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

Sandbox FAQ


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

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

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


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

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

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

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

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


Search the sandbox / Browse your pending proposals

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".

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

4730 Answers 4730

114 115
117 118

Building a Structured Outline

In a fictional document management system, input documents are structured with weighted headings. Lowest weighted headings are the most important.

This is a sample document:

H1 All About Birds

H2 Kinds of Birds

H3 The Finch

H3 The Swan

H2 Habitats

H3 Wetlands

From this document we would like to produce an outline using nested ordered lists in HTML, which would look like this when rendered:

  • All About Birds

    Kinds of Birds

    The Finch 
    The Swan 



Your code only needs to take the input list and transform it into the expected data structure, which will be printed and checked against the expected output.

  • \$\begingroup\$ What's the winning criteria? (did you know what does "winning criteria" mean? Read the on topic page of the help center) \$\endgroup\$
    Commented May 1, 2018 at 15:41
  • \$\begingroup\$ 1. Are the blank lines in the sample document going to be in actual input, or are they just there to avoid the lines merging? (If the latter, ending a line with two spaces inserts a line break; and formatting as code using four spaces at the start of each line is a better option for sample input anyway). 2. Will all lines begin with H, a number, and a space? 3. What is the range of possible numbers? 4. Does whitespace matter in the output? 5. When you say "expected data structure", does that mean that the output is something other than a string? \$\endgroup\$ Commented May 1, 2018 at 16:21
  • \$\begingroup\$ 1. The child items are indented (as as result of parsing them as HTML later) 2. yes, exactly as in the sample input. 3. <h1> to <h6> 4. no, it will be parsed as HTML 5. yes, List \$\endgroup\$
    – Tlink
    Commented May 1, 2018 at 16:44
  • \$\begingroup\$ Unrelated to this challenge, but I've just created this Sandbox post based on your deleted Time challenge. Let me know if you want to post it yourself in 72 hours, since it was originally your challenge. If you don't want to post it anymore I'll post it myself (with your permission). Either is fine by me. :) \$\endgroup\$ Commented May 2, 2018 at 9:50
  • \$\begingroup\$ Please edit the post to add more information instead of answering in a comment. \$\endgroup\$
    Commented May 2, 2018 at 11:11

Given equations, determine values (what would be a good title?)

Given these equations as input, write a program that determines which variable has the greatest, and which has the smallest value.


x             <   y
2x            <   y + z
2x + 2y + z   <   2x + y + z
x,y,z         !=   0


  • Your code must mathematically determine which variable has the greatest/smallest value.
  • As this is the shortest answer in bytes wins
  • 5
    \$\begingroup\$ 1. These aren't equations: they're inequalities. 2. One example does not constitute a specification. What limits are there on the number and type of variables, equations, terms, etc? 3. The given example doesn't have a solution. I think the only basic relationships that can be deduced are x < y < 0. z could be smaller than x or greater than 0. \$\endgroup\$ Commented May 1, 2018 at 16:13

The true spirit of "Hello World!"

A hello world program is traditionally the first program you write in a language you've never tried before. So we're gonna make some true hello world programs here. Your challenge is to golf a hello world program in a language you've never used before.

Once you've posted your best attempt, you are welcome to accept help from others in the comments, including those who've used that language before, but please leave a copy of your original attempt, and use strikethrough on previous byte counts.

Rules based on honesty of participants:

  • Really pick one you've never used before. Different versions (python 2 / python 3) don't count.

  • Don't look at any other codegolf pages other than "tips for golfing in [your language]". Hello world has been golfed in many languages on PPCG but copying would be boring. The point is for each participant to have fun trying something new.

Objective rules, copied from the Hello World catalog challenge (who's answers you are not allowed to look at):

  • Each submission must be a full program.
  • The program must take no input, and print Hello, World! to STDOUT (this exact byte stream, including capitalization and punctuation) plus an optional trailing newline, and nothing else.
  • The program must not write anything to STDERR.
  • Submissions are scored in bytes, in an appropriate (pre-existing) encoding, usually (but not necessarily) UTF-8.

Notes for the Sandbox:

-Will people understand the point of this challenge? I want people to realize that, while PPCG is unlikely to see any novel solutions here, this challenge will still be a challenge for each participant. In fact, even the top-scoring master golfers will have a challenge: to find a language they haven't used ;).

-Is the title obnoxious? Suggestions?

-This is supposed to be a fun challenge for learning and teaching. Will people get that, or will people get really competitive and look for loopholes in the honesty-based rules?

-Should I tag it popularity-contest despite asking for golfed code, just so that people realize it's not supposed to be super-competitive?

  • 7
    \$\begingroup\$ Although the idea is interesting, I don't think this should be hosted on this site (because the answer length would be >= length of existing post anyway). Perhaps in chat? Or make a chat room for that? \$\endgroup\$
    Commented May 3, 2018 at 16:58
  • \$\begingroup\$ I see what you're saying with the >= existing answers thing. Would that still be an issue if it was tagged popularity contest? \$\endgroup\$
    – Jared K
    Commented May 3, 2018 at 18:29
  • \$\begingroup\$ Popularity contests are notoriously difficult to get right. Only 3 of the past 15 have been accepted as good by the community. As-is, this would likely get closed as Too Broad because it's basically "Do X Creatively." \$\endgroup\$ Commented May 4, 2018 at 12:29
  • 2
    \$\begingroup\$ It would probably also get tagged as a dupe of the original Hello, World! question \$\endgroup\$
    – Jo King Mod
    Commented May 5, 2018 at 0:25

Infer the type

(This is not finished yet.)


Implement the type inference algorithm for Hindley-Milner lambda calculus.

Data Types

The data types in this challenge is (in Haskell):

data Kind  = Star | KindFunc Kind Kind
data Type  = TypeName String  | TypeVar String | App Type Type
data Value = ValueName String | ValueFunc String Type | App Value Value 

If your language doesn't support ADT or if using it will make the program longer, you can use any isomorphic representation.

You can assume that there is no name conflict, nor unbound name.



In Hindley-Milner system,every value has a single most general type. For example the type of Just 5 is Maybe Int (Note that in Hindley-Milner type system, unlike Haskell, doesn't have any typeclass).

If a function value has type A -> B, you can call it with argument with type A to produce B. Additionally, you can use a lambda. A \x -> y has type type(x) -> type(y). (type(x) means the type of value x). However, if you try to apply a value of type A to function with type B -> C type unification between A and B occurs (explained below)


Like a value, type is composed from smaller type, so that if K has kind "a -> b", and a has type "a", the type K a has kind "b". The example of the type is String, ->, Maybe, Either String, and EitherT String which has kind *, * -> * -> *, * -> *, *, and (* -> *) -> * (Assuming EitherT has kind * -> (* -> *) -> * -> *. Every value has type *. However, the type can be generic. So a value of Nothing has type Maybe a. Here, a can stand for any type.

Type Unification

(TODO : To be filled tomorrow)


The input is a:

  1. list of pair of type name and the its kind
  2. list of pair of value name and its type
  3. and a value to be infered


Either the type of the supplied input or error if there is type mismatch.


(Here we use Haskell notation for the input, it is not recomended in actual challenge because it will unecessarily make the program longer)

> Types  : Maybe :: * -> *, Bool :: *
> Values : Just :: a -> Maybe a, Nothing :: Maybe a, True :: Bool, False :: Bool, maybe :: (a -> b) -> b -> Maybe a -> b, compose :: (a -> b) -> (c -> a) -> (c -> b)
> Infer  : \f -> maybe (compose Just f) Nothing
> Result : (a -> b) -> Maybe a -> Maybe b
> Infer  : \f -> maybe (compose Just f) True
> Result : Type error
  • \$\begingroup\$ 1. A question should be self-contained. What is the specification of the type inference? What are a kind, a type, and a value? 2. A question should avoid imposing non-observable requirements. In other words, the specification should say what types should be inferred, but not dictate the algorithm. 3. It's not obvious how to map the example to the data types given above. I don't see any constructor names. \$\endgroup\$ Commented May 3, 2018 at 10:39

Page size changing

Given positive integer m and n both > 1, and array(maybe-empty ordered list) A consist of integers in [0,m)(0-based) or (0,m](1-based), output an array B consist of integers in [0,n)(0-based) or (0,n](1-based) such that, when given input m'=n, n'=m, A'=B, the output is A.

You are not allowed to use built-in large number. To be clear, the number you use should be at most polynomial to (m+n)*arrlength. (discussing, ban/penalty/just allow)

Example: if you output [1,2,3] for m=6, n=8, A=[4,3,2,1], then you have to output [4,3,2,1] for m=8, n=6, A=[1,2,3].

Shortest code win.

  • \$\begingroup\$ Is there any way simpler than implementing bijective base conversion? Otherwise it's just "implement big integer division"? \$\endgroup\$
    Commented May 4, 2018 at 13:59
  • \$\begingroup\$ I think there be \$\endgroup\$
    – l4m2
    Commented May 4, 2018 at 14:01
  • \$\begingroup\$ you use "array" presumably this means a non-empty ordered list .. a tuple .. thus A and B are each in a space isomorphic to Z but with representations in base m and n respectively .. why not say so \$\endgroup\$
    – jayprich
    Commented May 5, 2018 at 11:10

Palindromic Sum

For some all bases g ≥ 5, we prove that any positive integer can be written as a sum of three palindromes in base g.

This is the theorem proven in this paper by Cilleruelo, Luca, Baxter.


Given a positive integer n and a base g ≥ 5 find three palindromic numbers in base g that sum to n.


  • You cannot use leading zeros to make a number count as a palindrome, that means the most significant digit of a palindrome must be nonzero. (In base 10, the number 01210 = 1210 is not considered as a palindrome.)
  • The submissions must be able to process any aribtrarily large input n (within the memory limits). It is not sufficient to use finite precision integers, i.e. 32-bit ints.
  • The integers in the input and output can be in the format of native integer types, as strings in any fixed base (not dependent on g), or as list of digits in any fixed base (not dependent on g) or in the base g or as number in the base g.
  • 2
    \$\begingroup\$ Forbidding finite precision integers doesn't sit well with me, and it doesn't seem particularly vital to the challenge. \$\endgroup\$ Commented May 5, 2018 at 15:10

Pressure unit conversion

In my line of work I often need to convert between four different pressure units: bar, psi, MPa and kgf/cm2.


Write a program or function that does the following:

  • The user enters a value, the original unit and the conversion unit
  • The program or function makes the conversion
  • It then prints out the result in the form: [Starting value] [Starting unit] = [Converted value] [Converted unit] with spaces between all items.

For example with inputs 15.25, psi and kgf/cm2, the program would print out the following:

15.25 psi = 0.07030695796 kgf/cm2


  • The allowed range of input values can be in your language's standard floating-point type.
  • Input can be arguments, come from STDIN, or whatever is your language's preferred form of input.
  • The input units don't have to be literal strings - you can use other pre-defined values if you want.
  • However, the output must use the literal of the units: bar, psi, MPa and kgf/cm2.
  • Both kgf/cm² and kgf/cm2 are acceptable.
  • If printing out the result isn't feasible then you can return the string that would have been printed out.
  • This is code golf - shortest answer wins!

Conversion values

The conversion values are:

1 bar = 14.50377377 psi
1 bar = 0.1 MPa
1 bar = 1.019716213 kgf/cm²
1 psi = 0.06894757293 bar
1 psi = 0.006894757293 MPa
1 psi = 0.07030695796 kgf/cm²
1 MPa = 10 bar
1 MPa = 145.0377377 psi
1 MPa = 10.19716213 kgf/cm²
1 kgf/cm² = 0.980665 bar
1 kgf/cm² = 14.22334331 psi
1 kgf/cm² = 0.0980665 MPa

Some test cases:

33.4 bar = 484.426044 psi
19 kgf/cm2 = 1.8632635 MPa
65 MPa = 650 bar
-59.002 kgf/cm2 = -839.2057018 psi
23 bar = 23 bar
0 psi = 0 kgf/cm2
1 MPa = 1 MPa

I took these sample values from here. You can use it too for more test cases.

Sandbox Questions

  • Is four pressure units all right, or is it too many? My gut tells me that four makes it just complex enough, but I'm also worried that it's just adding extra information without any extra challenge.
  • I'm not certain if I should restrict the input as well as the output. I think restricting the output format adds to the challenge, but restricting the input would make the challenge too, well, restrictive.
  • I should display the conversion values in a more machine-readable form, but I'm not sure how. Any ideas?


Many fans of high-powered editors like to learn new tricks. For example, vimgolf.com: "Real Vim ninjas count every keystroke".


Produce this diagonal matrix in a text editor:


Winning criterion

Fewest keystrokes wins


  • Need to do it in a text editor
  • All pre-existing packages are allowed
  • Must start from an empty page
  • No copy/pasting external content


  • Say which editor
  • Present the total number of keystrokes
  • Provide a list of keystrokes, and say what each one does
  • Link to any packages used, so we can check them out!

See Is editor golf on-topic? for more info on editor golf.

  • \$\begingroup\$ -1 for text editor restriction, -1 for pre-existing packages, "must start from empty page" and "copy/pasting" are common sense/standard loopholes. "say which language/number of keystrokes/provide a list of keystrokes" is required in all code-golf challenges \$\endgroup\$
    – ASCII-only
    Commented May 8, 2018 at 8:09
  • \$\begingroup\$ Hi, welcome to PPCG! First of all good job for using the Sandbox, since a lot of new users do not. The tags you've added both doesn't exist, [tag:matrix] does exist however, but editor-golf not (tag lists). I'm also a bit confused about the challenge. Do we need to use any programming language, and create that output in a .txt file? Or do we actually use text editors like Notepad, Notepad++, VIM editor, etc. and use their default commands to get the result in as few commands / key-pressed as possible. I'm assuming the second if I read correctly. \$\endgroup\$ Commented May 8, 2018 at 8:09
  • 1
    \$\begingroup\$ re: packages - for all we know someone has already made a package to do it in one key :P \$\endgroup\$
    – ASCII-only
    Commented May 8, 2018 at 8:09
  • 3
    \$\begingroup\$ See codegolf.meta.stackexchange.com/questions/4816/… for more info on editor golf. \$\endgroup\$
    – Hatshepsut
    Commented May 8, 2018 at 8:12
  • \$\begingroup\$ @KevinCruijssen Right, the intent is to use the commands available in a text editor like vim or emacs. \$\endgroup\$
    – Hatshepsut
    Commented May 8, 2018 at 8:19
  • \$\begingroup\$ @ASCII-only If there's a package for doing cool stuff like that in one key, I want to know about it! : ) \$\endgroup\$
    – Hatshepsut
    Commented May 8, 2018 at 8:22
  • \$\begingroup\$ What determines whether something is a "text editor" or not? Is Microsoft Word one? Because that would allow you to use VBA macros. How about a programming environment like Eclipse? You can edit text documents in such a program. This needs to be a little better defined before it gets my upvote. \$\endgroup\$ Commented May 8, 2018 at 13:11
  • \$\begingroup\$ Challenges should not limit possible languages. I can't see any problem from allowing e.g. Python to compete. \$\endgroup\$
    Commented May 8, 2018 at 16:09
  • \$\begingroup\$ Although, let people vote. Note Please upvote if you think that limiting it to editors only is good (despite the fact that it can be ambiguous, as AdmBorkBork said in the comment above), or downvote it if you think that this can be made language agnostic. \$\endgroup\$
    Commented May 9, 2018 at 11:36
  • \$\begingroup\$ This is the latest well-received language-specific challenge, if I recalled correctly. \$\endgroup\$
    Commented May 9, 2018 at 11:36

How Many Cards Can You Play This Turn?

Given a deck of cards L, where each card has an nonnegative integer mana cost, and two nonnegative integers x, representing the mana you have this turn, and n, representing the number of cards you draw from the deck, output the average highest number of cards you can play this turn.

i.e. given an array of nonnegative integers L and nonnegative integers x and n, create the subset N by choosing n random elements of L. Find the average of the maximum number of elements in N that sum to no greater than x.


For example, L = [0, 1, 2, 3, 4, 5, 6], x = 1, n = 1. You get 1 mana this turn and draw 1 card at random, from a deck of cards with mana cost 0...6. Since there is a 1/7 chance of drawing any card and you only have 1 mana, there is a 2/7 chance you can play the 1 card you draw (0 or 1) and a 5/7 chance you can’t play the card you draw (2...6). Therefore the average max number of cards that can be played is 2/7 = 0.286

For a more complicated example, L = [0, 1, 2, 3, 4, 5, 6], x = 10, n = 4. You get 10 mana and draw 4 cards from the same deck as above. There are 7 choose 4 = 35 different possible hands. Of this, there are only 2 hands in which only two cards can be played (3,4,5,6 and 2,4,5,6). There are 22 hands where 3 cards can be played and 11 hands where 4 cards can be played. Therefore, the average max number of cards that can be played is (2*2 + 3*22 + 11*4) / 35 = 114/35 = 3.257


  • The input must be a list of nonnegative integers, L, and two nonnegative integers, x and n.
  • You may assume n <= len(L), i.e. you cannot draw more cards than are in the deck
  • The output must be either a decimal which has at least 3 digits of precision or an exact fraction
  • The distribution for each element of L must be equal
  • Standard I/O methods apply and standard loopholes forbidden
  • This is code golf so shortest answer in bytes wins

Test cases

Input: L (the deck), x (mana), n (cards drawn)
-> Output: average max # of cards played this turn (fraction for reference)

[0, 1, 2, 3, 4, 5, 6], 1, 1 -> 0.286 (2/7)
[0, 1, 2, 3, 4, 5, 6], 2, 2 -> 0.810 (17/21)
[0, 1, 2, 3, 4, 5, 6], 3, 3 -> 1.457 (51/35)

[0, 1, 2, 3, 4, 5, 6], 10, 3 -> 2.686 (94/35)
[0, 1, 2, 3, 4, 5, 6], 10, 4 -> 3.257 (114/35)
[0, 1, 2, 3, 4, 5, 6], 10, 5 -> 3.762 (79/21)

[0, 1, 2, 3, 4, 5, 6], 1, 0 -> 0
[0, 0, 0, 0, 0, 0, 0], 1, 3 -> 3
[2, 3, 4, 5, 6, 7, 8], 1, 3 -> 0

[0, 16, 4, 9, 3], 7, 2 -> 1.2 (12/10)
[11, 0, 3, 19, 6, 8, 15], 20, 4 -> 2.686 (94/35)
[14, 3, 2, 11, 5, 3, 9, 2, 10], 20, 6 -> 3.976 (334/84)

Take an expression containing variables and these functions, check whether it's true:

1 int  one()          1
- int  neg(int)       -a
+ int  add(int, int)  a+b
- int  sub(int, int)  a-b
* int  mul(int, int)  a*b
> bool gt (int, int)  a>b
< bool lt (int, int)  a<b
= bool eq (int, int)  a==b
& bool and(bool,bool) a&b
| bool or (bool,bool) a|b
! bool not(bool)      !a
 bool nand(bool,bool) !(a&b)
^ bool xor(bool,bool) a^b, or (a&!b)|(!a&b)
  bool imp(bool,bool) !a|b
... (any bool function, with any amount of input)
E bool any(var, expr) whether a be any value make expr true
A bool all(var, expr) whether a be all value make expr true

You can take input in any reasonable format. You can define what "variable" look like as long as there can be infinite possible different variables. Removing some functions is allowed as long as it doesn't reduce the ability of expressing, i.e. some calculatable function can convert expression with all functions possibly exist into the model you removed something. Redefining the function name or merging some symbols is also allowed.

To solve the problem, you are given another input source G. You need to solve the problem correctly when G[0] -> inf, G[1] > any_function(G[0]), G[2] > any_function(G[1]) ....

Sandbox Notes:

it's quite a fix of this one, but it's using haltable as output while I use another input source to help solve.

  • 2
    \$\begingroup\$ I understand what you're doing here. If you're OK with it, I'll adapt my version and post it, since it's more complete at the moment. \$\endgroup\$ Commented May 12, 2018 at 0:20
  • \$\begingroup\$ Now it looks similar to your previous "solve the halting problem" challenge. \$\endgroup\$
    Commented May 14, 2018 at 7:10

Source in n bytes

Write a program A and an integer K that, when given a number n>=K, your program generates a program(in same language) of length n that generates the source program A and don't if some characters are removed.

Smallest K for each language win.

  • \$\begingroup\$ What do you mean by "and don't if some characters are removed"? Any example? \$\endgroup\$
    – tsh
    Commented May 14, 2018 at 5:13
  • \$\begingroup\$ @tsh generates the source program A and don't generates the source program A if some character(s) of your generated source are removed, so you don't just fill the code with unnecessary space/etc \$\endgroup\$
    – l4m2
    Commented May 14, 2018 at 5:48
  • \$\begingroup\$ So it's similar to a pristine program? \$\endgroup\$
    Commented May 14, 2018 at 6:58
  • \$\begingroup\$ What do you mean by "don't generate program A if some characters removed"? Do you mean "for any sub character sequence S of source A, program S have different behavior to program A"? \$\endgroup\$
    – tsh
    Commented May 14, 2018 at 7:13
  • \$\begingroup\$ @tsh your program generates a program(in same language) of length n called P that outputs A, for any sub character sequence S of source P, program S have different behavior to program P(so don't output A) \$\endgroup\$
    – l4m2
    Commented May 14, 2018 at 10:00
  • \$\begingroup\$ This looks to me like a generalised quine. If a simple quine diagonalises f(Y):print Y, this diagonalises f(Y):if len("....")>X:print Y. There are some subtleties around the length of X, but I'm not sure how much this adds to existing quine questions. \$\endgroup\$ Commented May 14, 2018 at 10:01
  • \$\begingroup\$ @PeterTaylor For example, we have function () { source = ...; if (length(source)=10000) return source; else "9900 bytes padding"; } then its sub sequence function () { source = ...; if (length(source)=1000) return source; else "900 bytes padding"; } should just work. \$\endgroup\$
    – tsh
    Commented May 15, 2018 at 2:44
  • \$\begingroup\$ @tsh, I have no idea what you mean. I'm not even sure whether your example is meant to support my point or contradict it. \$\endgroup\$ Commented May 16, 2018 at 8:57
  • \$\begingroup\$ @PeterTaylor I think tsh can support or contradict your idea, depending on your meaning \$\endgroup\$
    – l4m2
    Commented May 16, 2018 at 11:00



Calculator tape

There is an infinite amount of calculators in a row (supporting the four basic operations, +, -, *, and /), all initially displaying "0". They have a peculiar property that any calculator which has a result lower than either the one to its left or right stop accepting any button presses except "=".

Besides the calculators is an operator who follows certain instructions and presses their buttons accordingly.

Unfortunately, the operator is a bit lazy and has asked you to write a program which would simulate all of this for him and then output a list of all nonzero calculator results.

Calculators and operations

Every calculator has three registers, one which stores the input, one which stores the last result, and one which stores the currently selected operation (+, -, *, /).
In the beginning, the input and result registers are unset, acting as 0 if adding/subtracting or 1 if multiplying/dividing. The operation starts as +.

The operations available are the following:

  • 0..9 - Append the digit to the input of the selected calculator, without changing its result.
  • # (±) - Change the sign of the input. Note that this might be executed even when the input is 0.
  • +, -, *, / - Evaluate the current operation, then set the operation accordingly and reset the input. As it changes the result, it might change the disabled statuses of calculators.
  • = - Evaluate the current operation and reset the input. Don't reset the operation.
    Unlike the other operations, this operation works even when the calculator has a lower result than its adjacent calculators.
  • <, > - Select the previous or the next calculator, similar to BF.

Input and output

Input is going to be a string or list of operations to be done. The characters used to represent the individual operations need not be the same as outlined above, but must be distinct.

Output should be a list (in any format) composed of nonzero calculator results and their indices. The indices might start from any number, as long as it is identical between runs.


Input  ->  Output # comment
1  ->  # The calculator's result is 0
1=  -> 0: 1 # The = operation evaluates result + input = 0 + 1 = 1
1#=  -> 0: -1
#1=  -> 0: -1 # Note that this results in input being -0 for a moment
1=>2=  ->  0: 1 # The second calculator does not work while the first is larger
1#=>2=  ->  0: -1, 1: 2 # Results start at 0, so negative results don't disable
1>2=<=  ->  0: 1, 1: 2 # = Works on otherwise-disabled calculators
4*4>1+2=<=  ->  0: 16 # * changes the result to 4, thus disabling others


  • As it currently stands, there are too few complicated examples, mainly because it is too easy to disable a calculator. Would be nice to have some which have multiple adjacent calculators with the same result.
  • There is too little inter-calculator interaction past the disabling mechanic. Maybe some way to read a result as input?
  • The third point might be yours! Comment to claim it ;)
  • \$\begingroup\$ What character encoding is ± in? Can we assume Windows-1252 where ± is 0xB1? (I suggest using an ASCII-only substitution, like #) \$\endgroup\$
    Commented May 15, 2018 at 6:38
  • \$\begingroup\$ > "The characters used to represent the individual operations need not be the same as outlined above, but must be distinct." -- the idea was that golfers might pick a different character, for example $ or #. Anyway, I will replace ± with # so it would be less confusing. \$\endgroup\$ Commented May 15, 2018 at 11:51

Tired Sumo Fighters

This is a simple KOTH challenge in which you program a bot to push other bots out of a ring. Each round consists of two bots, and every bot will compete against each other bot at least 10 times. The reason that this KOTH is different from other similar KOTHs is that the bot only is very tired, so it only has a little bit of energy, and it cannot waste it.


Your bot can move around the ring (An 11x11 square) using one of 10 commands:

moveForward() //Moves forward
moveLeft() //Moves left
moveBackward() //Moves backward
moveRight() //Moves right
moveToward() //Moves toward enemy
moveAway() //Moves away from enemy
approachCenter() //Moves toward center of ring
approachEdge() //Moves away from center of ring
moveCounterClockwise() //Moves counter-clockwise around ring
moveClockwise() //Moves clockwise around ring

However, each movement takes energy. As a parameter, set the energy consumption (Defaults 1). If one bot stays still and the other runs into it, the moving bot pushes the still bot. If the bots run into each other, the bot who allotted more energy pushes. If there is a tie, the bots do not move.


Say that bot 1 and bot 2 are fighting. If bot 1 is at position (-4,5) and bot 2 is at (-2,5), there is one space between them. If each bot moves toward the opponent, the one who spent more energy moves forward. If there is a tie, the bots do not move. If a bot has any coordinate with an absolute value greater than 5, it loses. If any bot has energy under 0, it loses. It is possible to live with 0 energy, but you will be quickly pushed out of the ring. Every bot starts with 200 energy


To find your opponent or yourself, use these commands:

locateSelf() //Returns as array [x,y]
locateOpponent() //Returns as array
energyLevel() //Returns your energy level
opponentEnergy() //Returns opponent's energy


function sumoChamp() {
    if (energyLevel() > opponentEnergy()) {
    } else {

As you can see, a basic sumo bot takes just a few lines. However, a smart sumo bot would be able to overpower it, and a genius bot could even figure out its pattern and destroy it. You cannot build systems designed to target specific bots that have already been designed. To save information for later, store it in your bot's function, like this:

sumoChamp.opponentResearch.avgEnergy = 4


Standard loopholes not allowed

Entries can be in any language that supports functions and arrays, but all entries will be converted to JavaScript before the game starts.

  • \$\begingroup\$ There should be more specification. What's the exact distance the bot will be pushed? \$\endgroup\$
    Commented May 19, 2018 at 16:46
  • \$\begingroup\$ @user202729 1 space \$\endgroup\$ Commented May 21, 2018 at 2:26
  • \$\begingroup\$ Perhaps my understanding is not complete, however would there be an optimum solution that would either guarantee a win or tie? i.e. move to the centre of the ring and then subequently push the opponent toward the outside of the ring with half of the remaining energy that you have each step. If the opponent uses less energy they are pushed, otherwise they will have wasted more energy than the first bot and eventually run out of energy prior to the first bot, thus get push out \$\endgroup\$
    – Moogie
    Commented Jul 20, 2018 at 4:09


Bananagrams is a tile-based word game where you race to make crosswords and use all your letters before everyone else does. In the original game, finishing your crossword allows you to say "peel", causing everyone to draw another letter tile. Whoever finishes their crossword when there are no more letter tiles to draw is the winner.


You will be given a list of letter tiles and your task is to arrange them into a crossword using this list of alphabetic words as your dictionary. In the spirit of the board game, you need to do this as quickly as possible.

Input and Output

Input to your solution will be a list of letters and you should output a grid of letters. You will need to read in the list of words.

As an example: given abcdeiloopswy, your program/function might produce

  a l

Test cases for timing

  • aabcdeeefghiijklmnoopqrssttuuvwxy

more TBA


  • No abusing standard loopholes
  • Input and output may be in any convenient format.
  • You must load the linked word list exactly, not a stripped down version.
  • You may assume that you will be given only lowercase letters
  • You may assume that there is at least one possible crossword from your given set of letters.
  • This is a challenge, so the answer with the shortest execution time in each language wins.
    • Be sure to post execution time and CPU as part of your solution.
  • 1
    \$\begingroup\$ Why do solutions need to be written in Python? What if a solution calls out from Python to another language? \$\endgroup\$
    – isaacg
    Commented May 7, 2018 at 21:23
  • \$\begingroup\$ @isaacg: one way or another, there needs to be a way to compare solutions on performance. The other option would be to make a test driver for several languages, but I don't want to write 10 test drivers. \$\endgroup\$
    – Beefster
    Commented May 8, 2018 at 2:15
  • \$\begingroup\$ I don't see why you need a test driver like that. Just require submissions to read from a file and write their outputs to a file, and have your test script read their file to evaluate their solutions. \$\endgroup\$
    – isaacg
    Commented May 8, 2018 at 5:12
  • \$\begingroup\$ @isaacg: how would I be able to score on execution time? I guess I could ditch that as a scoring requirement in favor of a time limit. \$\endgroup\$
    – Beefster
    Commented May 8, 2018 at 18:17
  • 1
    \$\begingroup\$ Time the execution of the submission, with something like the time command. Or require submissions to time themselves. I'm not suggesting you change your rules at all, just use different timing. \$\endgroup\$
    – isaacg
    Commented May 8, 2018 at 18:29
  • \$\begingroup\$ @Beefster (time command is a Linux utility, which can be used to measure the execution time of any process, which is also what TIO used to measure the time, however do note that TIO is not acceptable for measuring time) \$\endgroup\$
    Commented May 9, 2018 at 13:36
  • \$\begingroup\$ @isaacg: How would I adjust for differences in language speeds? Solutions in C would have a massive advantage over solutions in pretty much any other language. \$\endgroup\$
    – Beefster
    Commented May 9, 2018 at 19:28
  • \$\begingroup\$ @user202729: See above comment \$\endgroup\$
    – Beefster
    Commented May 9, 2018 at 19:29
  • \$\begingroup\$ @Beefster A lot of the time, algorithmic speed is more important than complilation quality, so it'll matter what language the program is easiest to write fast code in, which is not always C. Take a look at past fastest-code contests. C++, Rust, and Nim are popular. \$\endgroup\$
    – isaacg
    Commented May 9, 2018 at 19:39
  • 1
    \$\begingroup\$ @isaacg: After looking around, you have me convinced. I thought language advantages were a bigger deal than they really were. \$\endgroup\$
    – Beefster
    Commented May 9, 2018 at 20:13
  • \$\begingroup\$ I'm not a fastest-code expert (ask Dennis for more details?), but I'm afraid that using a "sample benchmark program" doesn't solve the "on which machine" issue, because on some machines some operations can be done faster than other machines. \$\endgroup\$
    Commented May 11, 2018 at 1:05

Construct any Sporadic Simple Group

There are 26 sporadic simple groups. Your task is to write a program which defines an operation isomorphic any of these groups.

Input: Two symbols that represent elements of the group. You can choose how these symbols will be represented (characters, numbers etc.).

Output: The result of the operation on the input.

Taken as a whole, the operator defined must be isomorphic to one of the 26 sporadic groups.

I don't actually know anything about group theory; I haven't even taken a course in it yet, just read some Wikipedia and thought this could be interesting. If it isn't trivially solvable, maybe someone could post it?

  • \$\begingroup\$ I think this has potential but needs a bit of work. 1. As it stands, it's almost 26 questions in one. Intuitively I would expect the smallest group to be the most golfable, but to really be sure I'd have to make a semi-serious effort with all 26. I suggest picking one group or at most one family of groups. 2. "Symbols" is a bit vague. I'm not sure whether I'd be allowed to use an array which must be a permutation of the numbers 0 to N for suitable N (which is the obvious way of doing some if not all of the groups). 3. You haven't specified a winning condition. I assume code-golf. \$\endgroup\$ Commented May 20, 2018 at 21:59
  • \$\begingroup\$ 4. (Hadn't thought of this when I wrote 2). Since every finite group is a permutation group, with a suitable input format this can be reduced to codegolf.stackexchange.com/q/1490/194 . So if you want to tackle it as group action, you need to constrain the input representation and running time quite harshly. However, it reminds me of codegolf.stackexchange.com/a/26423/194 where golfing a representation of a simpler perm group was an interesting subtask. The solution might be to ask to generate all elements of the group in a non-trivial representation (as perms or reduced words). \$\endgroup\$ Commented May 20, 2018 at 22:07

K-th largest element

Find the k-th largest element of n elements in at worst O(n)

It shouldn't be affected by

  • element arranging
  • element range
  • RNG (if you use)

  • \$\begingroup\$ This can be done with a heapify algorithm in O(n). Not that complicated. \$\endgroup\$
    – Beefster
    Commented May 19, 2018 at 20:33
  • \$\begingroup\$ @Beefster But (1) it's not trivial (at least for golfing languages, C++ has built-in nth_element) and (2) your algorithm is wrong (it takes O(n+k log n), which is about O(n log n) for k in O(n)) \$\endgroup\$
    Commented May 20, 2018 at 2:36
  • \$\begingroup\$ "O(n)" implies "at worst O(n)". Big-O notation only upper-bound the complexity, it doesn't lower-bound. (so an algorithm that is O(n) is also O(n²), O(n!), ...) \$\endgroup\$
    Commented May 20, 2018 at 2:37
  • 5
    \$\begingroup\$ Also, please "Write your challenge just as you would when actually posting it." -- people find it hard to understand your posts, so it would have a lower change of them not understanding your challenge. \$\endgroup\$
    Commented May 20, 2018 at 2:38
  • 1
    \$\begingroup\$ To the two people who have upvoted this answer: @user202729's point is relevant to you too. Do you seriously think this is ready to post with just a title and tags? \$\endgroup\$ Commented May 20, 2018 at 22:14
  • \$\begingroup\$ @PeterTaylor No because of the character limit xd (I thought it was a 0 vote, didn't know it's a +2/-2) \$\endgroup\$
    – l4m2
    Commented May 21, 2018 at 1:20
  • \$\begingroup\$ @user202729 nth_element is average O(n), worst O(n^2) iirc? \$\endgroup\$
    – l4m2
    Commented May 21, 2018 at 1:21
  • \$\begingroup\$ ... I think so. cppreference says "O(n) on average", and doesn't specify the worst case behavior. You may want to add "the algorithm must have deterministic runtime". \$\endgroup\$
    Commented May 21, 2018 at 1:23


Polyomino detection itself is a duplicate. And there was also a very similar question concerning simple connectedness of a tiling pattern. So I won't submit.

Holey polyomino

A polyomino is a plane polygon consisting of equally-sized squares connected at edges. A polyomino might have a hole if it contains a region with a square on all four sides.

The challenge

Determine if a collection of squares is a polyomino with a hole.


The input will be a matrix of 0s and 1s, where a 1 indicates a square in that position. For example, the top left nonomino in the picture above could be:


A truthy or falsey value for whether the input represents a polynomino with a hole.

Test cases

Arrays of rows:

[[1, 1, 1], [1, 0, 1], [1, 1, 1], [0, 1, 0]] => TRUE

[[1, 1, 1, 0], [1, 0, 1, 0], [1, 1, 1, 0], [0, 1, 0, 1]] => FALSE
  • \$\begingroup\$ Are holes of size greater than 1 considered? For example (as a list of rows since formatting is limited in comments) [[1111] [1001] [1111]] \$\endgroup\$ Commented May 22, 2018 at 20:21
  • 1
    \$\begingroup\$ @Kamil Drakari yes, that counts. I'll add a test case. And there's no need for the input to be square, so I'll change that too. \$\endgroup\$
    – ngm
    Commented May 22, 2018 at 20:24
  • \$\begingroup\$ Proposed test case [[0, 1, 1, 1], [1, 1, 0, 1], [1, 0, 0, 0], [1, 1, 1, 1]] => FALSE to cover the situation where there's an empty square with non-empty squares in all four directions but which still has a path to the exterior. \$\endgroup\$ Commented May 23, 2018 at 7:42
  • \$\begingroup\$ I don't think this challenge is likely to get much traction. I've just proposed a simpler "Is this a polyomino?" question which I think should go first, and might open up possibilities for other challenges. \$\endgroup\$
    – ngm
    Commented May 23, 2018 at 14:37
  • \$\begingroup\$ The simpler one is, as I wrote in a comment there a duplicate. This one is not a duplicate AFAIK, so you should stick with this one (if any). \$\endgroup\$ Commented May 23, 2018 at 18:54
  • \$\begingroup\$ Can there be two (or more) holes? \$\endgroup\$ Commented May 23, 2018 at 18:58
  • \$\begingroup\$ @StewieGriffin I've deleted the other one. And this one is very close to codegolf.stackexchange.com/questions/69030/… so I don't think I would proceed. \$\endgroup\$
    – ngm
    Commented May 23, 2018 at 18:58
  • \$\begingroup\$ @StewieGriffin the two-or-more hole thing could work, but I would be inclined to make it a question about topology rather than polyominoes. As in "calculate the genus of this surface" where the surface is represented by the binary array as in my examples. \$\endgroup\$
    – ngm
    Commented May 23, 2018 at 19:07

Go shopping!

You're shopping for four specific items. Your town contains fifteen shopping malls. Unfortunately you can't always find the item you want at the nearest mall. You want see whether you can purchase the four items on a single shopping trip.

For each item, there is a list of malls (given as coordinates) where the item can be purchased. Some malls however will only sell you a given item if you visit that mall first. Furthermore, the entrances and exits to the malls are badly placed and you can only visit malls in increasing order of coordinates.


  1. Item 1 can be purchased at mall (7, 2) but only if you visit that mall first. It can also be purchased at mall (3, 1).
  2. Item 2 can be purchased at mall (7, 1).
  3. Item 3 can be purchased at mall (3, 2), (4, 2), (5, 2) or (6, 2).
  4. Item 4 can be purchased at mall (7, 2).

With this scenario, it's not possible to purchase both items 2 and 3 in one trip.

  1. Item 1 can be purchased at mall (3, 3), (4, 3), (5, 3), (6, 3).
  2. Item 2 can be purchased at mall (3, 2), (4, 2), (3, 3) or (4, 3).
  3. Item 3 can be purchased at mall (6, 1), (7, 1), (6, 2) or (7, 2).
  4. Item 4 can be purchased at mall (4, 1), (5, 1), (6, 1) or (7, 1), but only if you visit that mall first.

In this secnario, you would visit mall (4, 1), (4, 2), (6, 2), and then (6, 3), and successfully purchase all four items on a single trip.

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

  • \$\begingroup\$ 1. "(given as coordinates)": always 2D and in integers? 2. "Some malls however will only sell you a given item if you visit that mall first." How is this indicated in the input? 3. "in increasing order of coordinates." What's the order in dimensions higher than 1D? Lexicographic? \$\endgroup\$ Commented May 24, 2018 at 13:49
  • \$\begingroup\$ I think your two examples need more separation. It was difficult to tell whether the middle "With this scenario..." line connects to the first, second, or both lists. I would add "Example 1:..." and "Example 2:..." to show the distinction. \$\endgroup\$ Commented May 24, 2018 at 15:03
  • \$\begingroup\$ @PeterTaylor 1. Yes. 2. I don't mind how that special case is indicated in the input; you could have two separate lists, or a boolean with the coordinates. 3. You can choose which coordinate(s) to increase. \$\endgroup\$
    – Neil
    Commented May 24, 2018 at 19:04
  • \$\begingroup\$ How come in the first scenario I can't go (3,1) (3,2) (7,1) (7,2) and purchase all four items? Do items need to be bought in order? If so, that should be explicitly stated. \$\endgroup\$ Commented May 25, 2018 at 14:03
  • \$\begingroup\$ @AdmBorkBork You can't go from (3, 2) to (7, 1) because 1 < 2. (And you can't go from (2, 3), to (1, 7) either, for the same reason.) \$\endgroup\$
    – Neil
    Commented May 25, 2018 at 15:40
  • \$\begingroup\$ Ah, both coordinates need to be non-decreasing. That could probably be made a little more clear. \$\endgroup\$ Commented May 25, 2018 at 15:58
  • \$\begingroup\$ This should be reflavored as an IKEA trip. :P \$\endgroup\$
    – Beefster
    Commented May 29, 2018 at 19:01


Softcoding is an anti-pattern defined by “the practice of removing ‘things that should be in source code’ from source code and placing them in some external resource”, which usually are config files, database entries, or both. “At the extreme end, soft-coded programs develop their own poorly designed and implemented scripting languages”, as can be seen by one of the most egregious examples of the practice, the Enterprise Rules Engine.

Thus, programmers who fall to the habit of softcoding, after tons of work, find themselves at the very point they were starting from: they have developed “some sort of COmmon Business-Oriented Language that’s generic enough to code any rule”. But unfortunately for them, there already is such a thing. “It’s called C++. And Java. And C#. And Basic. And, dare I say, COBOL.

For the sources of the above quotations are more information of the practise, see: Soft Coding on The Daily WTF, Softcoding on Wikipedia and The Enterprise Rules Engine on The Daily WTF.

The Challenge

Softcode! Make an actual, Turing-complete language that will reside in config files!

More specifically, you should write a compiler / interpreter / anything in between that will define a Turing-complete programming language, whose every valid source code will also be a valid JSON file. (The reason why JSON has been chosen is the simplicity of syntax and wide avalability of JSON parsers, so that you won't have to write a parser yourself.)

Input/output format is up to you.

The catch

Not your interpreter code should be golfed. Your JSON code should be golfed. Or, more specifically, golfable. That is: you will have to present a JSON file that, if fed to your program, will solve a given golfing challenge and will have smallest size out of all JSON files presented here. This will determine the winner; the size of the interpreter code is not relevant.

Technical details

Unfortunately, given how this challenge is defined, if I would define any particular problem your JSON-program should solve, one could get away with a simple trick of defining one particular command in one's JSON programming-language that solves this particular problem. To instead encourage creating a language that is as concise as possible, this challenge will be split into two phases. Phase 1 will last a month since posting this challenge; you will then write your compilers or interpretes. Then I will, on random, choose one challenge from this site that has at least 5 upvotes. (If this challenge proves unsuitable for this problem, I'll roll the dice again.) Afterwards, Phase 2 will begin, when you will present a JSON-program that solves this challenge.

Be creative.

Example interpreter

For reference, below is a simple JS interpreter of a Turing-complete JSON programming language. Of course, it does not strive to define a language suitable for golfing. (I hope there are no bugs in this interpreter, but I don't feel like testing it now.)


// This interpreter implements GOTO-programs, which are Turing-complete.

var stdin = new BufferedReader( new InputStreamReader(System['in']) );
var progsrc = stdin.readLine();
var prog = JSON.parse(progsrc).prog;

var vars = {};
vars.in = Number(stdin.readLine());

for(var i = 0; i < prog.length; i++) {
    switch(prog[i].command) {
        case 'ADD':
            vars[prog[i].varname] = vars[prog[i].varname] || 0;
            vars[prog[i].varname] += prog[i].constant;
        case 'SUBSTRACT':
            vars[prog[i].varname] = vars[prog[i].varname] || 0;
            vars[prog[i].varname] -= prog[i].constant;
        case 'GOTO':
            i = prog[i].index-1;
        case 'IF':
            vars[prog[i].varname] = vars[prog[i].varname] || 0;
            if(vars[prog[i].varname] == prog[i].constant) {
                i = prog[i].index-1;
        case 'HALT':
            i = prog.length;

    vars.out = vars.out || 0;
  • \$\begingroup\$ What's the winning criterion? \$\endgroup\$
    – feersum
    Commented May 26, 2018 at 23:29
  • 1
    \$\begingroup\$ @feersum Size of the JSON program. But nevermind. I realized how hard did I screw. Trivially, the correct solution is to pass the sole property name to an already-existant golfing language interpreter. I'll post a take 2 in a while... \$\endgroup\$
    – gaazkam
    Commented May 27, 2018 at 8:12

You have been given the following tree structure make from dictionaries and lists

{'snapshot': {'outer': {'inner': {'dir1': {'blob': 
          [{'file': '8ke45'}, {file1: '72d4kl'}]}, 
          'dir2': {'blob': [{'file1': 'fa65e'}]}}, 
           'something': {'blob': [{'thing1': '8447v'}]}}}

The dictionary contains all the directories and blobs(arbitrary file) in a given directory, in this case 'snapshot,' which contains the directory outer and no blobs. 'Outer' has all of its corresponding directories and blobs and so on until the end of the branch. Blobs objects are a dictionary with the key 'blob' and the value of a list of dictionaries containing the filename and content hash.

the tree looks like:

        |__ inner 
        |     |__dir1
        |     |   |__file
        |     |   |__file1
        |     |__dir2
        |     |   |__somefile
        |     |__another_file
        |__ something

the objective to create trees files, wherein each directory is the hash of the hash of the underlying directories or blobs.

e.g the 'inner' file:

    name: 7j3429ds 
    contents: tree 8340dnwh28a          --di1
              tree sh2991ka86n          --dir2
              blob 12046bshs63          --anotherfile

You may hash the file using any known checksum ie. sha1, md5 ect. This is an example for the hash of the contents of a file:

def hash_(name):
    hash_object = hashlib.sha1(bytes(str(name), encoding='utf-8'))
    hex_dig = hash_object.hexdigest()

An Example of the final output given the dictionary:

{'snapshot': {'inner': {'dir1':{'blob': [{'file': '8ke4528dgsk'}},  
                        'dir2': {'blob': [{'file1': 'fa65ek97n37'}]}}} 

should look as such:

        |__ ak37ka729sm
        |     |__193bal7q8gl
        |__ o8pdn279ax8

note the names of the files are just random hashes in the example when actually computing the hashes, the hash should consist of the hashes or all trees and blobs contained within that tree

45334423112 contents:

tree ds498kdh72b

ds498kdh72b contents:

tree ak37ka729sm
tree o8pdn279ax8

ak37ka729sm contents:

blob 8ke4528dgsk
blob fa65ek97n37

note that the blobs are the value of the filname key in the lowest level directory of the tree ie.

'dir1':{'blob': [{'file': '8ke4528dgsk'}]}

The winning criterion is the shortest solution.

If the question is seemingly vague, please comment on what you're unfamiliar with. Good luck ;)

  • 1
    \$\begingroup\$ would any which way mean returning the first character of the file name is fine? :P \$\endgroup\$
    – ASCII-only
    Commented May 27, 2018 at 11:08
  • 1
    \$\begingroup\$ "any way you want" is too open to abuse. x => 0 is a valid hash function, just have a lot of collisions. \$\endgroup\$
    Commented May 27, 2018 at 11:10
  • \$\begingroup\$ Cool thanks, very valid feedback :) \$\endgroup\$
    – LuckyPants
    Commented May 27, 2018 at 11:20
  • \$\begingroup\$ Your example has a mismatching [. (besides -- actually, the sandbox is relatively inactive, it's hard to get feedback) \$\endgroup\$
    Commented May 27, 2018 at 13:41

Given some lists of strings containing only lowercase letters, slice the first few (1 to length) characters for each string in each list, so that when each list is joined without separating symbol, it can't be any other list's possible expression.

For example, for {[ab,cd],[abcd]}, {ac,a} is valid, but {abc,a} is not because abc can also mean [abcd].

You can assume the result exist. If more than one solution, either output all of them or one of them. Shortest code win.


Link the pairs of points

Given some pairs of points on a plane, connect each pair with a simple polygonal chain such that any two different chains don't have any common points. The points won't necessarily be integral. Any two points in the input won't overlap.


[{(0,1.2),(1,2)}] -> [{(0,1.2),(2.6,1),(1,2)}] # Output doesn't need to be optimized
[{(0,0),(1,1)},{(1,0),(0,1)}] -> [{(0,0),(1,1)},{(1,0),(9999,0),(0,999),(0,1)}]

Reasonable I/O method allowed. Code-golf, shortest code win.

  • \$\begingroup\$ I think you want to describe a simple polygonal chain. \$\endgroup\$
    Commented May 28, 2018 at 5:37
  • \$\begingroup\$ Is there any two points in the input that have the same coordinate? \$\endgroup\$
    Commented May 28, 2018 at 5:38
  • \$\begingroup\$ @user202729 The chain shouldn't intersect with itself and other chains \$\endgroup\$
    – l4m2
    Commented May 28, 2018 at 10:23
  • \$\begingroup\$ Is it a sufficient answer to sort the points by angle and join them all up in order, and if not why not? \$\endgroup\$ Commented May 29, 2018 at 14:13
  • \$\begingroup\$ @PeterTaylor You are to link each pair, not to link all of the points together \$\endgroup\$
    – l4m2
    Commented May 29, 2018 at 14:22

Shortest code to generate a random ethereum address with matching private key pair

Ethereum is a blockchain platform. An Ethereurem public address consist of 256 bytes based on ECDSA encryption.

It’s very simple challenge about a service available on many websites without registering (which most of the time involve going on website main page and click the generate button) : generate a random ethereum address !

What is an Ethereum address ?

An Ethereum address is a public/private key pair hence the public address is just the public part of the key pair. Here’s an example of Ethereum address with private key after :

0x7bbaf5E447B089C5e0a802d07A2bc63870e90f05 651bfdb66c839ae0aa4ed59bbada63c13876feb60a89ca6dacc63a7b04cf4b60

Every private key and public key is valid as long as the length above is the same. But you need to output a private key which match a public key (It’s possible to retrieve the public key from the private key).
Example online generators : https://vanity-eth.tk/ https://forkdelta.github.io https://etherdelta.com https://walletgenerator.net/

Example code to generate an address in bash :


keys=$(openssl ecparam -name secp256k1 -genkey -noout | openssl ec -text -noout 2> /dev/null)

# extract private key in hex format, removing newlines, leading zeroes and semicolon
priv=$(printf "%s\n" $keys | grep priv -A 3 | tail -n +2 | tr -d '\n[:space:]:' | sed 's/^00//')

# extract public key in hex format, removing newlines, leading '04' and semicolon
pub=$(printf "%s\n" $keys | grep pub -A 5 | tail -n +2 | tr -d '\n[:space:]:' | sed 's/^04//')

# sha3 is a subset of the keccak familly function. Thus keccak is included in most sha3 tools.
addr=0x$(echo $pub | keccak-256sum -x -l | tr -d ' -' | tail -c 41)

echo 'Private key:' $priv
echo 'Address:    ' $addr


  • The resulting public address shouldn’t be fully random : the last byte should be 0. This can be achieved by generating about 256 random address and selecting the first which has a trailing null byte (the probability is 1∕256).
  • Print or return : the public key along the matching private key as a string.
  • The solidity has native functions to generate an address, so you don’t have to understand how to generate an address


The code which uses the fewest bytes.

  • \$\begingroup\$ Please explain what is unclear : codegolf.stackexchange.com/q/165724/50239 \$\endgroup\$ Commented May 30, 2018 at 8:21
  • \$\begingroup\$ Fetching the result of an online generator though an HTTP request is valid if you can get a public key with 1 or several null bytes at the end. Fetching results from an online source is usually a loophole forbidden by default. If you allow it here one could write their own generator which already handles the trailing zero requirement, put it on a website and post curl <some link shortener> as answer. \$\endgroup\$
    – Laikoni
    Commented May 30, 2018 at 9:22
  • \$\begingroup\$ @Laikoni : yes. I’m accepting curl answers… What’s the problem. This question does it to : codegolf.stackexchange.com/q/152844/50239 \$\endgroup\$ Commented May 30, 2018 at 9:36
  • \$\begingroup\$ The linked question explicitly states Standard loopholes are obviously not allowed, so I don't think that would be allowed. The general problem is that code-golf is about performing a certain task in the fewest number of bytes. Outsourcing the code which solves the task on a website and submitting code to the challenge which just gets the result from this page is thus generally not allowed, as your submitted code actually performs a different task, while the actual code that solves the task is not part of the score. \$\endgroup\$
    – Laikoni
    Commented May 30, 2018 at 9:50
  • \$\begingroup\$ @Laikoni is it fixed now ? \$\endgroup\$ Commented May 30, 2018 at 11:43

Create random recurrences with their solutions

(This is a work in progress and I would love any help please. First, how do I copy the wiki entry into this question without making a mess of the math as I have below? PPCG doesn't appear to support math rendering like the wiki does.)

The task is to create random recurrence relations that are solvable using the Akra-Bazzi method along with the solutions. This method solves recurrences of the form:

enter image description here

The conditions for usage are:

  • sufficient base cases are provided
  • a_i and b_i are constants for all i
  • a_i > 0 for all i
  • 0 < b_i < 1 for all i
  • |g(x)| = O(x^c), where ''c'' is a constant and ''O'' notates Big O notation
  • | h_i(x) | = O(x/(log x)^2) for all i
  • x_0 is a constant

The asymptotic behavior of T(x) is found by determining the value of p for which enter image description here and plugging that value into the equation

enter image description here

  • 1
    \$\begingroup\$ can you dumb this down so that people like me can understand it? maybe some examples? \$\endgroup\$
    – don bright
    Commented Jul 7, 2018 at 2:48
  • \$\begingroup\$ @donbright Yes absolutely. I need to rewrite using Mathjax too now it is supported. I will work on it next week. \$\endgroup\$
    – user9207
    Commented Jul 7, 2018 at 7:21
  • 1
    \$\begingroup\$ I have rewritten your post with code-golf delimited mathjax, here on pastebin: pastebin.com/TLM4D0Ri \$\endgroup\$
    – don bright
    Commented Jul 7, 2018 at 14:54

Divide two strings

One day, I saw the challenge to multiply two strings and I thought I might be able to do one better.

That challenge was fake. It was elementwise maximum. It was not real multiplication. So I set out to make something real. Real division between two strings.

I quickly realized that this would make an amazing challenge, as the algorithm was surprisingly complex and interesting to implement.

I then realized that it was actually easily reduced into a mere few operations. I'm still doing the challenge, though.

Enough with the backstory. Let's go.


To divide two strings, do the following, where x is the first string and y the second:

  • If x does not contain y, return a space and a period concatenated to x.
    • For example, testx and blah would become .textx, with a space at the beginning.
  • Otherwise, return every occurrence of y in x, a period, then y divided by x with every occurrence of y removed, with all the periods removed.
    • For example, eestestst and est would become estest.est.


Write a program or function that, given two strings via standard input, returns the first string divided by the second.

You may assume that neither input string contains a space, newline or period, and that the operation does not require more than 10 layers of recursion.

Test cases

test, es => es. es
test, blah =>  .test
okayye, y => yy. y
testes, es => eses. es
battat, at => atat. at
see, es =>  .see
see, e => ee. e
same, same => same. 
aabb, ab => ab.ab 
eestestst, est => estest.est
aheahahe, aheah => aheah.aheah ah


As this is , the submission with the least amount of bytes wins.

Sandbox questions

  • Is this a duplicate?
  • Have I missed anything?
  • Does anything need further explaining?
  • Is there an issue with the concept of the challenge?
  • \$\begingroup\$ What for aabb / ab? \$\endgroup\$
    – l4m2
    Commented Jun 1, 2018 at 12:42
  • \$\begingroup\$ okayye, y => yy. okay typo? \$\endgroup\$
    – l4m2
    Commented Jun 1, 2018 at 12:42
  • \$\begingroup\$ The test case battat, at => atat. by seems like it should be battat, at => atat. bt instead. \$\endgroup\$ Commented Jun 1, 2018 at 13:47
  • 1
    \$\begingroup\$ It seems odd to me that when no characters are matched the output format is {matched characters (empty)}<space>.{unmatched characters} while the format when there are matches becomes {matched characters}.<space>{unmatched characters}. I would much rather see consistent ordering of the . and <space> \$\endgroup\$ Commented Jun 1, 2018 at 13:50
  • \$\begingroup\$ @KamilDrakari 2 / 4 is 0.5, not .05, when 4 is not contained within 2 at all. Thus it makes no sense for test / blah to be . test when blah is not contained within test. We treat a space essentially like a zero would be with normal numbers. \$\endgroup\$
    – LyricLy
    Commented Jun 1, 2018 at 22:15
  • 1
    \$\begingroup\$ @LyricLy 6/4 is 1.5 not 1.05. If you're treating space as 0 then there shouldn't be any space after the dot in any of the test cases. \$\endgroup\$ Commented Jun 1, 2018 at 22:58
  • \$\begingroup\$ @KamilDrakari Actually, you're right. I was entirely mistaken because you get a fraction from 1 / remainder, not original number / remainder. Meaning this entire thing is wrong and I need to figure out a good replacement for 1 in string form. \$\endgroup\$
    – LyricLy
    Commented Jun 2, 2018 at 3:32
  • \$\begingroup\$ Fixed the issues. \$\endgroup\$
    – LyricLy
    Commented Jun 2, 2018 at 3:50
  • \$\begingroup\$ I accept downvotes, but I'd appreciate suggestions on how to improve the idea, or at least an explanation of what's wrong with it, so I know why I shouldn't post it. \$\endgroup\$
    – LyricLy
    Commented Jun 2, 2018 at 8:18
  • \$\begingroup\$ @LyricLy Please, post it if you haven't done it yet. It's so good! \$\endgroup\$
    – enzo
    Commented Jan 31 at 18:40
  • \$\begingroup\$ @enzo This challenge was posted. \$\endgroup\$
    – LyricLy
    Commented Feb 1 at 5:35


Often computers are used to perform consecutive jobs. Sometimes the order of doing these jobs does not matter. Other times there are dependencies between jobs that restrict the order that jobs can be performed. Additionally, jobs may require significant resources to store results, so it may be necessary to first schedule those jobs that will consume previous results before performing jobs that produce even more results. Furthermore, there may be significant costs to change from one type of job to another. The challenge then becomes finding the optimal execution order of jobs.


Allow me to illustrate the problem with an example. Consider a list of 10 jobs, numbered 0 to 9. Each of these jobs can be dependent on zero or more other jobs. (Assume there are no circular dependencies.) For example:

Job : Dependent on
---   ------------
0   : 8
1   : 6
2   :
3   : 0 6 9
4   : 7 9
5   : 2
6   : 4 8
7   :
8   : 9
9   :

This can be illustrated graphically as follows:


One possible solution to this particular example is to perform the 10 jobs in the order: 9 8 7 4 6 0 2 1 3 5 While this solution satisfies the dependency requirements, it is not ideal. To see why, let us perform the jobs one at a time:

Job : Results stored in memory
---   ------------------------
9   : 9
8   : 8 9
7   : 7 8 9
4   : 4 8 9
6   : 6 8 9
0   : 0 6 9
2   : 0 2 6 9
1   : 0 1 2 6 9
3   : 1 2 3
5   : 1 3 5

After performing the first job (job 9), we only have the result of job 9 in memory. After performing job 8 we need to store the results of 8 and 9, and after job 7 we have the results of jobs 7, 8 and 9. After performing job 4 we find that the results of job 7 is no longer needed and we can discard that, leaving only the results of job 4, 8 and 9. Carrying on we find that the worst case memory usage is after job 1 when we have to store the results of 5 different jobs (0, 1, 2, 6 and 9).

As it turns out there is a better solution to this example, and that is to do the jobs in the order: 7 9 4 8 6 0 3 2 5 1 The memory usage is then:

Job : Results stored in memory
---   ------------------------
7   : 7
9   : 7 9
4   : 4 9
8   : 4 8 9
6   : 6 8 9
0   : 0 6 9
3   : 3 6
2   : 2 3 6
5   : 3 5 6
1   : 1 3 5

This time we only ever needed to store the results of 3 jobs in memory, instead of the 5 in the previous solution.

Finally, consider that each job is of a certain "type", and changing from one type to another is something that should be avoided. For example, let's say we have 3 types of jobs: A, B and C, and our 10 jobs in the example have the following types: Jobs 0-3 is type A, jobs 4-7 is type B, and jobs 8 and 9 is type C.

Job : Results stored in memory : Job Type Change
---   ------------------------ : ---------------
7   : 7                          B
9   : 7 9                        C
4   : 4 9                        B
8   : 4 8 9                      C
6   : 6 8 9                      B
0   : 0 6 9                      A
3   : 3 6                        .
2   : 2 3 6                      .
5   : 3 5 6                      B
1   : 1 3 5                      A

As you can see, we had to change the job type 8 times: B C B C B A B A

When minimizing job changes, while still keeping maximum memory usage to 3, we now find that the optimal solution is: 9 8 7 4 6 0 3 1 2 5

Job : Results stored in memory : Job Type Change
---   ------------------------ : ---------------
9   : 9                          C
8   : 8 9                        .
7   : 7 8 9                      B
4   : 4 8 9                      .
6   : 6 8 9                      .
0   : 0 6 9                      A
3   : 3 6                        .
1   : 1 3                        .
2   : 1 2 3                      .
5   : 1 3 5                      B

This solution require only 4 job type changes (C B A B), while still only needing memory to store 3 results.

The solutions to this example that I presented here was found using a brute force search, ie going through every permutation of job order. From doing this I can tell you that this example has 1485 solutions that satisfy the dependency requirements. Of these 1485 solutions only 15 solutions require memory for no more than 3 results. Of these 15 solutions only 2 require only 4 job changes. The one is shown above (9 8 7 4 6 0 3 1 2 5). The other is: 9 8 7 4 6 0 3 2 1 5.

Unfortunately, brute force is not viable when the total number of jobs increase beyond 10, eg 100. The challenge is to come up with an algorithm that can find the optimal solution (or any one of them if there are more than one) using an algorithm that significantly outperforms brute force.

Example Input and Output

The input to the algorithm is a list of N jobs numbered 0 to N-1, each with a job type (A-Z) followed by a list of zero or more dependencies, eg:

0, A, 8
1, A, 6
2, A
3, A, 0 6 9
4, B, 7 9
5, B, 2
6, B, 4 8
7, B
8, C, 9
9, C

The output of the algorithm is the optimal execution order, eg:

9 8 7 4 6 0 3 1 2 5

Answers may be presented in any programming language or even pseudo code. I will translate them all to C++ and test them for correctness against the brute force algorithm using randomly generated inputs. The winner will be the algorithm that is fastest (and correct).

  • \$\begingroup\$ So, is this fastest-algorithm or fastest-code? \$\endgroup\$
    – wastl
    Commented Jun 9, 2018 at 18:35
  • \$\begingroup\$ What if there is a solution with memory 2, job changes 3, and one with memory 3, job changes 2? In other words, which one is given priority? \$\endgroup\$
    – wastl
    Commented Jun 9, 2018 at 18:39
  • \$\begingroup\$ @wastl I think fastest-code would be the easiest because it might get difficult determining which is the fastest-algorithm. Least memory usage has a higher priority than least job changes. \$\endgroup\$
    – Barnett
    Commented Jun 12, 2018 at 9:36

I want to do this, Render an ASCII maze but with ascii pipe characters.
I know this is similar, but I think the extra logic required to figure out which character to put at each position makes this sufficiently different.

here is what I am thinking of asking.

Write code that replaces non whitespace characters in a string with ascii pipe characters depending on adjacency to other non-whitespace characters.
this will create something like a maze where non-whitespace characters are the walls, and whitespace is abscence of walls here are the rules

your code should take its input from stdin and output to stdout

This table shows the correct output character for each combination of adjacent walls in directions: Left, Down, Up, and Right

 L D U R -> Output
 0 0 0 0 -> any non-whitespace character
 1 0 0 0 -> ─
 0 0 0 1 -> ─
 1 0 0 1 -> ─
 0 1 0 0 -> │
 0 0 1 0 -> │
 0 1 1 0 -> │
 1 1 0 0 -> ┐
 1 0 1 0 -> ┘
 0 1 0 1 -> ┌
 0 0 1 1 -> └
 1 1 1 0 -> ┤
 1 1 0 1 -> ┬
 1 0 1 1 -> ┴
 0 1 1 1 -> ├
 1 1 1 1 -> ┼

if characters are on the edge of a string then they should treat the outside as whitespace

This will be a code golf challenge

  • 1
    \$\begingroup\$ codegolf.stackexchange.com/q/80634/194 ? \$\endgroup\$ Commented Jun 6, 2018 at 8:51
  • \$\begingroup\$ I didn't see that one, but I still think this is different, because this does not require specific characters as input, the input can be any string \$\endgroup\$
    – JoshM
    Commented Jun 6, 2018 at 13:59
  • \$\begingroup\$ Can you add some test cases? \$\endgroup\$
    – 3D1T0R
    Commented Jun 6, 2018 at 18:18
  • 1
    \$\begingroup\$ Although there are some minor differences, this was probably be closed as a duplicate. \$\endgroup\$
    – wastl
    Commented Jun 6, 2018 at 19:23
  • \$\begingroup\$ i'll try to think of something to make this one different \$\endgroup\$
    – JoshM
    Commented Jun 6, 2018 at 19:30

N represented with only 1s in any base from 2 to N-2?

This is mostly a less interesting special case of Repdigit Base Finding so I won't submit.

Any positive integer N can be expressed as 11 in base N-1 or as N concatenated 1s in base 1. Let's call those the trivial cases in which N can be represented using only 1s. Let's call any other "all 1s" representation non-trivial. Some examples:

  • 7 has an "all 1s" representation in base 2, so 7 has a non-trivial "all 1s" representation.
  • 16 has no non-trivial "all 1s" representation.
  • 11 itself has no non-trivial "all 1s" representation by my definition because it is "all 1s" in base 10, which is 11-1.


Provide a program/function that produces one of two distinct, consistent values for whether or not an integer N > 3 has a non-trivial "all 1s" representation in a base between 2 and N-2.

  • \$\begingroup\$ I was thrown off by the 11 example, because there is quite clearly a way to express that using only 1, but then I realized that it's trivial because base 10 is base n-1 in that situation. That might be a good note to add. \$\endgroup\$ Commented Jun 6, 2018 at 15:54
  • \$\begingroup\$ This should generate the truthy numbers up to 1000 \$\endgroup\$ Commented Jun 6, 2018 at 20:37
  • \$\begingroup\$ @KamilDrakari very nice, thanks. I wonder how many numbers have more than one such base (like 31)? That's an entirely different challenge. \$\endgroup\$
    – ngm
    Commented Jun 6, 2018 at 20:42
  • \$\begingroup\$ I updated the code to search for duplicates, but could only find 31 and 8191. Golfed to fit the link in a comment \$\endgroup\$ Commented Jun 6, 2018 at 21:22
  • \$\begingroup\$ Table seems wrong \$\endgroup\$
    – l4m2
    Commented Jun 7, 2018 at 4:20
  • \$\begingroup\$ Not an exact dupe, but closely related: codegolf.stackexchange.com/q/75623/194 . \$\endgroup\$ Commented Jun 7, 2018 at 8:01
  • \$\begingroup\$ Note that these are numbers of the form (b**x - 1) / (b-1) for integer b > 1 and integer x > 2. \$\endgroup\$ Commented Jun 7, 2018 at 8:03
  • 1
    \$\begingroup\$ Also: OEIS A053696. Either your reference implementation was broken or you didn't remove duplicates. \$\endgroup\$ Commented Jun 7, 2018 at 8:10
  • \$\begingroup\$ @PeterTaylor thank you for alerting me to the other challenge, which is close enough for me to say this one is a dupe (at least and easier and less interesting special case.) \$\endgroup\$
    – ngm
    Commented Jun 8, 2018 at 18:24
  • \$\begingroup\$ Thanks for using the Sandbox! \$\endgroup\$
    – JayCe
    Commented Jun 9, 2018 at 23:10

# Proposed Question #

List the amount of factors for a given number.

For Example:

If given 1936, the output should be 15 (as 1936 has 15 factors)
If given 196, the output should be 9 (as 196 has 9 factors)

Test cases:

  • 64 - Outputs 7
  • 196 - Outputs 9
  • 1936 - Outputs 15


Code Golf - Least number of bytes for each language wins (No answer)

I have made my own in JavaScript - 143 Bytes.
Available here: https://glot.io/snippets/f1tl0vrnmy
To use different numbers, change the console.log(f(64).length) to console.log(f(###).length) where ### is your desired number.

BTW, if JavaScript or similar, only the defining function is considered in code length. The method of outputting (eg. console.log) does not count.

  • \$\begingroup\$ I'm not sure as I'm not a native speaker, but isn't that called "divisor" (not factor)? \$\endgroup\$
    – wastl
    Commented Jun 9, 2018 at 16:26
  • 1
    \$\begingroup\$ I fear it would get closed as a duplicate of this one \$\endgroup\$
    – JayCe
    Commented Jun 9, 2018 at 23:08
  • \$\begingroup\$ Exact duplicate. \$\endgroup\$
    Commented Jun 10, 2018 at 3:30
  • \$\begingroup\$ @wastl depends which country you are in (America or the rest of the world). \$\endgroup\$
    – Suda
    Commented Jun 11, 2018 at 1:41
  • \$\begingroup\$ @JayCe i don't think so the, as that is the prime factorisation of the number, where i am asking for the factor count. But, as with user202729, it is an exact duplicate. So, Thanks guys :D \$\endgroup\$
    – Suda
    Commented Jun 11, 2018 at 1:43
  • \$\begingroup\$ Thanks for using the Sandbox :) \$\endgroup\$
    – JayCe
    Commented Jun 11, 2018 at 2:06
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