructors (and is also likely to get longer and longer over time, due to the need for each program to be able to reconstruct the previous entry, and thus implicitly all previous entries). \$\endgroup\$
    – user62131
    Commented May 23, 2017 at 8:51

Quine without a character

Write a program in any language that takes as input any character, and outputs a quine in the same language, that does not contain that character.

For instance (for some made up language):

Input: b
Output: s(fg;fg)
Run "s(fg;fg)"
Output: s(fg;fg)

Input: (
Output: s[fg;fg]
Run "s[fg;fg]"
Output: s[fg;fg]

Your program must handle as input every character within the range of characters that are valid in the source code of the language you're using, including new lines, punctuation, etc.


This is with penalties. Your score is L + 1000xC where:

  • L is the length of your program in bytes
  • C is the number of characters it fails to meet the requirements on.

So, if you produce a 50-character program that passes every character except for ( and ), your score is 2050.

Standard loopholes are forbidden, and standard methods of input/output are ok. Outputs must be a proper quine, whatever that is.

  • \$\begingroup\$ Basically a duplicate. The two challenges aren't quite the same (this one is more lenient), but the solution technique will be the same in both cases. \$\endgroup\$
    – user62131
    Commented May 23, 2017 at 6:17
  • \$\begingroup\$ Ah, I didn't see that one. I think that one is much harder, because your program has to output the whole quine factory. In my version, it's just a quine factory that outputs quines to order. \$\endgroup\$ Commented May 23, 2017 at 7:35

Add a language to a quine

Add a language to a quine program. Your program must output itself in all languages used so far.

Second to last answer wins after no answers have been posted for two weeks.

  • \$\begingroup\$ this is hard mode of existing challenge? \$\endgroup\$ Commented May 24, 2017 at 3:58
  • \$\begingroup\$ @DestructibleLemon Basically. \$\endgroup\$
    – anna328p
    Commented May 24, 2017 at 3:58
  • \$\begingroup\$ @Okx So that you can't win by posting an answer which is impossible to add onto. \$\endgroup\$ Commented May 25, 2017 at 15:12

Build a simple 2D game engine

Write a program that take input specifying specs of a simple 2D game regarding the following:

  • GUI: a space where you can print text
  • Scene: a 2D stage where player\enemy move
  • Input Scheme: a way to check for input
  • Player: one object
  • Enemy: one or more objects
  • Coin: one object
  • Engine: manage running scenes

The program should offer the following instructions:

  • generate: create an object & place it in the scene
  • destroy: destroy an object or hide it from the scene
  • move: move an object to a different location, or apply motion
  • collision: check if two objects are colliding or interlacing
  • write: write to GUI
  • param: create, modify, or read an integer
  • terminate: stop running update, and run end
  • a way to do an if-else

A Scene should at least have the following functions:

  • start: sequence of instructions, applied once when the scene is fired
  • update: sequence of instructions, applied N-times per second
  • end: sequence of instructions, applied once when the scene is terminated


Input the game spec



START: GEN(p1,PLAYER,5,1); GEN(e1,ENEMY,4,4); GEN(c1,COIN,3,6); PARAM(r1,0)

          IF(INPUT(ARROW-UP), MOVE(p1,-1,0));
        IF(INPUT(ARROW-DOWN), MOVE(p1,+1,0));
        IF(INPUT(ARROW-LEFT), MOVE(p1,0,-1));
       IF(INPUT(ARROW-RIGHT), MOVE(p1,0,+1));


Output an interactive game

[ 0        ] <----- G:GUI (1 x 10)
|          | <--- S:Scene (5 x 10)
|          |
|      C <-|----- C:Coin
|   E <----|----- E:Enemy
|P <-------|----- P:Player
  • game starts with player, one enemy, one coin
  • you can control the player with arrow keys
  • if player come in contact with enemy, player disappear then it ends
  • if player come in contact with coin, coin disappear then it's counted
  • when game ends, if coin count is 1 you win, otherwise you lose


  • What tags to use?
  • How can I simplify the requirements so that it's possible to solve it with languages of simple means?
  • I haven't seen many challenges like this, to write a program which parse instructions to create an interactive program within it, what do you think?

Distinct strict partition counts


The Task

Write a function or program that, given a positive integer n, returns or prints an array (or list, set, etc.) of length l such that each index i contains the count of distinct partitions of size i of n (one-indexed) and l is the maximum size of partitions for n.


Let f(n) implement the task described.

Consider f(10)

10 may be broken into the following distinct partitions:

10         // Size 1
9,1        // Size 2
7,2,1      // Size 3
4,3,2,1    // Size 4

There are:

  • 1 distinct partitions of size 1.
  • 4 distinct partitions of size 2.
  • 4 distinct partitions of size 3.
  • 1 distinct partitions of size 4.

Therefore, f(10) returns [1, 4, 4, 1].

Test Cases


  • \$\begingroup\$ A008289. \$\endgroup\$ Commented May 24, 2017 at 21:25
  • \$\begingroup\$ @PeterTaylor Thank you. I was dreading the larger test cases. \$\endgroup\$ Commented May 24, 2017 at 21:35
  • \$\begingroup\$ in my opinion this suggestion is quite clearly defined and simple, I like it - but it didn't attract votes nor critique .. do people find it too mathematical? I was going to propose something similar related to enumerating Standard Young Tableaux given a partition shape \$\endgroup\$
    – jayprich
    Commented Aug 22, 2018 at 10:14

Show key code

Inspired by the showkey command.


You have to output the key and if is a key-up or a key-down event. I exemplify:

enter image description here

The output I want:

Key X Down
Key X Up

for both the instants you press or release a key.

You can output it on a console or on a GUI.

It is , so shorter code wins.

  • 1
    \$\begingroup\$ I don't think this is currently clear. What keys correspond to what key codes? You should include either a list, or some source where one can determine which keys correspond to which codes. \$\endgroup\$
    – Wheat Wizard Mod
    Commented May 27, 2017 at 0:18
  • \$\begingroup\$ @WheatWizard i think, there is standard. Also, if not, there will be keyboard/language-related key code table \$\endgroup\$ Commented Jul 31, 2017 at 11:48

Set The Shapes

Updated on 23/05/17 with new shapes & questions


Let s be the sequence of polygonal numbers with S sides and t be the sequence of polygonal numbers with T sides. Take the set union of the first n elements of s and the first n elements of t.


3 integers, n, s & t where:

  • n>0, 2<(s,t)<13 and s!=t
  • The values of s & t each represent a different polygonal number type:
  3 = Triangle
  4 = Square
  5 = Pentagon
  6 = Hexagon
  7 = Heptagon
  8 = Octagon
  9 = Nonagon
 10 = Decagon
 11 = Hendecagon
 12 = Dodecagon


An array of the set union of the first n numbers of shapes s and t in ascending order.


(assuming 0-based indexing)

  • Input: n=5, s=3, t=4
  • The first 5 triangular numbers are 0, 1, 3, 6, 10
  • The first 5 square numbers are 0, 1, 4, 9, 16
  • Output: [0, 1, 3, 4, 6, 9, 10, 16]


  • Input must be 3 separate integers
  • Output must be an array (or equivalent in your chosen language)
  • If your chosen language isn't capable of handling either or both of the above then standard I/O methods apply
  • s & t are guaranteed to be different but you must be able to handle them being input in either order - smallest first or largest first
  • This is so lowest byte count wins

Test Cases

Input: 5, 3, 4
0-Based Output: [0, 1, 3, 4, 6, 9, 10, 16, 20]
1-Based Output: [1, 3, 4, 6, 9, 10, 15, 16, 25]

Input: 10, 9, 8
0-Based Output: [0, 1, 8, 9, 21, 24, 40, 46, 65, 75, 96, 111, 133, 154, 176, 204, 225, 261]
1-Based Output: [1, 8, 9, 21, 24, 40, 46, 65, 75, 96, 111, 133, 154, 176, 204, 225, 261, 280, 325]

Input: 1, 7, 6
0-Based Output: [0]
1-Based Output: [1]

Input: 8, 3, 3
Invalid input as both shapes are the same

Input: 0, 2, 17
Invalid input as n<1, s<3 and t>12

Bonus Idea 1

Brownie points if you want to increase the range for s & t and include some other polygons (e.g., star, dodecagon).

Bonus Idea 2

Alter the challenge to only require that solutions be able to handle 5(?) different shapes with a score reduction for each additional shape, up to a maximum of 10(?).


This is my first challenge (inspired, in part, by this) so all feedback very much welcome.

  • Should I add or remove any tags?
  • Is the above sufficiently different from this and this?
    (Votes: 2 for this not being a dupe, 2 for it being a dupe of the second linked challenge)
  • Does everything read OK & make sense?
  • Should I include the formula for each shape in the question?
    I'm leaning towards "yes" as:
    1. For those that know them, figuring them out won't be a challenge
    2. For those that don't, figuring them out wouldn't add to the challenge
    3. As soon as one person posts a solution containing the formulas, that makes them available to everyone else anyway.
  • Should there be more (e.g., star, tridecagon+) or less possible shapes?
  • Would it improve the challenge if I allowed solutions to pick a smaller subset of 5(?) shapes to work with? How about if that subset had to be sequential (e.g., 3-7 or 5-9 but not 3,5-8)?
  • What test cases should I add?
  • Do the bonus ideas add to or detract from the challenge?
    Answered: Bonuses are generally bad.
  • Is there a better name I can give this challenge?
  • Should I be asking any other questions?!
  • \$\begingroup\$ Yeah if you want to make it bigger just add the polygons and require them. But right now it looks to be a big enough handful. \$\endgroup\$
    – user63187
    Commented May 17, 2017 at 10:14
  • 1
    \$\begingroup\$ The two linked challenges don't seem similar enough to make this a duplicate (to me). \$\endgroup\$ Commented May 17, 2017 at 10:18
  • 2
    \$\begingroup\$ 1. I don't find "the first n numbers of both shapes" very clear. In fact, I can't find a single English phrase which cleanly expresses the meaning I reverse engineered from the example. I think it takes a few sentences or a mathematical expression. 2. The example looks wrong: what happened to 15? 3. I disagree with trichoplax: IMO this is a duplicate of the second linked question. \$\endgroup\$ Commented May 17, 2017 at 10:51
  • \$\begingroup\$ Thanks for your comments, @PeterTaylor. 1. Could you suggest some wording for the intro to make it clearer? 2. Yep, I messed that up in an edit - it's fixed now. 3. Fair enough, that's one vote for it being a dupe and one vote against - I'll update to reflect that. \$\endgroup\$
    – Shaggy
    Commented May 17, 2017 at 10:55
  • 2
    \$\begingroup\$ Let S be the sequence of polygonal numbers with s sides and T be the sequence of polygonal numbers with t sides. Take the set union of the first n elements of S and the first n elements of T. \$\endgroup\$ Commented May 17, 2017 at 11:16
  • \$\begingroup\$ Many thanks, @PeterTaylor; I've edited that in. \$\endgroup\$
    – Shaggy
    Commented May 17, 2017 at 11:21
  • \$\begingroup\$ I agree with Peter - this is too close to the second linked challenge. \$\endgroup\$
    – user45941
    Commented May 29, 2017 at 10:23
  • \$\begingroup\$ Thanks for the feedback, @Mego. That ties the vote; guess this stays in the sandbox a little longer, so. \$\endgroup\$
    – Shaggy
    Commented May 29, 2017 at 10:28

Dodge your death!


"Muhuhuhahahah!" The mad scientist laughs. "You're trapped in my own little game!"

In front of you is a deadly pit of snakes, while behind you is a bottomless chasm. There's no way out, you're stuck!

"Two steps in front of you is the snake pit, and two steps behind you is the chasm. But! Before you move, you MUST write down a sequence of steps, forwards and backwards, and give them to me. But! because I'm feeling a bit evil today, I can make you take, instead of every step, every nth step, where n is less than your sequence length!

Choose wisely, now."

What's the maximum number of steps you can take before your imminent death?


The intro above is a twist on the Erdős discrepancy conjecture, which was recently proven true (if you want to understand more about this, go to this video, by James Grime - I "stole" the twist question off of him).

The answer to the intro is 11 steps, but I won't go too in-depth with a proof. The answer, if the distance between you and the two "dangers" were 3 steps, is 1160 steps, although that isn't validated properly yet.

Your task is to make a program that generates the longest sequence of steps you can for a larger x, where x is the number of steps between you and the two dangers. For the purposes of this challenge, + represents a step forward, and - represents a step back.

So, an output for an input 2 is:


Which works, no matter what n the mad scientist chooses. For our challenge, x = 5.


  • Your entire program should fit into your answer. However, if it doesn't fit, please provide an additional Github repository, or something similar.
  • You may update both your answer and program, if you can get a better score via optimisation.
  • In your answer, you must have:
    • Your program, in its entirety
    • The amount of steps generated - this will be your final score.
      • You must also provide an online version of the sequence in a Pastebin, or something similar. This is so we can check your answer.
    • The time your final score was last updated, so I don't have to check your history
  • You may NOT hardcode sequences beforehand
  • Your program must work for all x (where x is the number of steps between you and the pit & chasm), but you only need to provide the score for x = 5.

The answer with the largest score wins!


  • What tags should I put in?
  • What things should I fix / elaborate?
  • For the "link to text file containing sequence" thing, I'm worried that the files might eventually get really big, as answers are more and more sophisticated. Is there any way to counteract that?
  • 1
    \$\begingroup\$ 1. "Your task is to make a program that generates the longest sequence of steps possible for a larger x" doesn't actually seem to be true: later text implies that you want something that terminates in a reasonable time rather than something which finds the actual optimum. 2. "The accepted answer will be the answer with the highest score, a week after the challenge is posted" is something which we've been discouraging for about the past five years. 3. For n=3 isn't the maximum possible actually 1160? \$\endgroup\$ Commented May 31, 2017 at 11:52
  • \$\begingroup\$ @PeterTaylor I got confused with what you meant by 2 for a while... you mean that the answer shouldn't be determined in time at all, right? \$\endgroup\$
    – clismique
    Commented May 31, 2017 at 12:04
  • \$\begingroup\$ Yes, exactly. Accept the best answer after a week if you want, but don't imply that there's a deadline to submit answers. \$\endgroup\$ Commented May 31, 2017 at 13:34
  • \$\begingroup\$ Very closely related. The rules aren't quite the same, but this may nonetheless be similar enough to be a duplicate. Also, "your program must fit into your answer" is a meaningless length restriction; these sequences are highly compressible, so there's no reason not to calculate the best possible answer for n=5, hardcode it, and still have length left for a feeble attempt at other lengths. \$\endgroup\$
    – user62131
    Commented May 31, 2017 at 21:47
  • \$\begingroup\$ @ais523 The maximum length for n = 5 is extremely high, though, if I'm not mistaken. That would certainly make it extremely difficult to compress, and even if it was compressed, there's a very high chance of being beaten by some other answer. Also, that challenge's primary goal is code-golf, whereas mine doesn't focus on code golf at all, rather, generating the longest sequence possible. I can make n higher if you want. \$\endgroup\$
    – clismique
    Commented Jun 1, 2017 at 8:00
  • \$\begingroup\$ @ais523 I got rid of the "program must fit into your answer" thing. \$\endgroup\$
    – clismique
    Commented Jun 1, 2017 at 8:20
  • \$\begingroup\$ You go back and forth between x being the input and n being the input (the number of steps). It'd be better to be consistent. Also, instead of saying your program generates the longest sequence of steps possible, which can't be done because we don't know how long that sequence is, you should instead say that it generates as long a sequence as you can, or something like that. \$\endgroup\$
    – isaacg
    Commented Jun 1, 2017 at 9:40
  • \$\begingroup\$ @isaacg Fixed both issues! \$\endgroup\$
    – clismique
    Commented Jun 1, 2017 at 11:24

Data Categorization with a Si(g)n


Machine learning is a very powerful tool for categorizing data. It can help find a function that splits some already known data so that unknown values can be predicted. Typically, this function should be simple so that it does not "overfit" the data. As it turns out, previous studies have found out that the simple function f(x) = sgn(sin(ax)) can split most data into two categories with only a single coefficient to adjust! This will definitely (not) prevent overfitting, and should (not) be used for every application of machine learning.

sgn(x) is the sign function of x. As Wikipedia says,

         / -1 if x<0,
sgn(x) = |  0 if x=0,
         \  1 if x>0.

Data overfitting demonstration


Two ordered sets/arrays/lists of integers {m1, m2, m3, ...} and {n1, n2, n3, ...}.

You can take input as 2 lines of input, an array with the two arrays m and n as its elements, or any other reasonable method of input.


A value of a such that sgn(sin(ax)) equals 1 when x = m1, m2, m3, ..., and -1 when x = n1, n2, n3, ...

If no such value of a exists, you can output any non-numerical value.

Test cases:

Example inputs will be in the form [[m1, m2, m3, ...], [n1, n2, n3, ...]]
Input                  | Output
[[1, 5],    [2, 3]]    | 1.75
[[2, 3, 6], [5]        | 4.5
[[1, 3, 5], [2, 4, 6]] | 3.1415925
[[1, 6],    [2, 3]]    | Random junk
[[2, 3, 4], [1]]       | <Anything non-numerical>
  • 1
    \$\begingroup\$ Is there a way to determine whether it is possible to find such a value? If yes, could you please add that to the challenge, if no, then I think this challenge is impossible to solve. Other than that you should probably define what happens for sign if the argument is zero. Is it enough if it we get a solution for a finite precision implementation of sin (but there would actually be no solution)? \$\endgroup\$
    – flawr
    Commented May 30, 2017 at 18:27
  • \$\begingroup\$ I guess with a fast enough oscillation, there should be a solution for any data set in which there are not two points of opposite categories with the same x value. \$\endgroup\$
    – feersum
    Commented May 31, 2017 at 16:29
  • \$\begingroup\$ @flawr When I designed the challenge, I didn't know if there was a solution. But, that's what made it interesting. While actually making the test cases and verifying them, I found a good method of finding possible values, and also test it they exist. However, I thought that it was the analysis of the function that made the challenge interesting, and so I wanted people to find their own methods of finding an answer. I'll maybe add some bounds to where you need to check, but I don't want to just straight up give away a possible answer that everyone will copy. \$\endgroup\$
    – K Zhang
    Commented May 31, 2017 at 21:07
  • \$\begingroup\$ @feersum I had previously thought that that was the case, but it turns out that some data sets do not have a solution. I put 2 of them in the test cases section. \$\endgroup\$
    – K Zhang
    Commented May 31, 2017 at 21:20
  • \$\begingroup\$ "Two sets/arrays/lists of numbers": define numbers. All of the test cases use integers, but restricting it to integers vs permitting doubles makes a big difference. \$\endgroup\$ Commented Jun 1, 2017 at 11:20
  • \$\begingroup\$ @PeterTaylor I've restricted the inputs to be integers only \$\endgroup\$
    – K Zhang
    Commented Jun 1, 2017 at 11:54
  • \$\begingroup\$ @feersum It is relatively easy to see that the last test case does not have a solution. \$\endgroup\$
    – flawr
    Commented Jun 1, 2017 at 12:30

Odd with Odds, Even with Evens

Given an input integer n, express it as a sum of numbers with the following properties:

  • If n is even, the numbers used to sum should be even.
  • If n is odd, the integers used to sum should be odd.
  • Any integer used (x) should be 0 < x <= ceiling(n/2).
  • All numbers used in the summation should be unique.
    • If there are no set of distinct odd/even integers that sum to n, return -1.
  • Output may be as an array or delimited string.


Input | Output       | Comments
2     | -1           | 0 < ? <= 1 = [1] (No evens, not possible)
12    | [6,4,2]      | Order is arbitrary, [2,6,4] is fine.
24    | [8,4,12]     | [2,4,6,12] works too.
9     | [1,3,5]      | Odd cases are a bit trickier to think about.
      |              | ceiling(9/2) = ceiling(4.5) -> 1 < x <= 5 -> [1,3,5]       
1     | -1           | c(1/2) = c(.5) -> 1 < x < 1 -> []
13    | [1,5,7]      | c(13/2) = c(6.5) -> 1 < x <= 7 -> [1,3,5,7]
27    | [13,11,3]    | Etc...

The numbers you use for each answer and how you get to the numbers you used do not matter, the only thing that matters is that they adhere to the spec. The order of the numbers does not matter, and you may have different results than provided.

This is , first person to mail Bill the contents of Hillary Clinton's private mailserver wins.

  • 1
    \$\begingroup\$ "no unique sum of odd/even integers" --> "no set of distinct odd/even integers that sum to" \$\endgroup\$ Commented May 26, 2017 at 5:21
60 61
63 64

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