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 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.


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!


3422 Answers 3422

28 29
31 32

Is this number part of a Collatz prime sequence?

A fast step of the Collatz sequence is defined as

$$s_\mathrm{Collatz}:\mathbb{N}_\mathrm{odd}\to\mathbb{N},\quad n \mapsto \frac{3\cdot n+1}{2}.$$

Given an odd positive integer \$n\in\mathbb{N}_\mathrm{odd}\$, your task is to decide whether or not both \$n\$ and \$s_\mathrm{Collatz}(n)\$ are prime numbers.

Your program should output two distinct and unique values to represent truthiness and falseness, whereby falseness may also be represented by signalling an error.

Since \$n+\frac12(n+1)=n+\lceil\frac n2\rceil\$ for odd \$n\$, the sorted sequence of all numbers which result in truthiness in the above sense is equal to the tail of A158709.

Test cases

-8 -> -        ; undefined behavior
1  -> false    ; (3*1 +1)/2 = 4 is not prime
3  -> true     ; (3*3 +1)/2 = 5 is prime
5  -> false    ; (3*5 +1)/2 = 8 is not prime
7  -> true     ; (3*7 +1)/2 = 11 is prime
11 -> true     ; (3*11+1)/2 = 17 is prime
15 -> false    ; 15 is not prime
91 -> false    ; (3*91+1)/2 = 137 is prime, yet 91 is not
97 -> false    ; (3*97+1)/2 = 146 is not prime
  • \$\begingroup\$ What is the definition defining? Is \$p\$ a Collatz prime iff it's a prime and \$\frac{3p+1}2\$ is a prime? If \$p\$ a Collatz prime iff it's a prime and \$\frac{2p-1}3\$ is a prime? Or is a prime Collatz if either of those conditions hold? \$\endgroup\$ – Peter Taylor Sep 13 '19 at 10:56
  • \$\begingroup\$ @PeterTaylor are they not the same thing? \$\endgroup\$ – Pureferret Sep 13 '19 at 11:09
  • \$\begingroup\$ Alternative phrasing: which of \$p_1\$ and \$p_2\$ are you calling a Collatz prime? \$\endgroup\$ – Peter Taylor Sep 13 '19 at 11:11
  • \$\begingroup\$ @PeterTaylor I mean \$p_1\$ \$\endgroup\$ – Pureferret Sep 13 '19 at 11:22
  • \$\begingroup\$ So neither the input prime nor the Collatz prime need to be prime? \$\endgroup\$ – Jonathan Frech Sep 17 '19 at 7:56
  • \$\begingroup\$ "Input MAY be non-prime, in which case the output is always false (see 15)" -- did you specifically define this behavior or why is the above the case? \$\endgroup\$ – Jonathan Frech Sep 17 '19 at 8:00
  • \$\begingroup\$ @JonathanFrech I'm (personally) only interested in going from prime to prime, but I wanted to define the haviour for non-prime input. When I say 'may be non-prime' I mean, it should be able to handle it, it's an allowed/expected input but not one that gives a True output. \$\endgroup\$ – Pureferret Sep 17 '19 at 10:55
  • \$\begingroup\$ To me it feels like an unnatural extra constraint. \$\endgroup\$ – Jonathan Frech Sep 17 '19 at 12:54
  • \$\begingroup\$ @JonathanFrech it's just defining how to behave with certain inputs. \$\endgroup\$ – Pureferret Sep 18 '19 at 9:12
  • \$\begingroup\$ Citing Exceptional edge cases; such out-of-place definitions are generally frowned upon. \$\endgroup\$ – Jonathan Frech Sep 18 '19 at 9:56
  • \$\begingroup\$ @JonathanFrech I've changed the initial ask, and now it shouldn't be an exception \$\endgroup\$ – Pureferret Sep 18 '19 at 10:23
  • 1
    \$\begingroup\$ Cf. A158709. \$\endgroup\$ – Jonathan Frech Sep 18 '19 at 10:53
  • \$\begingroup\$ @JonathanFrech Do you think this is postable? \$\endgroup\$ – Pureferret Apr 27 at 1:17
  • 1
    \$\begingroup\$ cf. Things to avoid when writing challenges: Prime numbers. Unless there's a mathematical way to avoid any of the existing golfed primality test methods, a task involving primality test is not very interesting. \$\endgroup\$ – Bubbler Apr 28 at 6:09
  • 1
    \$\begingroup\$ @Pureferret I would not post it -- unless you can think of an interesting golf opportunity. You could, however, improve the problem at hand: there are many questions one can ask about Collatz trajectories. And if you ask for something along the lines of "decide if the given number is a multiple of the length of its Collatz trajectory", you get a less arbitrary connection of concepts whilst preserving the topic of divisibility. \$\endgroup\$ – Jonathan Frech Apr 28 at 14:28

Reject tab, return to linefeed

  • 3
    \$\begingroup\$ How exactly are "printing" and "returning" distinguished? Are submissions required to be a function that also outputs to STDOUT in addition to returning a value from the function? Also, I would strongly advise against subjective criteria like having an error message related to tabs, unless they are purely for brownie points; point bonuses, especially ones that can't be judged objectively, are discouraged. \$\endgroup\$ – hyper-neutrino Mod Apr 17 at 17:26
  • \$\begingroup\$ Printing and returning can be considered identical to each other for the purposes of this challenge. \$\endgroup\$ – StackMeter Apr 17 at 18:58
  • \$\begingroup\$ I've edited this down to a stub now that it's been posted to save space \$\endgroup\$ – Dude coinheringaahing Apr 28 at 14:15

Solve the Alien Probe puzzle


  • 1
    \$\begingroup\$ The description was clear to me, without watching either video. Doesn't seem like the wording needs much improvement. \$\endgroup\$ – Redwolf Programs Apr 28 at 13:54

Drawing the Mathematics Stack Exchange logo (ASCII Logos 2)

The goal is to create the Mathematics Stack Exchange logo using ASCII characters.

It looks like this.

Mathematics Stack Exchange

The exact output should look like this:

                /=/ \=\
             /=/       \=\
            |             |
            |\=\       /=/|
            |   \=\ /=/   |
            |      |      |
       _    |      |      |    _
    /=/ \=\  \=\   |   /=/  /=/ \=\
 /=/       \=\  \=\|/=/  /=/       \=\
|             |    |    |             |
|\=\       /=/|         |\=\       /=/|
|   \=\ /=/   |         |   \=\ /=/   |
|      |      |         |      |      |
|      |      |         |      |      |
 \=\   |   /=/           \=\   |   /=/
    \=\|/=/                 \=\|/=/
 /=/   |   \=\           /=/   |   \=\
|             |         |             |
|\=\       /=/|         |\=\       /=/|
|   \=\ /=/   |         |   \=\ /=/   |
|      |      |         |      |      |
|      |      |    _    |      |      |
 \=\   |   /=/  /=/ \=\  \=\   |   /=/
    \=\|/=/  /=/       \=\  \=\|/=/
       |    |             |    |
            |\=\       /=/|
            |   \=\ /=/   |
            |      |      |
            |      |      |
             \=\   |   /=/

You must make sure the cubes should be symmetrical just like the original logo, and must make space to create a 6-point star in the middle. Yes, the output looks messed up, but it's all I can do to really make a shape.

Remember, 6 boxes aligned as a hexagon.

Can you draw it with the least bytes possible?

Tags: ,

Any suggestions?

If this question reaches a score of 4, it will be posted on 12:00 PM UTC, 1 month after Stack Overflow's.


KoTH - JS WarBots

Based on King of the Hill: Robot Battle

Rules and Instructions

Program a bot as an object in JavaScript. The functions that can be called are:

  • left(name) Moves left 1 unit (x position -1)
  • right(name) Moves right 1 unit (x position +1)
  • up(name) Moves up 1 unit (y position -1)
  • down(name) Moves down 1 unit (y position +1)
  • forward(name) Moves forward 1 unit (z position -1)
  • backward(name) Moves backward 1 unit (z position +1)
  • mine(name) Places a mine at current position (bots can be blown up by their own mines, with a 1 tick delay before priming)
  • scan(name) Scans for mines and players in adjacent cells (cells a bot can move to). Returns an array [left, right, up, down, forward, backward]. Each element can be 0 (nothing there) 1 (mine) 2 (other bot).
  • pass(name) Do nothing

name is the name of the bot. Any bot whose action's name is not the name of the bot will be disqualified (if your bot is called "Foo" you must have pass("Foo") and not pass("foo") or pass("Bar"))

The 3D arena is a 1000x1000x1000 cube, centered at (0,0,0). Each turn of a game, every bot must select one of the above actions to perform. Every 10 turns (subject to change) minesweeping will occur, removing 50% of the existing mines to reduce camping.

A bot is eliminated from a game if:

  • It is in the same position as a mine.
  • It exits the game area.
  • It collides with another bot (both bots are eliminated).
  • The cell that it occupies does not have integer coordinates ((0,0,0) is valid, (0.5,0,0) is not)


After 1000 turns, if a bot is not eliminated, it receives Math.floor(1000/bots) points, where bots is the number of bots left. If a bot eliminates another bot by mining it, it receives 100 points. If there are no bots left after 1000 turns, nobody gets any points.

Formatting your bots

Here are examples of bots (These will be playing too!):


var RandomBot = { // Same name as the name property
    name: "RandomBot",

    x: Math.floor(Math.random() * 1000 - 500), // If unspecified, defaults to 0.
    y: Math.floor(Math.random() * 1000 - 500), // If unspecified, defaults to 0.
    z: Math.floor(Math.random() * 1000 - 500), // If unspecified, defaults to 0.

    turn: () => {
        var action = Math.ceil(Math.random() * 9)
        if (action == 1) { left("RandomBot") }
        else if (action == 2) { right("RandomBot") }
        else if (action == 3) { up("RandomBot") }
        else if (action == 4) { down("RandomBot") }
        else if (action == 5) { forward("RandomBot") }
        else if (action == 6) { backward("RandomBot") }
        else if (action == 7) { mine("RandomBot") }
        else if (action == 8) { scan("RandomBot") }
        else { pass("RandomBot") }

May the best bot win!

The challenge will be posted later when it receives sufficient votes and the controller is developed. If there are any questions regarding this challenge, please comment about it.

  • 2
    \$\begingroup\$ Is there anything preventing a bot from placing six mines around itself, then never moving? \$\endgroup\$ – Redwolf Programs Apr 30 at 16:03
  • \$\begingroup\$ @RedwolfPrograms Do you have any suggestions on preventing that? \$\endgroup\$ – fasterthanlight Apr 30 at 16:33
  • \$\begingroup\$ @RedwolfPrograms I've created another rule regarding camping. \$\endgroup\$ – fasterthanlight Apr 30 at 16:44
  • 1
    \$\begingroup\$ I think the problem is less camping itself, and more just that I don't think there's very much strategy possible. The current restriction is trivially beatable by just enclosing yourself in a 1x1x2 space and moving back and forth. \$\endgroup\$ – Redwolf Programs Apr 30 at 17:41
  • \$\begingroup\$ @RedwolfPrograms Maybe there should be a limit on how many mines you can place? \$\endgroup\$ – fasterthanlight Apr 30 at 17:57
  • 1
    \$\begingroup\$ That could lead to some interesting gameplay, but might actually make the camping problrm worse because it takes very few mines to do. I'd recommend writing a few rough bots on a sheet of paper, and running some games in your head, then seeing what makes it fun and what could use some improvement. I think there needs to be more room to do interesting things; currently there's not really much strategy beyond "don't run into mines". \$\endgroup\$ – Redwolf Programs Apr 30 at 18:20
  • 1
    \$\begingroup\$ @RedwolfPrograms I came up with an idea regarding the issue that you brought up. There could be a minesweeper that runs every 10 or so turns where 50% of mines are removed. \$\endgroup\$ – fasterthanlight Apr 30 at 19:15
  • 1
    \$\begingroup\$ I think the idea behind that is good, but it has the same problem of being easily fixable by campers, and also removing plenty of mines set up by other players. \$\endgroup\$ – Redwolf Programs Apr 30 at 20:39
  • 1
    \$\begingroup\$ @RedwolfPrograms I will consider new methods for the issues in the challenge. \$\endgroup\$ – fasterthanlight Apr 30 at 20:43
  • 1
    \$\begingroup\$ Should I move this to chat? \$\endgroup\$ – Redwolf Programs Apr 30 at 20:48
  • \$\begingroup\$ Sure, I moved it.. \$\endgroup\$ – fasterthanlight Apr 30 at 20:51
  • \$\begingroup\$ Just wondering, why use functions rather than return values? e.g. having the bot return "left" rather than call left("name"). Also, right now, this just seems like a 3D version of Robot Battle. The three-dimensionality might actually be a bad thing, though, since it makes it harder to trap bots using mines. Like Redwolf, I also encourage you to think about the rules and the strategy of the game to try to allow for more strategy, and to differentiate it from the challenge it's based off of. \$\endgroup\$ – Andrew Li May 4 at 2:37
  • \$\begingroup\$ Also, the camping can be reduced to scanning, going into a corner, placing three mines around you, and waiting. \$\endgroup\$ – StackMeter May 4 at 5:54

xkcd 2385

  • \$\begingroup\$ Now that this has been posted, it should probably be edited down and deleted. \$\endgroup\$ – Redwolf Programs May 3 at 1:38

How many Faro Shuffles for a cycle?

  • \$\begingroup\$ @Pureferret it has been! \$\endgroup\$ – pxeger May 3 at 13:27

Produce a range

Your task is to take a list of integers and find inputs to a Python range call to produce that list. That is, output three values (start, stop, step) so that range(start, stop, step) equals the given list.

You can assume that this is possible, which means that consecutive numbers in the list all have the same nonzero difference. Be careful that your code works for negative step sizes, as well as for empty or singleton inputs.

How range works

Python's built-in range produces a list* of equally-spaced numbers. Called as range(start, stop, step), it counts from the start value in increments of step like

[start, start + step, start + 2 * step, ...]

This list continues as long as the value is below stop given positive step, or above stop given negative step. If the start value already fails this test, an empty list is produced. Note that the stop value itself is never included in the list, giving a half-open interval.

range(0, 5, 1)   = [0, 1, 2, 3, 4]
range(0, -5, -1) = [0, -1, -2, -3, -4]
range(0, 2, -1)  = []
range(0, -2, 1)  = []
range(3, 4, 10)  = [3, 7]
range(3, 4, 11)  = [3, 7]
range(3, 4, 12)  = [3, 7, 11]
range(1, -2, 0)  = [1]

*In Python 3, it actually makes a range object, but we'll ignore that distinction.

Test cases

Note that there can be multiple valid inputs. Different stop values can cut off the result at the same point when the step is not ±1. A singleton or empty list can be produced in many ways.


  • \$\begingroup\$ Is this return {a[0], a[-1]+sign(a[1]-a[0]), a[1]-a[0]}? \$\endgroup\$ – the default. May 7 '20 at 15:43
  • \$\begingroup\$ @mypronounismonicareinstate I think that works for inputs with 2+ elements, though the empty list and singleton list also need to be handled. Is this too simple for a challenge? \$\endgroup\$ – xnor May 7 '20 at 19:32
  • \$\begingroup\$ (I just noticed this might work in languages with modular indexing) This is probably not too simple, and I like the idea, but it seems like most of the complexity here comes from these special cases. \$\endgroup\$ – the default. May 8 '20 at 2:46
  • \$\begingroup\$ Both Python 2 and Python 3 disagree with step=0 in range. range(1, -2, 0) would raise ValueError instead. Is this intend behavior in this challenge? Also, some languages (Matlab) would support range with floating point values, which some floating point errors may introduced. If you are not aiming to work on floating point errors, maybe add a integer tag. \$\endgroup\$ – tsh May 11 at 2:24

Are You My Mother? (very WIP)

(coincidentally, thought up around mother's day)

Oh no! All the newly-hatched ducklings have been mixed together, and they all look the same! The ducklings want to reunite with their own mother. But the mothers are also selfish and want as many ducklings as possible to go home with them. Since all the ducks look the same, the only way to tell who your mother is is by how she acts.

Your task is to write two bot algorithms: one for the mother duck and another for the ducklings.

The game

There are 5 ducklings for each mother duck

All submissions compete at the same time

Finding your mother

All of the ducklings line up in a 1-dimensional array in the pond, as do the mother ducks. They do not move around.

Each turn, each duck (either duckling or mother duck) can do one of the following actions:

  • Do nothing
  • Quack
  • Flutter its wings
  • Splash
  • Spin around
  • Shake its head
  • Put its head underwater
  • some other action

Each duckling will see the actions of each mother duck on each turn

After 20 turns, each duckling guesses which half of the mother duck array contains its mother (the upper half will get the mother duck in the middle when there are an odd number of mothers) and the process continues until each duckling has narrowed down to a single mother duck.

Duck(ling) limitations

Ducklings are young and have poor memory. They cannot remember how long they have been lined up and can only remember the last thing each mother duck did when deciding what action to take. The only working memory available to ducklings is a single integer between 0 and 15, inclusive.

Mother ducks have a bit more memory and can remember five integers between 0 and 15, inclusive.


Score 1 point for each of your ducklings that went home with its mother. Score 1 point for each duckling (regardless of whose it is) that went home with your mother duck.

  • 1
    \$\begingroup\$ Controller/submission language? (suggesting ecmascript if you haven't already written one) \$\endgroup\$ – Wezl May 7 at 23:19
  • \$\begingroup\$ @Wezl yeah, probably Javascript \$\endgroup\$ – Beefster May 10 at 15:53
  • 1
    \$\begingroup\$ The narrowing logic might be a bit unfair if, say, a duckling is unsure about two ducks, but they are in different halves, while another duckling is unsure about two ducks that are next to each other, so has much more time to decide. Plus, I think, 4 bits are very little storage; I'd allow at least 16 or 32 bits, which is a bit more but still limiting. \$\endgroup\$ – FZs May 18 at 16:05
  • \$\begingroup\$ @FZs I've considered the alternative of letting each duckling have 1-on-1 time with each mother duck, after which they can decide whether to go home with her or not. But they do not get a second chance. Not sure how to handle orphan ducklings there though. \$\endgroup\$ – Beefster May 18 at 16:19
  • \$\begingroup\$ And what if the ducklings had to return a mother index on the choice-time? Or if the ducklings had to return an array of mother indices, and on the subsequent turns, it'd only receive info about the ones it chose previously; finally, after n turns, they have to decide? These are just ideas, so you don't have to accept them as they are... \$\endgroup\$ – FZs May 18 at 17:38

CGCC sings a song together

  • \$\begingroup\$ Does "languages can only be used once" mean that each user can only use each language once, or each language can only be used at most one time in the entire challenge? i.e. if Alice uses Language A, can Bob use Language A in his answer? \$\endgroup\$ – Andrew Li May 10 at 3:22
  • \$\begingroup\$ @AndrewLi Each language may only be used on a single answer, regardless of who posts that answer \$\endgroup\$ – Dude coinheringaahing May 10 at 15:23
  • \$\begingroup\$ That makes sense, although 74 different languages sounds like a lot, so maybe it would be hard to find "good" languages near the end? I guess that's kind of the point though. I haven't really done any of these challenges so I don't know whether it would get hard to find a language near the end. \$\endgroup\$ – Andrew Li May 10 at 16:14

Modify to Increment: Javascript Edition (WIP)

Your task is to insert bytes/characters into the previous answer such that it outputs the next number in sequence, spelled out in English, as its return value. The first answer must output one.

Coding restrictions

Your answer's code must be atomically irreducible, meaning that if it is possible to remove some subset of lexical tokens from the code and have it produce the same output, then it is an invalid answer. Note that this is a distinctly different concept from removing characters from the code, as it treats identifiers, string literals, numbers, and operators as indivisible entities. For the purposes of this challenge, a comment is considered a lexical token, so commenting out parts of the previous answer is not allowed by virtue of this restriction.

Some tokens are not subject to this restriction:

  • The declaration keywords let, var, and const
  • Semicolons

Note that removing an operator between two tokens does not merge the tokens together into one token, so removing the + from 1+1 would result in 1 1 rather than 11

SANDBOX NOTE: I'm debating whether this should be tightened up to character-wise irreducibility, but I'm thinking that may be too restrictive and be too difficult to chain. On the other hand, quoting and escaping into an ever-growing .substring call seems to be a pretty easily-exploited loophole here. Perhaps a simple bandaid solution would be to ban more than 3 consecutive backslashes. Though perhaps this isn't a problem at all because answers will be trying to make it as hard as possible to chain.

Multi-digit spelling

For numbers above 20, the result should be spelled with multiple words. You may use spaces, hyphens, or underscores to separate words and this does not need to be consistent. (e.g. twenty three can be followed by twenty-four, and one hundred-thirty four is valid even though the spelling looks weird)

Should this challenge reach more than 100 answers, the expected format should be like one hundred thirty six rather than a hundred thirty six. You may optionally insert and between hundred and the next word.


Answer 1


Answer 2

_=>'twone'.substring(0, 3)

Answer 3

_=>'threewone'.substring(0, 3+2)

Answer 4

_=>'thfoureewone'.substring(02, 3+2+1)

Answer 5

_=>'thfoureewone'.replace('our', 'iv').substring(02, 3+2+1)

Rules and Scoring

  • You may not comment out part or all of the previous answer.
    • This includes pseudo-commenting such as enclosing code in strings, using the comma operator to no-op parts of the code, or wrapping blocks in if(0)
  • The case of the output does not matter (e.g. OnE is just as valid as oNE)
  • No third-party libraries or vendor-specific features are allowed. The only features allowed are those found in the ECMAScript standard library as of May 14, 2021.

The first answer to go 7 days (i.e. 168 hours) without a successor is the winner.

  • 1
    \$\begingroup\$ Looks like an interesting challenge idea! I think more formally defining what "pseudo-commenting" entails would help. For example, would [a, b][0] be invalid? What about 0 ? a : b? Also, I think you need to formalize what answers need to output; there are a couple different ways to spell english words. For example, is 147 "a hundred forty seven", "one hundred forty-seven", "one hundred and forty-seven"? etc. \$\endgroup\$ – hyper-neutrino Mod May 13 at 19:09
  • 1
    \$\begingroup\$ I think an "irreducible" requirement may help this question. How should multi-digit numbers be spelled? \$\endgroup\$ – Razetime May 14 at 11:54
  • \$\begingroup\$ @Razetime Could you elaborate on irreducibility? The only thing that comes to mind is to require that the solution will not output the correct number spelled out if some subset of characters are removed \$\endgroup\$ – Beefster May 14 at 15:17
  • \$\begingroup\$ yeah that is pretty much the exact definition. \$\endgroup\$ – Razetime May 14 at 15:19
  • \$\begingroup\$ I think strict irreducibility might make it too hard to chain, though it does trivially ban comments and pseudo-comments. \$\endgroup\$ – Beefster May 14 at 15:28
  • \$\begingroup\$ Try x=>'Xforty-two'.slice((function(){/* w h a t e v e r p r e v i o u s a n s w e r w i t h s p a c e s i n s t e r e d i n t o e v e r y 2 c h a r a c t e r s */}+'').length===155) \$\endgroup\$ – tsh May 17 at 7:30
  • \$\begingroup\$ @tsh maybe irreducibility isn't the right solution. Maybe the limitation needs to be a maximum number of characters added. \$\endgroup\$ – Beefster May 17 at 14:38
  • \$\begingroup\$ An alternative would be to make adding to the solution also be atomic, so 'one' cannot become 'twone' \$\endgroup\$ – Beefster May 17 at 14:47

Just Enough Ones

  • \$\begingroup\$ Looks good to me. Not a big fan of OEIS questions in general, but this is one of the better ones. \$\endgroup\$ – Don Thousand May 12 at 19:09

An approximation for factorials of reals

One approximation for factorial function of reals is the following function:

\$ F_0(x) = \lfloor x \rfloor ! { \lceil x \rceil } ^ { x - \lfloor x \rfloor } \$

The function is less inaccurate for higher values of \$ x \$, meaning that you can get better approximations using the following recursive definition:

\$ F_{n+1}(x-1)=\frac1x{F_n(x)} \$


\$ F_0(\frac72)=3!\times4^{1/2}=12 \$

\$ F_1(\frac52)=12/\frac72=\frac{24}7 \$

\$ F_2(\frac32)=\frac{24}7/\frac52=\frac{48}{35} \$

\$ F_3(\frac12)=\frac{48}{35}/\frac32=\frac{32}{35} \$

\$ F_4(-\frac12)=\frac{32}{35}/\frac12=\frac{64}{35} \$

Given non-negative integer \$n\$ and real (well, floating-point) \$x>-n\$, please calculate \$ F_n(x) \$.

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

Bonus brownie points for using your code to approximate \$ \lim_{n\to\infty}F_n(-\frac12) \$:

\$ F_1(-\frac12)=2 \$
\$ F_2(-\frac12)\approx1.89 \$
\$ F_3(-\frac12)\approx1.85 \$
\$ F_4(-\frac12)\approx1.83 \$
\$ F_5(-\frac12)\approx1.82 \$
\$ F_6(-\frac12)\approx1.81 \$
\$ F_7(-\frac12)\approx1.80 \$
\$ F_{10}(-\frac12)\approx1.79 \$
\$ F_{18}(-\frac12)\approx1.78 \$
\$ F_{88}(-\frac12)\approx1.77 \$


Optimal addition subtraction chain


  • \$\begingroup\$ Maybe consider order testcases simply from -4 to 16? (Without sorted on their output length) Also, maybe add some larger testcases (no need to list all possible solutions if there are too many. maybe you can show one possible solution, and answers need to having same length output) \$\endgroup\$ – tsh May 14 at 4:01
  • \$\begingroup\$ @tsh I can add slightly bigger test cases, but there are no known efficient algorithms to calculate it, so I'm bounded by about 100 \$\endgroup\$ – Command Master May 14 at 4:14
  • \$\begingroup\$ Okay, after larger testcases added, it is not equal to oeis.org/A056792 now. :) \$\endgroup\$ – tsh May 14 at 5:30
  • \$\begingroup\$ I believe that one can construct the answer without brute force search. So don't worry about it bound. It is up to you to decide if brute force search is allowed: If allowed, you may say "your program should be able to calculate answers for |n| < 100, and your algorithm should apply to any size n in theory"; If not, you could say "your program should be able to compute up to \$\pm 10^8\$ in reasonable time (not time out on tio for example)" in the question. \$\endgroup\$ – tsh May 14 at 5:45
  • \$\begingroup\$ @tsh from wikipedia: "the determination of a minimal addition-subtraction chain is a difficult problem for which no efficient algorithms are currently known" \$\endgroup\$ – Command Master May 14 at 5:47
  • \$\begingroup\$ If i understand that statement correctly, it means an efficient (or polynomial) algorithms in \$O(P(\log_2 n))\$ is not exist. But A solution works under, say, \$O(n)\$ or maybe \$O(n^2)\$ is still possible. \$\endgroup\$ – tsh May 14 at 5:58
  • \$\begingroup\$ Maybe, but I'm also not aware of any pseudopolynomial algorithm which solves it \$\endgroup\$ – Command Master May 14 at 6:01

Convert version string to pack_format


  • \$\begingroup\$ @Arnauld thanks, fixed \$\endgroup\$ – Command Master May 16 at 16:52

Telephone Cipher Encoder

The telephone cipher is a relatively basic cipher originating from the book The Terrible Two. The cipher is explained here as follows:

Does this keypad (below) look familiar? It does? Good. You’ve seen a phone before.


Now, in the telephone cipher, there are two numbers for each letter. The first digit corresponds to the number on a telephone, while the second digit corresponds to the position on the key.

For example:

21 = A

Why? Because A is located on the number 2 on the keypad and A is in the first position of that particular key.

22 = B

B is located on the 2 key and is in the second position of that key.

53 = L

L is located on the 5 key and is in the third position on that key.

Given an input string s, the program should output the string encoded with the telephone cipher.

s may contain single spaces between words, but will not have leading or trailing whitespace and will use only lowercase letters a-z. s will always have a length greater than or equal to 1.

The output should contain no whitespace and be properly encoded with the telephone cipher as described above.

As an example, the following input:

hello world

Should result in:


All usual loopholes are disallowed. This is code golf so shortest answer in bytes wins. May the odds be ever in your favor!

  • \$\begingroup\$ What about the evens? :P \$\endgroup\$ – Makonede May 18 at 21:15
  • 2
    \$\begingroup\$ Back to the days mobile didn't have a touch screen, I typed 4433555 555666 instead of 4232535363 for hello... \$\endgroup\$ – tsh May 19 at 2:58

Memory KoTH

Memory is a game where a bunch of pairs of identical cards are laid upside down, and you try to find pairs while only looking at two at a time.

In this KoTH, the way it will work is:

The game will be played on a 4096-item array, and the "cards" will be integers 0-2047.

Each bot takes its turn in order. It has access to the results of previous moves (Up to its previous turn), but their only storage is a single integer.


The controller will be written in Javascript.

The bot has a move function, which must return two integers: The positions of both its guesses, in the form [g1, g2], where both are integers between 0 and 4095, and must not be gone already (see below).

The bot has access to:

The most recent move of every bot, including itself, in the form of an array of [g1,g2,r1,r2], where r1 and r2 are the first and second values revealed. The first item of this will be your bot's most recent guess, and the rest will be the other bots. This will be the global variable prev, and is readonly.

A picture of the entire grid, as a 4096-item array, left to right and top to bottom, where 0 means gone and 1 means still there. This will be the global variable grid, and is readonly.

An array of values that are gone. This will be the global variable gone, and is readonly.

A single ArrayBuffer(50), a 50-bit set of raw binary data. See the docs for help on how to access this. This will be the property this.storage, and can be used to store data.

Writing to globals is banned.


The bots will take their turn in a predetermined randomised order.

If a bot's moves are two tiles with the same value, those tiles are removed from the game area, and that bot gets a point.

The game ends when all tiles are gone, and the bot with the most points wins. In case of a tie, the bot that is last in the randomised order wins.

Bots should be a Javascript object like:

  name: "A bot",
    // insert code here


Should I change storage limit or grid size?

  • \$\begingroup\$ Shouldn't the bot be able to see the values of gone items? \$\endgroup\$ – Command Master May 15 at 9:03
  • \$\begingroup\$ @CommandMaster True, will add. \$\endgroup\$ – emanresu A May 15 at 21:02
  • \$\begingroup\$ This doesn't really matter too much but if it were up to me, personally, I would change the storage limit from \$[1,2^{50}]\$ to \$[0,2^{50}-1]\$ so it's a bitstring of length 50 \$\endgroup\$ – hyper-neutrino Mod May 18 at 6:10
  • \$\begingroup\$ @hyper-neutrino Oh yes, that's what I intended. \$\endgroup\$ – emanresu A May 18 at 6:11
  • \$\begingroup\$ What if two bots tie? \$\endgroup\$ – StackMeter May 18 at 8:18
  • \$\begingroup\$ @StackMeter The one that is last in the randomised order wins. I'll add that. \$\endgroup\$ – emanresu A May 18 at 8:41
  • \$\begingroup\$ Wouldn't it be easier to use a Uint8Array rather than a Bigint? Purely for ease of use. \$\endgroup\$ – EnderShadow8 May 20 at 12:51
  • \$\begingroup\$ Also need to make it clear that writing to globals is banned, since that's a legal play right now \$\endgroup\$ – EnderShadow8 May 20 at 12:52
  • \$\begingroup\$ @EnderShadow8 Yes, writing to globals is banned. \$\endgroup\$ – emanresu A May 20 at 20:07
  • \$\begingroup\$ @EnderShadow8 How would I format this Uint8Array? Like how many items would it need to be restricted to? \$\endgroup\$ – emanresu A May 20 at 20:07
  • \$\begingroup\$ Up to you. It's just a more convenient method of storing data that's a certain size. \$\endgroup\$ – EnderShadow8 May 20 at 23:32
  • \$\begingroup\$ @EnderShadow8 Actually, I think a BigInt would be better for this challenge, as remembering a single tile is 23 bits, which would be confusing to fit into a Uint8Array. \$\endgroup\$ – emanresu A May 21 at 4:22
  • \$\begingroup\$ Just make it a plain ArrayBuffer and DataView. That's easier. I don't like using bit operations to extract data from integers. \$\endgroup\$ – EnderShadow8 May 21 at 4:42
  • \$\begingroup\$ @EnderShadow8 Ok. \$\endgroup\$ – emanresu A May 21 at 4:43
  • \$\begingroup\$ Are you considering giving the bot the result of its first guess before asking for its second? That would introduce a new layer to the strategy. Eg. explore on the first move and defend or attack on the second \$\endgroup\$ – EnderShadow8 May 21 at 5:04

Calculate \$ \lfloor n \log_2(n) \rfloor \$, exactly

Given an integer \$ 2 \le n \$, you need to calculate \$ \lfloor n \log_2(n) \rfloor \$, assuming all integers in your language are unbounded.

However, you may not ignore floating-point errors - for example, in python lambda n:int(n*math.log2(n)) is an invalid solution, because for example for n=10**15, int(n*math.log2(n)) is 49828921423310432, while the actual answer is 49828921423310435.


Test cases

2 -> 2
3 -> 4
4 -> 8
5 -> 11
6 -> 15
7 -> 19
8 -> 24
9 -> 28
10 -> 33
100 -> 664
1000 -> 9965
10000 -> 132877

10 brownie points for beating my 4 byte 05AB1E answer.

This is code golf, so the shortest answer wins. Good luck!


Symmetrical Triangles, posted

Meta Questions

  • Dupe?
  • More tags?
  • Should I allow/disallow more output formats?
  • Is "An equilateral triangle array" clear, or is there a way I can clarify it?
  • Especially relevant test cases?
  • Is there an easy way to take up less vertical space and still have a good amount of test cases?
  • \$\begingroup\$ This might not actually be more clear, but I think you can describe it as "output w lists of sizes [1, 2, ..., w] of 1/0s such that the total number of 1s is equal to n and each sublist is a palindrome". \$\endgroup\$ – hyper-neutrino Mod May 17 at 20:51
  • 1
    \$\begingroup\$ open-ended-function? \$\endgroup\$ – pxeger May 19 at 15:29
  • \$\begingroup\$ @hyper-neutrino it doesn't have to have each sublist a palindrome to be symmetrical \$\endgroup\$ – pxeger May 19 at 15:30
  • \$\begingroup\$ When I first read this, I thought that it only took one input n, and had to output a n-sized triangle with n 1s. Now I realise that the size is a different input, but I think it could be more interesting if n == w? \$\endgroup\$ – pxeger May 19 at 15:31
  • \$\begingroup\$ @pxeger I don't think that makes it more interesting, especially since it removes the point of the challenge (arranging the 1s), by allowing them to be always placed at the bottom of the triangle. \$\endgroup\$ – Wezl May 19 at 15:34
  • \$\begingroup\$ @Wzl ah, I didn't think of that \$\endgroup\$ – pxeger May 19 at 15:43

Prime Factorization - but on the exponents too

  • \$\begingroup\$ There seems to be a stray double-quote here: "2^(2^2^2)*(3^2). Should it be removed, or should there be a matching one at the end? \$\endgroup\$ – pxeger May 28 at 8:57
  • \$\begingroup\$ There shouldn't be in the output - it was just to show the string format \$\endgroup\$ – StackMeter May 28 at 8:58
  • \$\begingroup\$ I've edited this down to a stub now that it's been posted to save space \$\endgroup\$ – Dude coinheringaahing Jun 1 at 23:12

Create an ascii line given length

  • \$\begingroup\$ Can answers use vertical tabs instead of space padding? Also, I'd suggest saying that "\$n\$ will be a positive integer", as "integer" could be zero/negative \$\endgroup\$ – Dude coinheringaahing May 31 at 15:22
  • \$\begingroup\$ edited: hopefully makes sense \$\endgroup\$ – PyGamer0 May 31 at 15:37
  • \$\begingroup\$ We usually allow a wide range of I/O methods. It is recommended to rethink your I/O restriction (mainly because it will get downvotes if people think the I/O restriction is unnecessary). Also, is it allowed to print trailing spaces after the backslashes on each line, or a trailing newline after the last line? (I guess yes to the latter from your code, but it's always better to be explicit) \$\endgroup\$ – Bubbler Jun 1 at 4:34
  • 1
    \$\begingroup\$ Strictly speaking, that line you are drawing actually has a length of sqrt(32). I think you should make it clear that the length refers to the number of non-whitespace characters. Also, "use a constant in your code" seems to be frowned upon as a method of input. \$\endgroup\$ – Recursive Co. Jun 1 at 15:57
  • \$\begingroup\$ how is the length \$\sqrt{32}\$??? \$\endgroup\$ – PyGamer0 Jun 2 at 13:53
  • \$\begingroup\$ Good challenge as a new user. \$\endgroup\$ – qwr Jun 2 at 14:14
  • \$\begingroup\$ The challenge is easy enough that it should not be necessary for answers to explain themselves. Answerers will do this voluntarily if they think it is necessary. \$\endgroup\$ – qwr Jun 2 at 14:16
  • \$\begingroup\$ @MaanasB pythagorean theorem. sqrt(4^2 + 4^2) = sqrt(32) \$\endgroup\$ – Recursive Co. Jun 4 at 17:36

Shuffle a subsequence

Thanks to rak1507 for the suggestion

"Random" in this challenge always refers to "uniformly random" - of all possible choices, each has an equal chance of being chosen. Uniform shuffling means that every possible permutation of the array has an equal chance of being chosen.

Given an array consisting of positive digits (123456789), select a randomly chosen, non-empty subsequence from the array, shuffle the elements and reinsert them back into the array in the former indices, outputting the result

For example, take L = [5, 1, 2, 7, 4]. We choose a random non-empty sublist, e.g. [5, 2, 4]. These are at indices 1, 3, 5 (1-indexed). Next, we shuffle [5, 2, 4] to give e.g. [2, 5, 4]. We now reinsert these into the list, with 2 at index 1, 5 at index 3 and 4 at index 5 to give [2, 1, 5, 7, 4].

You may also take the input as an integer or a string, and output it as such, or you may mix and match types.

This is , so the shortest code in bytes wins



Combinatorial Pipes

You're a plumber working on a house, and there's some pipes that must be connected at odd angles. You have 8°, 11.25°, 22.5°, 45°, and 90° fittings at your disposal, and you want to use as few as possible to match the angle as closely as possible.


  • Match the desired angle as closely as possible, with as few fittings as possible. It can be over or under the desired angle.
  • Accuracy is more important than the number of fittings
  • In the case of two different sets of fittings with the same resulting angle, whichever has the fewest number of fittings should be selected.
  • If the two sets use different fittings, match the same angle, and have the same number of fittings, either may be chosen.


Your input is a random integer between (non-inclusive) 0 and 180, which represents the desired angle.


Your output should be an array where [0]-># of 8° fittings, [1]-># of 11.25° fittings, etc. If your language does not support arrays, you may output a comma separated list, where the first value represents the number of 8° fittings, and so on and so forth.

Test Cases

90° ->[0,0,0,0,1]
24°-> [3,0,0,0,0] ([0,0,1,0,0] uses less fittings, but is less accurate and therefore incorrect)
140°->"2,1,2,0,1" acceptable if language does not support arrays


Lowest byte count for each language wins a high five from me if we ever bump into each other (and the challenge).

Sandbox Questions

Howdy! I feel like this could be an interesting golf, but I'm unsure if the language is clear and concise enough to get the idea across.

  • \$\begingroup\$ Might be interesting to make it more general, give a list of possible pipe fittings as an argument? \$\endgroup\$ – rak1507 Jun 16 at 9:12

Decode USB packets


  • \$\begingroup\$ Does this not basically boil down to base-conversion? \$\endgroup\$ – pxeger Jun 9 at 12:59
  • \$\begingroup\$ @pxeger Base conversion will not work. For one base conversion generally ignores leading zeros, while 00010 /= 10, so its not a bijection. But even if you ignore that it's nowhere near monotonic. 11 > 100 but f(11) = 10 < 11 = f(100). \$\endgroup\$ – Wheat Wizard Mod Jun 9 at 13:25

Is there a left-right connection?


Given a square array of 0s and 1s, determine whether or not there exists a path of 1s connecting the leftmost and rightmost columns. A path can take steps of one unit up, down, left or right, but not diagonally. Every symbol on the path must be a 1, and it must start somewhere in the first column and end somewhere in the last column.

Shortest code in each language wins.


111   ->  True

110   ->  False

010   ->  False

0     ->  False

1     ->  True

01110 ->  True

01100 ->  False


The array may be represented in any reasonable form, such as a list of lists [[0,0,0],[1,1,1],[1,1,1]] or a string '000 111 111'. It can optionally be in transposed form (so that the roles of rows and columns are exchanged; equivalently the code can instead determine whether there is a top-bottom connection). Any two distinct symbols can be used in place of 0 and 1. The output can be in any truthy/falsy form.


Consecutive Distance Rating


  • \$\begingroup\$ This is really neat! Simple, elegant, but nontrivial. It'd be interesting to look at the sum and its growth given an (OEIS) sequence. \$\endgroup\$ – AviFS Jun 12 at 1:25

Title: Make a WebCrawler dictionary writer.

Notice: for any text within <> tags, please ignore any .'s. These are simply added so that they are treated as plain text, and not code.

Your goal is to make a program that acts as a regular WebCrawler, but also writes a dictionary using the websites.

You should have a list that is used to store words that have been added called Dictionary, and another list to store website urls called NextSites. You should also have a variable called sites, and a variable called input2. Any other variables or lists are optional.

It should ask for input upon running the program, allowing you to input a website url, from where it will start web crawling. How it asks for input does not matter, so long as you are able to use any website as input. It should then ask for input again, allowing you to chose how many sites it should continue for. The variable much should be set to this input.

Next, it should check the website for any text contained within <.p> tags. It should separate the text contained within the tag at any space contained in it. Next, it will compare each part that has been separated against the current text in the list used to store words. Any part that is not already in the list should be added to it.

After it has checked all of the text within the <.p> tags on the site, it should check for any <.a> tags on the site. Any urls that it finds in the 'href' part of the <.a> tag should be added to the website url list, so long as the url list is not larger than 99 urls. (This means that the maximum amount of urls that can be in the url list at any one time is 100).

When all of the <.a> tags on the site have been checked for urls, it should change the current site it is on to the first site in the list of urls. It should remove this url from the list, and move all other urls to the spot one less than them on the list, so that there is no empty space at the beginning. (eg: If you had 3 urls: 1. google.com , 2. wikipedia.com , 3.stackexchange.com, then after switching to google.com, the list would now be 1. wikipedia.com , 2.stackexchange.com .)

It should then repeat the above steps, and continue to do so until the variables sites and much are the same.

Next, it should should output the entire list of words that it found. The method used to output it does not matter, so long as all words in the list are outputted, with a new line for each word. E.g.: You may have found the words cat, mouse, and food. The output would look as follows:




Scoring: The scoring follows regular code golf rules. The person with the smallest program in bytes wins. However, in the case of a tie between two programs, the program that can webcrawl and write the dictionary starting on wikipedia.com for 100 sites fastest will win.

  • \$\begingroup\$ running this type of program even once would probably ends up throttling the sites and blocking access to the crawler. Is there a good way to prevent that? \$\endgroup\$ – Razetime Jun 17 at 2:32

Largest Number with No Repeating Digit Pairs

Inspired by the problem with the same name on Puzzling SE.

You are to find the largest number that only uses every digit pair once, in a given base. For example, in ternary we have 2212011002. We can do this greedily by simply starting with the largest number, and then adding the next largest number we can without repeating a pair, until we’ve gotten them all.

Challenge: Given a base from 1-10, output the largest number in that base with no repeating digit pairs.

As long as the digits make the number, you may output with anything, or nothing, in between. Whether regularly delimited, or irregularly gibberished.

You may also take input in any reasonable way. For example, you might take the base, the max digit, or an ordered list representing the digits available. For octal, this would mean 8, 7 or 76543210. If you feel like a challenge, you can take octal as input. I won't complain!

Note that it need only work for bases from 1-10. Invisible points for doing alphanumeric bases like hex, but not at all required.

This is , so least bytes per language wins.

Test Cases

Decimal: 10

Octal: 8

Quaternary: 4

Ternary: 3

Binary: 2

Unary: 1
  • 1
    \$\begingroup\$ I was gonna suggest this be able to support even higher bases, up to 36 maybe. Though that might give some languages that have built-in support for higher number bases an unfair advantage? Because other languages might have to add special support for A = 9 + 1 rather than just using the ASCII values. Maybe just base 26, and use only letters, no numbers? Not sure if it's worth bothering... (Ah, I see you added that note after I posted this comment) \$\endgroup\$ – Darrel Hoffman Jun 16 at 19:27
  • \$\begingroup\$ @DarrelHoffman I did add the edit to make the question as is clearer, but whether or not to include higher bases is precisely my one hang-up! That's what we're discussing in TNB. Come join us! \$\endgroup\$ – AviFS Jun 16 at 19:37

KotH - Floating Point Prisoners Dilemma

  • 1
    \$\begingroup\$ Looks intriguing! How does each round work though? So one is coop, zero is defect, but what do the floats in between do? Look forward to hearing! \$\endgroup\$ – AviFS Jun 11 at 2:33
  • \$\begingroup\$ It uses an equation from the wonderful fellows at math.stackexchange.com to determine what the in-between states should do. I'm bad at explaining, so the equations in my (probably horrible) code are in the play function of the controller class in controller.py \$\endgroup\$ – 4D4850 Jun 11 at 2:46
  • \$\begingroup\$ @4D4850 Would you link to the question if you have it still? \$\endgroup\$ – math Jun 11 at 17:46
  • 1
    \$\begingroup\$ It was actually closed, but here it is: math.stackexchange.com/questions/4152360/… \$\endgroup\$ – 4D4850 Jun 11 at 18:27
  • \$\begingroup\$ Understood it now, good challenge! \$\endgroup\$ – math Jun 12 at 8:39
  • \$\begingroup\$ Should I post the question? \$\endgroup\$ – 4D4850 Jun 17 at 19:08
  • \$\begingroup\$ Posted on Main: codegolf.stackexchange.com/questions/229926/… \$\endgroup\$ – 4D4850 Jun 17 at 22:14
  • \$\begingroup\$ I've edited this down to a stub now that it's been posted to save space \$\endgroup\$ – Dude coinheringaahing Jun 18 at 14:33
28 29
31 32

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .