# Sandbox for Proposed Challenges

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

## Posting

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.

## Discussion

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 needs more feedback, but it's been ignored, you can ask for feedback in The Nineteenth Byte. It's not only allowed, but highly recommended!

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.

## Other

Search the sandbox / Browse your pending proposals

The sandbox works best if you sort posts by active.

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".

Get the Sandbox Viewer to view the sandbox more easily!

# The Great Code Golf Heist

Alternatively, the most literal cops'n'robbers you'll ever see

## Backstory

Sandbox Note: This part will be specific to each thread, so I've provided each version

### Robbers

For the last few years, you and your team of robbers have been planning to rob the International Code Golf Museum (ICGM) of some of it's most prized possessions (rumour has it, they have a rare transcript of Dennis being outgolfed!). Tonight is the night that your plans will be put into action... tonight you will walk away with glorious riches and legendary artifiacts of code golf history.

That is, if you can actually manage to get in, loot the rooms and get back out without getting caught.

### Cops

Tonight has been a quiet night at the Internation Code Golf Museum (ICGM)...too quiet for one's liking.

Of course, that's probably due to the fact that a bunch of robbers are rumoured to strike tonight and steal the most priceless artifacts from the ICGM's collections (we can't have them finding out the fact that Arnauld is a machine learning algorithm designed to answer challenges with Javascript, can we. [that's a joke we love you Arnauld!])

It's your job to stop the robbers and make sure that not a single thing leaves the museum.

## The Museum

Sandbox Note: This part is common to both cops and robbers

Here is a map of the museum:

You may be wondering "hang on, what are the dimensions of that map?". Well the answer is simple: you don't need to know.

Movement around the ICGM is analogous to a Henry Stickmin game: in each room, you can either move to an adjacent room, or perform an action in that room.

Robbers start at the Getaway Car and can enter through either Hallway A or Hallway B. They then move through Hallway A.A then Hallway A.B or move through Hallway B.A and then Hallway B.B. Either way, they end up at the exhibition room. From the exhibition room, robbers can move to any of the treasure rooms which is where all the valuables are stored. After that, they make their way back to the getaway car to, well, escape and get away.

Cops are randomly allocated a position at the start of the heist, and can move wherever they need to.

Sandbox Note: There's going to be a system where each room takes a certain number of "strides" to get from one end to another. This way, robbers with stolen items are "slowed down" a little and the cops have a bit of a chance to catch up

## Stealing Treasures

Sandbox Note: Robber specific

Once you reach a treasure room, you have the oppourtunity to get your hands on some of the finest works the ICGM has to offer. There will be a selection of items to choose from, each with different values. Sandbox Note: I might add a part where there is a limit on how much robbers can carry

However, items with more value impact how fast you can move through. Sandbox Note: Something relating to the stride system here.

But being the sneaky robbers you are, you have a few tricks up your sleeve(s). When moving through the ICGM, you can:

• Activate a trap that will slow down the cops (avaliable only once Sandbox Note: Subject to change)
• [Other things coming soon]

Once you get back to the getaway van, everything you have stolen is considered "safe" and counts towards your team's score. But once you're in the van, that's it... you can't go back for more.

Your team wins if you manage to steal items with a combined worth of insert value here

## Protecting the ICGM

Sandbox Note: Cop specific

In order to protect the artifacts of the ICGM from being stolen, you have a few abilities you can use against the robbers. You can:

• Apprehend a robber if they are in your proximity
• [Other things coming soon]

For your team to win, the total value of items stolen must not exceed insert value here

## The Catch

Sandbox Note: Common to both threads

The heist program will only be simulated once. That means that if your submission errors, you're out of the game for good. I mean, you don't see criminals replace the things they stole just to rerun the heist over and over to see how good they are ;P.

The heist will occur on insert date here. You can edit your submissions all you like until the time of running.

Coming soon.

# Feedback

• Seeing as how this is a first draft, there are bound to be flaws with things like the movement mechanics and cop/robber interactions. I'm more looking for first impressions and overall flaws in the challenge.
• I can't wait to see the valuables you place in the treasure room.
– RGS
Oct 13 '20 at 17:13

# Secret ">" Stacking Challenge: cheating code-golftetris

Sequel to Secret ">" Stacking Challenge: grading. You can skip the whole Background section if you already read the first one.

## Background

Tetris Grand Master 3 has a hidden grading system based on the shape of the stack at the end of the game, which is called Secret ">" Stacking Challenge. It consists of entirely filling the lowest rows except for the zigzag pattern which starts at the left bottom cell and spans the entire width:

#
.#########
#.########
##.#######
###.######
####.#####
#####.####
######.###
#######.##
########.#
#########.
########.#
#######.##
######.###
#####.####
####.#####
###.######
##.#######
#.########
.#########


The board is graded by how many lines follow this exact pattern from the bottom line. Note that the topmost hole in the pattern must be blocked by an extra piece of block. If you consider the #s and .s as the mandatory pattern (blanks can be anything), you can get the score of 19 only if the exact pattern above is matched from the bottom line. Analogously, if the board matches this pattern

   #
###.######
##.#######
#.########
.#########


but not

    #
####.#####
###.######
##.#######
#.########
.#########


then the score is 4.

For this challenge, consider a board of arbitrary size (other than 20 cells high and 10 cells wide). We can grade the board for the same pattern: for example, if the board has width 4, this is the pattern for score 3:

  #
##.#
#.##
.###


and this is the pattern for score 10:

   #
###.
##.#
#.##
.###
#.##
##.#
###.
##.#
#.##
.###


## Challenge

Given the board width and the desired score for Secret ">" Stacking Challenge, pick a sequence of tetrominoes and generate the sequence of moves that will achieve the score. The Tetris moves can be represented in any ways that clearly specify where each tetromino is placed in which orientation, optionally along with the board state after each placement.

For the Tetris movement rules, we use simple permissive rule as in this challenge: you can place a tetromino anywhere (even in closed rooms), as long as it doesn't float in the air or overlap with existing pieces. Therefore, if you plan to use coordinates, you need to specify both x and y coordinates (the y coordinate should be counted from the bottom, as the board can grow upwards without bound -- or count from the top by also outputting the field height).

You can assume the width is at least 5 and the score is nonzero. You should theoretically support arbitrarily high score. The generated sequence doesn't need to be minimal.

Standard rules apply. The shortest code in bytes wins.

## Output example

For board with 6 and score 2, one possible way is as follows: (As are the tetromino placed at each turn, # are existing pieces on the board, and . are empty cells)

......  ......  ......  ......  ......
A.....  #.....  #.....  .AA...  .##AA.
AA....  ##AA..  ####AA  #.AA..  #.##AA
.A....  .#AA..  .###AA  .#####  .#####


The above is a valid output format (you can choose any distinct chars/values in place of .A#). The following is also valid (although it is less obvious, it is indeed an unambiguous description of tetromino placements):

......  ......  ......  ......  ......
A.....  ......  ......  .AA...  ...AA.
AA....  ..AA..  ....AA  ..AA..  ....AA
.A....  ..AA..  ....AA  ......  ......


And this: (tetromino code, rotation, x and y coordinates from bottom left, 0 indexed)

S 1 0 0
O 0 2 0
O 0 4 0
Z 0 1 1
Z 0 3 1


And anything in between (e.g. showing tetrominos as a canonicalized matrix instead of a code). If in doubt, ask in comments.

# Calculate the inverse of a matrix

• n , nor n2, will never exceed the maximum value... and The elements of M−1 will never exceed... Even under those assumptions, intermediate computations can exceed the maximum. So maybe it's better to say something more general, maybe along the lines of this Oct 2 '20 at 23:18
• Also, matrix inversion notoriously causes floating-point error build-up when naïve algorithms (i.e. algorithms not specifically designed to avoid error accumulation) are used. This results in relatively large errors in the output. And those algorithms are likely to be the ones used in a golfing challenge (if the language doesn't have a builtin). So maybe you should add a note saying that it is sufficient if the algorithm works when arbitrary precision is assumed Oct 2 '20 at 23:18
• @LuisMendo I've edited in "It is acceptable if your program fails for some inputs due to floating point issues, so long as the underlying algorithm or method works for arbitrary matrices." which I think should cover both of those points Oct 4 '20 at 20:58

# Can I print my picture on {A,B,C}{0-10} paper?

The task is to find the smallest paper size on which it is possible to print a picture of the dimensions given in milimetres. The image will be printed without margins.

# Input:

Two numbers (bigger than zero) and a letter a, b, or c, for example:

290
200
A


# Output:

Paper size, for example:

A4

Another examples:

218,297,a      A3
1,1,c          C10
9999,9999,c    ??? (error)
74,52,A        A8
31,44,B        B10
26,1100,A       A0


• Upper- and lowercase variants of letters "A", "B" and "C" are allowed on input and on output.
• The image can be printed vertically or horizontally.
• The values can be passed as a parameter or entered by the user.
• Width and height of picture will be always > 0, and letters will be always 'a', 'b', or 'c'. You don't need to validate them.
• You need to handle paper sizes A0 - A10, B0 - B10 and C0 - C10. If the image is too large, you can throw an exception, print an error or whatever you want, as long as it is clearly different from valid result and the application will not hung up.

Paper sizes, please ignore inch values (source: Wikipedia) :

This is - fewest bytes wins.

• Perhaps you should inline what sizes the An, Bn and Cn papers are. Also, some more test cases (~5-10) would be nice for the final version. Oct 22 '20 at 0:03

# Brainfuck arbitrary precision multiplication

## Goal

The goal is to multiply two numbers in the shortest amount of cycle.

## The Input

The input is two numbers written decimally separated by a space. The number is NOT restricted by the size of the integer inside of the cells. The program must accept arbitrary sized integer

## The Output

The output is a single integer written decimally.

## Brainfuck variants used

Since the flavors of the Brainfuck is important in this challenge, you are required to use this flavor

### Memory

The memory is an array of cells, unbounded to the left and the right, with 8-bit integer as the contents.

### Input/Output

The input and the output uses ASCII symbol mapping. EOF is interpreted as \0 char.

### Looping

The [ means that "check the current cell. If it's zero, jump to the instruction after the matching ]" The ] means that "jump to the matching [." no cycle is taken in this instruction.

### Cycle

Every instruction takes a single cycle every time it's executed except for ]. ] is a free instruction.

### Reference implementation

For the reference implementation, use copy.sh with this option:

1. Cell size (Bits): 8
2. Dynamic (infinite) Memory: yes
3. End of input: char: \0
4. Count instructions

## Scoring

The winner is the program that is: (later number is for tiebreaker)

1. The program with lowest computational complexity (counted by using cycle metric as explained above, and in x where x is the length of the largest input in base 10) is the winner.
2. The program with lowest space complexity (counted by finding the rightmost cell reached by program)
3. The program that takes the least cycle to execute 1234567890*987654321
4. The program that takes the least memory to execute 1234567890*987654321
5. The shortest code

Computational complexity is determined in terms of the number of digits each arguments have.

• fastest-code is the winning criterion tag, so you don't need code-challenge. To lower the obstacle before starting to work on the challenge, I suggest to use the copy.sh online interpreter as the standard. (I think it satisfies the cycle count rule?) Sep 3 '20 at 7:02
• @Bubbler Actually, I designed my challenge with copy.sh as the reference. The only difference is that the memory tape is not unbounded to the left. Sep 3 '20 at 7:25
• You can set "memory overflow behavior" to "abort" for that. Sep 3 '20 at 7:39
• @Bubbler It's not possible to have both infinite memory and abort as memory overflow behavior. I changed the challenge so that the memory is also unbounded to the left. Sep 3 '20 at 7:42
• Can we make assumptions such as 'the number of digits in the numbers is less than X' or 'the number of digits in the number of digits in the numbers is less than X'? Sep 5 '20 at 14:32
• @thedefault. No. The program must handle arbitrary sized number. Sep 5 '20 at 15:57
• @Xwtek So it would be impossible to answer it in a flavor with 30000 cells of one byte each? I think the limit on the size of each number should be half of the number of cells (and thus infinite for flavors with infinite cells) Sep 9 '20 at 23:30
• @RedwolfPrograms Yes, it's impossible. Maybe I'll make a challenge to minimize memory use, but for now, you have to use infinite flavors. "I think the limit on the size of each number should be half of the number of cells" Not necessarily, I bet you need much more memory than that. Sep 11 '20 at 2:20
• "Given a list of strings of two-letter abbreviations (see below) for US states, determine whether they are connected." So can we expect a python list? Or should we expect a comma separated list? I think you should clarify the input. Giving the input as a python list gives python (and any other languages that have the same syntax) an advantage over, for example bash. Nov 8 '20 at 14:27
• What should the output be? Is it <input> is not connected. or <input> is connected.? Or can it just be true/false? Nov 8 '20 at 14:48
• @nthnchu I think they mean a list of strings, whatever that is in your language. I agree with using truthy/falsy for connected/not connected. Nov 8 '20 at 15:13
• @Keba Are we required to compress the list, or take it as an input? If we need to compress it, add kolmogorov-complexity. Nov 8 '20 at 15:14
• Thanks for your replies. My idea was that the exact input/output format should not matter to much. Accepting a list of strings is fine but I guess just a long string such as "AK WA OR" or even "AKWAOR" is also fine – whatever works well in your language. And output should be truthy/falsy, yes. Thought that was covered by standard I/O rules but tried to clarify it.
– Keba
Nov 8 '20 at 17:20
• @Razetime: Sorry, I am not sure whether I can follow. The code should output a truthy/falsey value for a given list of strings.
– Keba
Nov 8 '20 at 17:22
• @Keba They are asking whether the list of states is provided as input or if we must keep it in our code. If we should keep it in the code, you should add the kolmogorov-complexity tag. Nov 8 '20 at 22:20
• @nthnchu The input should consist of a list of strings such as ["AK", "WA"] (or something equivalent) and nothing more. The list of neighboring states, however, is not included in the input. Still, the output is not constant and hence that tag does not apply?
– Keba
Nov 8 '20 at 22:49
• @Keba It still has the large constant string of the neighboring states. Look at wikipedia and kolmogorov-complexity's info page for more info. Nov 8 '20 at 22:57
• Even though the output isn't constant is still relates to it. Nov 8 '20 at 23:04
• From that page: ”a post should be tagged kolmogorov-complexity iff the bulk of the challenge is to produce a string or a subset of a dataset which is given in the question.“ I do not thing that the bulk of the challenge is to produce that string. First, that string does not need to appear anywhere and second, the main problem should be to decide whether the corresponding graph is connected or not.
– Keba
Nov 8 '20 at 23:17
• And this question looks very different from the ones posted with that tag.
– Keba
Nov 8 '20 at 23:18
• Yeah you're kind of right. Just wondering, is ["AK"] considered connected? Nov 8 '20 at 23:41
• It is, I add it to the list of test cases.
– Keba
Nov 8 '20 at 23:50
• My case for the tag is that compressing the string is one of the only ways to win; I just completed the challenge in less bytes than the string in python because I compressed it. Nov 9 '20 at 0:01

# Is my triangle on the lattice? decision-problemcode-golfgeometry

Posted on the main site.

# Is it a Lobster Number?

## Posted

• I like the idea, but the fact that prime numbers don’t count seems a bit arbitrary.
– rues
Jan 23 at 21:11
• It definitely is a bit arbitrary, but I felt that it made sense as an edge case, since while it is a prime factor, it is already the number itself, and there are no other factors. The related (but not identical) OEIS sequence also seems to have made this distinction, so it seemed reasonable. It also has the simple explanation that lobsters are not needed to get the result for the prime numbers, and shouldn't be overly difficult to work around in golfing, as all you need is a single comparison (is there only one factor, or is the first factor equal to the number). Jan 23 at 21:35
• Yeah, that makes sense, and it's not a deal-breaker for me, it would just make writing an answer slightly more annoying :)
– rues
Jan 23 at 21:36
• @user - Posted since there seems to be no general problems with the question. Jan 24 at 15:04
• Well, I don't see any problems with it myself, but it would have been good to let it sit a few more days to get more feedback
– rues
Jan 24 at 15:07
• @user - I'll keep that in mind, this is my first question here, this site is quite a bit different from the other StackExchange sites. Jan 24 at 15:13

## BF memory layout optimizer (posted).

See the notes in the revision history.

[please review other sandbox posts]

• (upvote this comment if you want a variant with [fastest-code] instead.) Jan 31 at 2:11
• (upvote this comment if you want both a [code-golf] version and a [fastest-code] version.) Jan 31 at 2:11
• (upvote this comment if you want only a [code-golf] version.) Jan 31 at 2:11

Note:

• as you can probably tell, this is a simpler variation; however the poster of the other sandbox post has not been visiting the site for a while.
• because of the current situation, I recently (after the 2 upvotes is made) changed the winning criteria. Is there any problem with the description?
• What info should be included in the header?...

## Efficient table-lookup computation

• (ah, right. The sandbox is still as inactive as usual.) Jan 30 at 17:16
• Or it could be that the challenge is simply a little hard to read.) Jan 31 at 14:45
• If two answers have the same A+B, is the one with lower A or lower B considered better, or is there no tie-breaker? Feb 2 at 8:43
• @Dingus No, see the meta post about challenge having multiple winning conditions. codegolf.meta.stackexchange.com/questions/19041/… Feb 2 at 10:19
• There's no consensus on that meta question. I think you should state that if two answers have the same A+B then they score the same. Feb 2 at 10:37
• @Dingus +1 is not a consensus? Feb 2 at 10:42
• A post is considered as a consensus only if it's at >= +10 and upvotes >= 2*downvotes. So +5/-4 (at the time of writing) is closer to a controversy than a consensus. I guess many people have just feelings without enough justification about whether it's fine or not. I personally view it as a good challenge, so I upvoted it. Feb 8 at 23:20
• A minor nitpick: In your Python code, adding string to a number is invalid. I guess you meant to take the inputs as strings, e.g. m1 = "0" etc. Feb 8 at 23:21
• For the lack of sandbox feedback, I usually leave my challenges for at least a week, regularly asking for feedback in the chat (at least 2 or 3 times before moving to main). Feb 8 at 23:24
• @Bubbler Fixed. Feb 9 at 1:41
• to me it looks like the given method to generate a single score from A and B only adds confusion, especially the "combined score" part... what's the advantage compared to a simpler formula like A^2+B^2? (I would be in favour of just using A and B and letting all pareto-optimal solutions win, but I understand that at the moment there's not enough consensus for this)
– Leo
Feb 12 at 8:57
• @Leo Because it doesn't encourage these solutions, obviously? -- (I think that the solution with minimum A is pretty interesting, but I'd like to see a solution that generates reasonably-good parametrized solutions too.) Feb 12 at 9:53
• @Leo as explained in the linked meta post -- (did I explain it clearly enough?) that method would allow essentially the same "every solutions win" situation by letting every such "winning" solution improves the "combined" score. (and is the only one that I can come up with with that condition) Feb 12 at 9:54
• The problem with the combined score is that you have two scores again, the "normal" one and the "combined" one... Does only one or the other count for "victory"?
– Leo
Feb 12 at 10:29
• @Leo Obviously the combined one. Note that the criteria is chosen so that the combined score is always better than or equal to the normal score, and the combined score is always better than the previous combined score. Feb 12 at 10:32

# Generalise perfect numbers

• Better having a statement "in this challenge $\sigma^n(k)$ denotes...". It can be interpreted as $(\sigma(k))^n$ too. Feb 24 at 4:15
• Can answers assume that "the sequence is infinite"? Feb 24 at 4:16
• @user202729 Addressed both questions Feb 28 at 1:18

# Byte-sized Huffman Coding (WIP)

Huffman codings are a method to compress data with certain frequency properties, usually text. Normally, these operate on bits rather than bytes, but this challenge will instead operate on whole bytes instead. Since you wouldn't get any benefit otherwise, you can represent multiple consecutive characters with a sequence of one or more bytes, for instance '. ' (a period followed by a space) could be represented by byte 1, 'The' could be represented with byte 2, and 'Ishmael' could be represented by a 255 then a 7 (among many other sequence codings).

# Challenge

Create a program that compresses a plain-text version of a work of literature by returning a byte-wise Huffman coding table and a sequence of bytes that represents the text with that table.

## Rules and Assumptions

• You may assume that the text is written in English and uses only printable ASCII characters plus space, newline, and tab.
• It must be a proper Huffman coding; no mapping may be the prefix of another.
• Not all Huffman sequences need to be mapped to a particular character sequence; you could, for instance, not have 7 mapped to anything or not have 255, 39 mapped to something, but have every other 1 and 2 byte sequence mapped to something.
• The returned coding table must be able to encode every possible sequence of valid characters (as per the first assumption above). The simplest way to do this is to make sure that every individual character is mapped to a Huffman byte sequence.
• It can be possible to encode a body of text multiple ways using the returned encoding table. If both ca and at are mapped to byte sequences, cat could be encoded two ways. This is totally fine.
• Case must be preserved.
• Runs do not need to be deterministic, i.e. two runs of the same program with the same input could produce different Huffman tables and compressed output.
• Your program must return a result within a reasonable amount of time to be considered a valid solution. (If you want a hard limit, I'll say 5 minutes on a 2GHz Intel dual-core i5 with 16 GB RAM running Windows 10)

## Scoring

Your results will be run against a corpus of (TBD) 12 publicly available literary (and non-fiction) works. For each work of literature, your score will be the size, in bytes, of the compressed text, plus the total length of all text strings mapped to a byte sequence in the Huffman-coding table. Your overall score is the total score across all 12 works.

Lowest score wins.

### Literature list

• The King James Bible
• Hamlet by William Shakespeare
• Dracula by Bram Stoker
• Frankenstein by Mary Shelley
• On the Origin of Species by Charles Darwin
• Moby-Dick by Herman Melville
• Little Women by Louisa May Alcott
• Tom Sawyer by Mark Twain
• Pride and Prejudice by Jane Austen
• The Hound of the Baskervilles by Arthur Conan Doyle
• < Something that entered the public domain in 2019 because it was published in 1923 >
• < Something written in the last 20 years willingly released into the public domain or with a Creative Commons license that allows derivative works >

### Sandbox

At least one of the last two literary works should preferably be written by a female and/or non-white author to hopefully make writing styles diverse enough to make hard-coded Huffman tables ineffective. Each work should be comparable in length to the other works.

Links to these books (in plain text) would be appreciated. Substitution suggestions are welcome.

• If you want to vary writing styles, is there a non-fiction work that only contains the allowed characters? Sep 24 '19 at 23:21
• @trichoplax The most notable non-fiction book I can think of for that would be "On the Origin of Species" by Charles Darwin. I've also thought about throwing in the King James bible. Maybe I could bump up the total to 12 works. I also probably will drop War and Peace because the plain text version I found was machine converted and has issues. Sep 25 '19 at 15:21
• Regarding plain text books, have you checked Project Gutenberg? They have plaintext versions of many of their books. Sep 25 '19 at 18:35
• I have no idea how much sample text is a good amount for this challenge (number of books, length of books) but it's probably worth doing some kind of rough check that there is enough text to give variation between answers (no optimal solution) while still being little enough text that running in a reasonable time is realistic (doesn't require weeks of work before an answer is efficient enough to meet the time restriction). Maybe others can suggest good ways of approximating this? Sep 25 '19 at 20:16
• @trichoplax so basically you suggest sampling out, say, a few chapters instead of the whole book and then reducing the required runtime? I'm open to that, especially since the word count difference between Hamlet and the Bible is so big. The interesting thing about the Bible is that it has a ton of different authors, so it would almost be better to have the first chapter of each book instead of inserting, say, the entirety of Genesis. Sep 26 '19 at 4:41
• I couldn't guess at this point whether more text or less text would be better, and I don't have a way of estimating, just wondering if anyone else does. Sep 26 '19 at 7:32
• I see no problem with 5 minutes as a rough time limit. I'd lean towards a time limit that allows someone to write a quick answer and then improve on it gradually, to encourage more participants. How long that needs to be for the text you settle on I don't know. As long as you're confident an optimal solution can't be found, you could just time a naive approach and then choose a time that doesn't exclude that. Then you can get lots of early answers to get the competition going, but still have open ended improvement over the long term Sep 26 '19 at 7:38
• Not to say that this isn't an interesting challenge as it is but I do wonder if it wouldn't be more interesting without requiring that we use Huffman Encoding? Sep 27 '19 at 22:21

# What's the odd one out?

• Looks like this but a bit more interesting Mar 9 at 13:35
• Great challenge! Seems ready to be posted
– user100690
Mar 9 at 16:03

# Sociable sequences

• You probably need to describe what a proper divisor is. Separately, I'm not sure how great requiring infinite output is. I'd probably consider allowing another optional argument that limits how many sequences to output? Nov 12 '20 at 19:47
• @FryAmTheEggman I've updated the challenge to be closer to the normal [sequence] I/O rules Nov 26 '20 at 15:23
• So is 25 a 1-sociable number, since its divisors sum to 6, and then there's a cycle?
– rues
Jan 6 at 21:04
• @user No, $25$ is not a $1$-sociable number. "They are numbers whose proper divisor sums form cycles beginning and ending at the same number". $25$'s "cycle" is $25 \to 6 \to 6 \to \cdots$, which does not begin and end with the same numbers. I'll edit that in though Feb 22 at 17:50
• That "begin and end at the same number" is unclear. (it's a cycle, it neither begins nor ends) Perhaps "the initial number is inside the cycle". Feb 24 at 4:30
• Should that "given value" be interpreted as "value that will be given as input"? Feb 24 at 4:31
• That "does not diverge" sounds like a terribly hard conjecture... EDIT indeed it is. en.wikipedia.org/wiki/Catalan%E2%80%93Dickson_conjecture ("likely false"), and looks like that the sequence you came up with already have a name. Feb 24 at 4:32
• with that, it's problematic, you'd better specify that "If there are $n$ such numbers, the answer must provably finish printing them all without assuming the conjecture" -- although for most approaches it isn't that hard. Feb 24 at 4:38

# Write an interpreter generator

• Can an input contain one or more operations? like a + 1 - 4 * 8? Is input guaranteed to be valid? Mar 22 at 17:11
• @Wasif editing the answer
– user100690
Mar 22 at 17:12
• Another suggestion, you might make it a code-challenge, where the objective will be to generate as short output as possible (Which means to golf the generated interpreter as much as possible), where the original source code length will not matter, so people can work for their code more peacefully, But code-golf (original challenge) one is good too. Mar 22 at 17:14
• @Wasif hmm... not a bad idea at all. This challenge does seem more effective with the code-challenge tag, I will edit that into the answer.
– user100690
Mar 22 at 17:17
• @Wasif on second thought, I'm not really sure how you would actually test the length of the output... it would not be fair for one answerer to test with a 100-command language while another tests with a simple 1-command language. Thanks for the suggestion, but code-golf seems like the better option for now.
– user100690
Mar 22 at 17:19
• OK, also how would handle destructive input like a / 0, which attempts to divide the accumulator by 0, which would result into crashing the interpreter? Mar 22 at 17:22
• the input is valid doesn't seem like a good enough description, let me change that
– user100690
Mar 22 at 17:22
• "each command is one letter long." so, highest number of inputs are 26? Mar 22 at 17:33
• Can we pre-define the input array in the header section of Try It Online? Mar 22 at 17:35
• @Wasif yes, that can be inferred from that fact. Not sure what the header is, because I don't really use tio
– user100690
Mar 23 at 6:23
• Header section means the part where you write code, is not added to the byte count. Mar 23 at 6:52
• Let us continue this discussion in chat. (Automatically comments were moved there because lengthy conversations cannot be run in comments) Mar 23 at 6:52
• I've edited this to a stub now that it's been posted Mar 23 at 15:40
• @ChartZBelatedly thank you, forgot to do that
– user100690
Mar 23 at 16:05

# Keep PPCG running in Game of Life

• "Minimally destroying CGCC", "Keep PPCG running". Hmm, someone doesn't like the name change :). May I ask why you have this proposal too, though?
– rues
Mar 22 at 19:24
• @OriginalOriginalOriginalVI Because the two tasks are different, and I very much doubt answers to the two will be trivially similar Mar 22 at 19:38
• Right now you could just place a glider in the box and it would run forever. Maybe redefine 'fixed position'? Mar 23 at 8:16
• @Ausername "If any spaceships or patterns of infinite growth are generated, the board will never reach a "fixed state" and such cases are invalid submissions." A glider is a spaceship Mar 23 at 13:13
• Oh ok, didn't see that bit. Mar 23 at 19:37

## Limit of lists code-golf

You're given a never-ending sequences of lists, each of which appends some number of values to the end of the previous one. That is, each list is a prefix of the next.

3
3,1,4
3,1,4,1
3,1,4,1
3,1,4,1
3,1,4,1,5,9,2,6
...


While some steps may leave the list unchanged, its length grows unboundedly, giving an infinite list in the limit. Your goal is to output this infinite list.

Note that you can't know a-priori how many lists you must read to get, say, the 5th value in the infinite list, just that you'll eventually hit a list with 5 or more elements.

Input and output:

The list elements are digits 0-9. You may treat them as characters if you wish.

The input and output are both infinite lists. These can be represented in various ways, and may be different for the input and output.

• An infinite list or stream
• A stateful method or black-box function that produces a new value with each call
• Repeatedly reading from STDIN or writing to STDOUT, or file buffers or the like

A mapping from index to value isn't allowed for input or output. The output must be uniform, without chunks of digits grouped together.

Sandbox: Infinite list I/O is hard. Any suggestions?

• "Your goal is to output this infinite list" - can you clarify that it's by taking later elements from the lists that get later and later in the input list - I had to read this several times to understand that. Also, while I don't think there's any getting around the infinite input requirement, perhaps you could change the output to standard sequence rules to make it more flexible? Mar 27 at 18:04
• Is the first list guaranteed to be one element long? Or can it be any length? Can it (and any subsequent lists) be empty? Mar 27 at 18:09
• You should probably specify the intermediate output/s needs to be as soon as it's available from the input infinite lists, or at least in finite time, so noone can submit tail -1 Mar 27 at 18:21

# cadaddadadaddddaddddddr - linked list accessing

• Yay, a lisp challenge! Does each 'a' mean a car and each 'd' a cdr?
– rues
Mar 24 at 21:56
• @OriginalOriginalOriginalVI yeah, but I'm to lazy to have written a description in english so far. I guess I'll get to it eventually :P Mar 24 at 21:58
• Fortunately the common lisp hyperspec already describes the bulk of it well. Mar 24 at 22:22
• You might want to make the reference implementation a link to TIO (unless it doesn't work on TIO, of course)
– rues
Mar 24 at 22:23
• @OriginalOriginalOriginalVI Why? (I can't access TIO anyway) Mar 24 at 22:25
• Meh, I just don't like those code snippets, and it makes it easier to pull out of the question, modify the input, and stuff. These code snippets sometimes break for me. If you can't access TIO, though, it doesn't matter, of course.
– rues
Mar 24 at 22:26
• posted Mar 25 at 15:04
• I've edited this down to a stub now that it's been posted Mar 28 at 14:01
• It's unclear what kind of expressions we have to deal with, but it looks good, and interesting . E.g. do we have to handle expr |> function(with_one_parameter)? Mar 25 at 15:27
• @Wezl I was thinking the expression could be anything with printable non-whitespace ASCII but I realize now that could be problematic. I'll change it to be more restrictive. Mar 25 at 15:30
• @Wezl I specified that you may assume all functions will take only one parameter, so you do not have to handle expr |> function(with_one_parameter). Mar 25 at 15:39
• Why do some piped expressions have () and some do not? Are we meant to support both of these? Mar 26 at 4:45
• @Ausername It's to demonstrate that both are valid, but as I state in the rules you need only support one convention. Mar 26 at 13:17

# What's my TIO uniqueness?

• Oops, didn't read that
– rues
Mar 23 at 20:59
• Maybe you could say "undefined" instead of 0 in the sentence "The TIO uniqueness of a language for which this is impossible is 0". Also, do you want submissions to store all 680 language names in the code? Mar 24 at 23:58
• @Bubbler Changed to undefined, it fits better. Yes, submissions should have some way of storing the names, be it in an external file or in the program itself etc. Mar 25 at 0:02
• @FryAmTheEggman Given that there are 680, I think it may be impractical to include the full list in the question, which is why I linked to this gist with all languages and their outputs Mar 25 at 0:20
• Ah, sorry, I missed the link. Mar 25 at 3:44
• "subsequence" tag? Why?
– tsh
Mar 29 at 8:58
• @tsh Because the TIO uniqueness depends on the shortest substring that isn't common with any other language Mar 29 at 18:49
• but substring is not subsequence
– tsh
Mar 30 at 0:32
• @tsh No, but [substring] isn't a tag, and [subsequence] is the best alternative Mar 30 at 21:37

# Rejecting invalid IPv4 addresses

• You should generally include the definition of things like this directly in the challenge rather than just linking to a Wikipedia article on it, so people don't have to go to an external site Apr 8 at 8:42
• I think it would be clearer if you used boolean true/false for "is this valid", rather than "invalid" vs "valid". You might also want to change classification to decision-problem Apr 8 at 8:46
• @pxeger Thanks. Included both suggestions. Apr 8 at 15:55
• Is this means the input should always be a string / list of characters? May I take input as an array of 4 integers / a 32 bit unsigned integer / a built-in type for IP address (if there is one)?
– tsh
Apr 13 at 3:57

## Remove first encountered elements from a second list

• While this comes from an SE question, having two arrays seems a bit odd. A more standard task is simply to remove the first occurrence from the single array. (Though, it may be a duplicate then) Aug 17 '16 at 18:33
• @NathanMerrill I thought of limiting it to just one array, but in my opinion this way is more challenging. Aug 17 '16 at 19:01
• I agree, it does increase the difficulty of the challenge. Aug 17 '16 at 19:04
• The separator between the elements of the list needs to be distinct from the separator between lists -- it just means that the two output lists should be distinguishable (and not clumped into a single list, for example), right? The current wording doesn't make much sense if we can return two lists from a function. Apr 12 at 23:07
• @Bubbler, yes, that's right - I'll change it. Thanks. Apr 13 at 5:06
• I suggest using standard JSON in your examples, e.g. [2,3,3,5,5,4,3,7,1]
– Adám
Apr 13 at 6:58
• I think removing elements from L2 is unnecessary. The essence of the challenge is to remove elements from L1 based on the first occurrence of each unique element in L2.
– Adám
Apr 13 at 7:00
• Thanks @Adám for the comments. You're right - I edited accordingly. Apr 13 at 19:41

# Interpret control characters like a terminal

• As "your program does not need to interpret backslash escape sequences - the input will contain the literal control codes themselves.", I'd suggest actually including the characters in the test cases, or at least including a TIO link (or pastebin etc.) with the literal characters Apr 12 at 13:14
• @cairdcoinheringaahing \r isn't really usable on the web because it will be converted to a newline, and most languages have their own literal syntax for entering those characters anyway, so I think it wouldn't really help Apr 12 at 13:18
• Suggest a aaaaaaa\b\b\b\t case, do TAB fill them space?
– l4m2
Apr 15 at 4:17
• @l4m2 thanks - that helped me discover some subtle bugs in my reference implementation too Apr 15 at 9:01

# A Self-Referential Sentence

## The Story

One day, you decide that you want a sentence that tells you where in the sentence the letter T occurs (excluding whitespace and punctuation). Out of curiosity, you try to make one. Messing around a little you get

T is the first, fourth, eleventh, sixteenth, twenty-fourth, ....

Oh dear, this sentence appears to run forever. But you now think you have an interesting number sequence, so you slap it into the OEIS search bar and lo and behold you find sequence A005224, Aronson's sequence. And better yet, an interesting code-golf problem that no appears to have posed before!

## The Task

Your task is to write a program that takes in a single positive integer, n, as input and gives the position of the n-th "t" in the above sentence (indexing begins at 1 for the sake of this problem). For example, an input of 1 should return 1, while 2 should return 4. The input number will not exceed 4 decimal digits in length (i.e. the maximum input is 9999)

As always, the shortest code in bytes wins, and standard loopholes apply.

## The Meta

Ok, so I have a couple of questions, since this my first sandbox post.

1. What can I do to flesh out this problem? This seems short, especially for a CGSE prompt. Should I somehow flesh out the heading fluff? Or should I add something more to the task itself?
2. I was pretty thorough in my search of the sandbox and main site for similar problems, but I could always have missed something, so please let me know if this is a duplicate.
3. Is the 4 digit input limit reasonable? Should I raise it or lower it? Remove it entirely? Since I'm not providing a file with ordinal strings, it seems like having a restriction on the size of the input is quite important.
4. Finally, please let me know if there's any other glaring problems in this prompt, this is both a first draft, and my first attempt at a code-golf prompt (since high-school).
• Nice first challenge! I'd suggest following the standard sequence I/O rules and allowing programs to output either the first $n$, the $n$th term or all terms. Additionally, forcing 1-indexing (for the sequence) doesn't improve the challenge any more than allowing either 0 or 1 indexing. I cannot find any challenge that could be a dupe through some searching, so this looks to be 100% original. Finally, I'd recommend including either test cases or the first 10 or so terms in the challenge body Apr 24 at 16:21
• @cairdcoinheringaahing 1-indexing is fundamental to the recursive definition of this sequence, as “T is the zeroth, third, tenth, twelfth, seventeenth, twentieth, …” is quite a different sequence (not just off by one). Apr 25 at 18:25
• @AndersKaseorg I meant 0 indexing in the input, not in the position of the T (e.g. n = 0 would output 1, n = 1 would output 4, etc.) Apr 25 at 18:27

# diddly darn posted

• Tag chess? Apr 12 at 6:43
• My god, this is amazing. I can't wait to see the full version! Apr 12 at 9:48
• It's not clear to me what should be output in the non-deterministic cases. Do we have to output all possibilites?
– kops
Apr 22 at 23:35
• Addtionally, what do you want the result of this to be: ,v, \n >,A \n ^<B (pastebin; is multline code possible in a comment?) Rules as written I think it's a tie since the center cell is reached twice but it's not clear this is desirable.
– kops
Apr 22 at 23:37
• @kops it's okay for the board to result in a tie. Apr 22 at 23:43
• And the point is to output the result of the board, which may not be deterministic Apr 22 at 23:43
• So each possibility has to be output with the correct probability in the non-deterministic cases? And for the specific board in my second comment, it's very much morally an A victory, not a tie, but the technicality of passing through the same cell going in different directions makes it a tie in these rules which I find a bit weird.
– kops
Apr 22 at 23:46
• @kops no it is not the probability but the result of running it once. That result may vary. And even though that may seem like it should be a win for A, it could be the result of some clever play from B to trick A into thinking they've won Apr 22 at 23:50
• @Lyxal I didn't mean to say the probability itself should be output, but that for each possibility, the probability of that possibility being output has to be correct?
– kops
Apr 22 at 23:56
• @kops you only ever output one result - the winner of the game when evaluated. Because there are commands that change the direction, it can be impossible to 100% tell who wins. I was simply pointing out that there is more than one possible output for such boards. Apr 22 at 23:59