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4481 Answers 4481

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105 106

Give counterexamples to thesis that prime numbers are finite.

It is known since antiquity that there are infinite prime numbers, first proved by Euclid. An unorthodox yet brief formulation of this proof was recently posted on MathExchange:

There are infinitely many primes, if not, multiply all and add 1.


Given a prime number N, output a prime number created by multiplying it with all lower primes and adding one, i. e. (2*3*5*...*N) + 1. In other words, if someone claimed that N is the largest finite number, you have to give a counterexample Euclid would likely compute to prove them false.

You don't have to consider input number that is not a prime.

Your representation of a number must have a range of 32-bit signed integers or larger.

You can use floating point representation as long as your code uses int-specific operations (e.g. no double-specific bit-fu).

Shortest code wins, standard loopholes forbidden. You may use a lookup table of primes.

Example Input and Output

2 → 3          // because             2 + 1 = 3
3 → 7          // because           3*2 + 1 = 7
5 → 31         // because         5*3*2 + 1 = 31
13 → 30031     // because 13*11*7*5*3*2 + 1 = 30031
31 → 223092871 // (largest possible output within `int32` range)


This is very similar to, yet not identical with A006862: your solution must output a number belonging to that sequence. The difference is that your solution must work for inputs that are primes, while A006862 recognizes any natural number (including 0).

If your solution is function f(), the following equivalence must be true:

A006862(n) = M <=> f(A000040(n)) = M

A000040 is the sequence of primes, therefore A000040(n) can be read as "n-th prime number".

  • \$\begingroup\$ In its current state I would have to vote to close this as unclear what you're asking. 1. What should the output be for input 13? 2. Why is the output for 31 not 200560490131? 3. How does it differ from A006862? If these issues are fixed then it likely becomes a duplicate of codegolf.meta.stackexchange.com/a/11638/194 \$\endgroup\$ Apr 29, 2017 at 18:20
  • \$\begingroup\$ @PeterTaylor, I hope my edit clarifies your doubts. \$\endgroup\$
    – Szymon
    Apr 29, 2017 at 22:50
  • 2
    \$\begingroup\$ Do you realise that 30031 = 59 * 509 and is therefore not prime? \$\endgroup\$ Apr 29, 2017 at 22:59
  • \$\begingroup\$ Oh, that's a game changer: the challenge is obviously still valid, but now it has little meaningful interpretation. I guess I'll leave it abandoned for future reference of other people not understanding Euclid's theorem. \$\endgroup\$
    – Szymon
    Apr 29, 2017 at 23:21

Rewrite a number in the Munafo System


The Munafo Alphabetic PT System (created by Robert Munafo) is a system of concisely writing very large numbers such that ordered values become lexicographically ordered when written. A full description (and some examples) of the system can be found in the link, but here's a series of steps to compute the string M(N) from a positive real number N:

  • If N < 10, then M(N) is simply the single digit of N, possibly followed by an factional part; that is, _ followed by any digits after the decimal place of N.
  • If 10 ≤ N < 100, then M(N) is the letter a followed by the two digits of N, possibly followed by an factional part.
  • If 100 ≤ N < 1000, then M(N) is the letter b followed by the three digits of N, possibly followed by an factional part.
  • If 1000 ≤ N < 10 000, then M(N) is the letter c followed by the four digits of N, possibly followed by an factional part.
  • If 10 000 ≤ N < 100 000, then M(N) is the letter d followed by the five digits of N, possibly followed by an factional part.
  • If 100 000 ≤ N < 10300, then M(N) is:

    1. the letter e
    2. exactly three digits, representing the exponent of N
    3. an underscore _
    4. any number of digits, representing the significand of N in standardized form.

    More specifically, a string M(N) = ex1x2x3_y1y2y3... represents the value N = y1.y2y3... × 10x1x2x3.

  • Finally, if 10300N, then to determine M(N), we must first apply the following recursive process:

    1. Set T = 0 and R = N.
    2. If R < 10300, then break out of this process.
    3. Otherwise, set T = T + 1, set R = log10(R), and return to step 2.

    (Effectively, this sets N equal to 1010R with T nested exponents.) Now, with our values of T and R, the value of M(N) is written as follows:

    1. the letter e
    2. the substring M(T)
    3. an underscore _
    4. the substring M(R).

...and that's the entire system. I apologize if anything I said was unclear or vague; in that case, you should definitely check the link for more detail.

META: Is there anything here that can be clarified?


As input, your program or function is to take three nonnegative numbers X, Y, and Z. X and Z are guaranteed to fit in your language's default integer type, and Y can fit in the default floating point type. These three values represent the number N = 10Y × 10Z, with X initial exponents as before. From here, the challenge is simple: given X, Y, and Z describing a value N, output the Munafo string M(N).

Test cases

META: Still figuring these out. Will probably end up using at least a couple from the link. Suggestions are welcome.


Same Same But Different

This challenge involves two similar digit-based routines each of which, when given a non-negative integer seed, produce eventually periodic sequences.
The goal is to find the nth seed number (counting up) that does not define the two sequences to be the identical (by virtue of the order of its digits), yet does yield sequences of equal pre-periodic length.

Reverse & Absolute Difference Routine.

A reverse and difference sequence may be defined in any given base for a non-negative seed by iteratively taking the absolute difference between the number and itself with its digits reversed.

An example:

The number 54410 is 10001000002;
The reverse of which is 00000100012, which is 1710;
The difference between the two is 52710;
Repeating the process we have 52710 = 10000011112, reversed is 11110000012 = 96110, and a difference of 43410;
Repeating again, 43410 = 1101100102, reversed is 0100110112 = 15510, and a difference of 27910;
Repeating again yields 18610, then 9310, and then 0, which will continue to yield 0.

  • So, the pre-periodic reversal and difference base-2 trajectory length of 54410 is six.
    - since the iterative procedure may be performed six times before a periodic part, {0}, is reached:

54410, 52710, 43410, 27910, 18610, 9310, {0}

  • Similarly the pre-periodic reversal and difference base-2 trajectory length of 9310 is one
    ...and the pre-periodic reversal and difference base-2 trajectory length of 0 is zero (as it is in any base).

Note: Ignoring the absolute part of this definition, many low bases have their iterations listed at the OEIS: base-2, base-3, base-4, base-5, base-6, base-7, base-8, base-9, base-10, base-11, base-12, base-16, base-20.

Kaprekar Routine.

A similar, but different sequence is the Kaprekar sequence where the absolute difference above is replaced by the difference, and the reversal is preceded by a digit-sort.

An example:

The number 83638110 is CC31D16;
Which, reverse-sorted is DCC3116, which is 90424110;
And forward-sorted is 13CCD16, which is 8110110;
The reverse-sorted less the forward-sorted is then 82314010;
Repeating the process we have 82314010 = C8F6416, reverse-sorted is FC86416 = 103434010, forward sorted is 468CF16 = 28897510, and the subtraction yields 74536510;
Repeating again yields 67983010, then 67167010, then 80682010, then 75352510, then 67167010...

  • Notice that we have hit a periodic part of the sequence, {67167010, 80682010, 75352510}, so the pre-periodic Kaprekar base-16 trajectory of 83638110 is four:

83638110, 82314010, 74536510, 67983010, {67167010, 80682010, 75352510}

Note:: Once again the OEIS lists the iterations for some low bases: base-2, base-3, base-4, base-5, base-6, base-7, base-8, base-9, base-10.
Furthermore some pre-periodic sequences also exist: base-2, base-3, base-4, base-5, base-6, base-7, base-8, base-9, base-10.

Same same...

Any number whose digits are already either forward or reverse sorted in some base will have the same pre-periodic trajectory length for both sequences in that base, since the resulting procedure will be identical.

There are, however, numbers which have the same pre-periodic trajectory length in both sequences even though their digits in the base in question are not already sorted.

The first example in base-10 is 107 which has a pre-periodic trajectory length of three in both sequences:

Reverse & difference: 107, 594, 99, {0}
Kaprekar: 107, 693, 594, {495}

Aside: Any number with less than three digits will always be sorted or reverse-sorted, as will the first three digit number, so the only others to actually check in base-10 before finding 107 would be [102-106].

The challenge

Write a program or function which when provided with a positive* integer, n, and an integer base greater than one**, b, outputs the nth number (in the natural ordering) which has neither sorted nor reverse-sorted digits in the given base, b, but which does have the same pre-periodic trajectory length for both procedures defined above in the given base.

The input and output are as flexible as ever, however the input and output numbers (or string or list representations thereof) are to be in a single unchanging base, consistent between input and output - for most this will probably mean base ten, but any single base is acceptable, as long as it is consistent across any invocations of the code.

* Feel free to have the counting (i.e. n) be 0-based if you would prefer, just say in your submission.

** Note there are no such numbers in base-1, so the code does not need to handle b=1 as an input.

Test cases (one-based n, IO in base-10)

 n     b  result
 1     2      17
 2     2      33
 3     2      51
 4     2      65
 5     2      73
 6     2      85
17     2     297

 1     3      28
 2     3      29
 3     3      32
 4     3      48
 5     3      51
 6     3      55
17     3     101

 1    10     107
 2    10     160
 3    10     161
 4    10     172
 5    10     186
 6    10     187
17    10     329

 1    16     264
 2    16     266
 3    16     355
 4    16     373
 5    16     400
 6    16     401
17    16     522

 1   257   66179
 2   257   98949
 3   257   98951
 4   257   98953
 5   257   98955
 6   257   98957
17   257   98979

 1  1234 1523375
 2  1234 2242181
 3  1234 2243417
 4  1234 2244655
 5  1234 2245897
 6  1234 2247147
17  1234 2260770

Formula 1 2016 standings

The Formula 1 2017 season is well underway, but I thought that the final standings could be good idea for a challenge.

The task is to print the standings at the end of 2016 season. This is how the table looks like:

1  N Rosberg    385
2  L Hamilton   380
3  D Ricciardo  256
4  S Vettel     212
5  M Verstappen 204
6  K Raikkonen  186
7  S Perez      101
8  V Bottas     85
9  N Hulkenberg 72
10 F Alonso     54
11 F Massa      53
12 C Sainz      46
13 R Grosjean   29
14 D Kvyat      25
15 J Button     21
16 K Magnussen  7
17 F Nasr       2
18 J Palmer     1
19 P Wehrlein   1
20 S Vandoorne  1
21 E Gutierrez  0
22 M Ericsson   0
23 E Ocon       0
24 R Haryanto   0

The table is divided into 3 columns: position at the end of season, driver name and points. In each of the columns, the contents can be aligned to left or to right for each column independently. If all columns are aligned to right, the output will look like this:

 1    N Rosberg 385
 2   L Hamilton 380
 3  D Ricciardo 256
 4     S Vettel 212
 5 M Verstappen 204
 6  K Raikkonen 186
 7      S Perez 101
 8     V Bottas  85
 9 N Hulkenberg  72
10     F Alonso  54
11      F Massa  53
12      C Sainz  46
13   R Grosjean  29
14      D Kvyat  25
15     J Button  21
16  K Magnussen   7
17       F Nasr   2
18     J Palmer   1
19   P Wehrlein   1
20  S Vandoorne   1
21  E Gutierrez   0
22   M Ericsson   0
23       E Ocon   0
24   R Haryanto   0

The columns should be separated exactly by 1 space. Trailing spaces are allowed as well as 1 trailing newline.

This is competition - shortest answer wins.

  • \$\begingroup\$ Thanks for posting in the Sandbox before posting on the main site ;-) In its current state, the challenge seems like it's just about finding the shortest way to encode all the drivers' names and how many points each had at the end of the season. While this may be an interesting challenge for some, many users on the site may find it fairly boring with not much room for real innovation. (You can tag it kolmogorov-complexity, btw) \$\endgroup\$ May 2, 2017 at 15:30
  • \$\begingroup\$ While reading this, I had an idea for a challenge where you take in a driver's last name and output the three-letter abbreviation, but that would be rather boring because that's currently just the first three letters of each name... \$\endgroup\$ May 2, 2017 at 15:34
  • \$\begingroup\$ @ETHproductions That could be more interesting with MotoGP or Superbike drivers \$\endgroup\$
    – AlexRacer
    May 2, 2017 at 16:54

Golf me a golf-lang

In any programing language of your choice write me an interpreter (or compiler targetting a reasonable pre-existing binary) for a golfing language. You will be scored based on a combination of the length of the interpreter and the golfiness of programs in your (user created language). In particular your goal will be to minimize the following formula.

max((Interpreter Length in bytes),512)/16-25*("checkmarks")

A good answer will not only contain an interpreter, but also contain code samples and documentation to encourage use of your language.

The number of "checkmarks" a language has earned is the number of times an answer in your language has beat or tied with the best answer (out of languages which are submissions to this challenge) to an open, positively voted (score of +1 or higher) question (whatever the win criterion) which postdate your language and were not asked by its creator(s) (in the event of a community wiki submission).

Your language will now be in a meta-competition around stack exchange, and you can earn checkmarks if other people choose to win other challenges with your competition.

Note that use of eval (or similar) is banned. You may write your interpreter/compiler in any preexisting programming language, and your language that you implement may be as similar as you wish to currently existing programming languages.

  • \$\begingroup\$ I'm worried that crafting a good solution to this will take a lot of time (thus meaning that late answers are fairly common) – it is language-design, after all – but the victory condition means that there's a hard limit to how long answers can arrive. Perhaps the languages should be somehow tested on how they fare on questions that postdate the languages themselves? \$\endgroup\$
    – user62131
    Apr 12, 2017 at 21:23
  • \$\begingroup\$ If a challenge is selected which a language can't implement (e.g. one which requires a GUI, but the language only does I/O via stdio), how is it scored? \$\endgroup\$ Apr 12, 2017 at 21:27
  • \$\begingroup\$ @PeterTaylor I decided to go a completely different way, challenges your language cant complete neither hurt or help you. \$\endgroup\$ Apr 12, 2017 at 22:26
  • \$\begingroup\$ @ais523 I restructured the challenge, I liked the idea of being tested based on how you fare on languages that post date yourself, this way later answers are at a disadvantage, but they can still compete (and win). \$\endgroup\$ Apr 12, 2017 at 22:27
  • 2
    \$\begingroup\$ There are plenty of good reasons to not award an accepted answer tick (e.g. some challenges have a reason to preserve the sort order), so not all challenges will have one. Change that to count questions on which your language has the best answer or tied for the best answer (based on the challenge's victory condition), and I like the new scoring. \$\endgroup\$
    – user62131
    Apr 12, 2017 at 22:29

How many times will you have to golf a quine?

For this challenge, you must create a programme which takes an integer x and outputs its source x many times.


  • This is codegolf, the programme containing the least amount of bytes wins.

  • If you submit a function, the function must take x as a parameter and either return or print the body of the function x many times to STDOUT

  • If you submit a lambda, it is not required for you to assign it to a variable

  • Standard loophole restrictions apply.

  • Empty programmes are forbidden

  • \$\begingroup\$ There's really no reason not to use code formatting for x. The image doesn't even load for me, I had to check the markdown to see what you're trying to say. \$\endgroup\$
    – Riker
    May 2, 2017 at 17:53

Steal my credit card

recently a lovely man named Boris that I met online sent me a copy of an album that I didn't even know was released yet, the 16kb '.exe' ('EXtremely Excellent-music' apparently) file didn't open, but I did notice that it had managed to make some charges against my credit card, now I'm normally very secure in my banking, I don't write passwords down at all, and I keep my bank details in a '.txt' (Truly eXceedingly proTected) file, so I'm not sure how the hackers managed to get access, to confirm how secure I am:

  • I keep a .txt file named 'info.txt' in my documents folder
  • There's lots of unimportant information in this
  • I store the credit cards in a specific format
  • All of my credit cards have the same number of digits in the number and cvv
  • the 'Type' of card may be different, but it will always be in the format Cardtype - MM/YY
  • I don't format anything else in a similar manner, you can assume that this is the only two lines together.


Visa - 10/18

(not a real credit card, i'm not an idiot)

which corresponds to:

Type - MM/YY (Expiry date)

The format will always match:

[a-zA-Z]+ - \d{2}/\d{2}\r?\n\d{16}/\d{3}

so it's very difficult to be read by any automatic parsing program.

my insurance doesn't believe me, so I'm here to prove that it's basically impossible, and since we all know programmers are actually just hackers with day jobs, I figured you would all be able to help me.

after adding all of the default Java libraries together and doing some math I believe there's almost no room left for any actual code in the exe file he sent me, so please try and keep the program as short as possible.

The Challenge:

write a program which takes no arguments, and when run will:

  • read in a file named 'info.txt' from the same directory the program is run in.
  • somewhere in the file, two lines will correspond to the above format
  • return only those two lines
  • any amount of leading or trailing white-space is allowed.
  • you can assume one credit card per file
  • you do not need to do any validity checks on the card number etc.

attached is my original info.txt if you want to test against it, but it should work with any credit card number, that way it will be more believable.

[gdrive link to info.txt]

This is so shortest code in each language wins.

Any code, interpreted or compiled, on any system, in any language is valid.

  • \$\begingroup\$ It's probably best that the victory condition isn't mentioned in just the fluff at the start; I missed it first time I read this. You probably need to confirm what counts as a credit card number or as a CVV. Given the talk about .exe files in the fluff, you should also confirm what languages are allowed (it's likely best if you allow any language, even interpreted languages). \$\endgroup\$
    – user62131
    May 5, 2017 at 12:06
  • \$\begingroup\$ @ais523 added some more clarification there, and a regex to match the two lines. \$\endgroup\$
    – colsw
    May 5, 2017 at 12:23
  • \$\begingroup\$ Minor correction: the second Z in the regex should be capitalized (should be [a-zA-Z] not [a-zA-z]) \$\endgroup\$ May 5, 2017 at 13:56
  • \$\begingroup\$ @BusinessCat oopsie, thanks. \$\endgroup\$
    – colsw
    May 5, 2017 at 15:14

Is the line closed?

Given input consisting of only whitespace characters and -/\|, tell if the line created by -\/| is closed.

For this challenge, \ points one character up and to the left, as well as one character down and to the right. / points up and to the right, and down and to the left. | points one to one character above and below it. - points one character to the left and right of it.

# means it is being pointed to.





A line is a bunch of /\-| characters that point to each other.

\    |   |
 \---/   \

The characters - and | cannot validly point to each other. Similarly, / and \ cannot point to each other either.





A closed line is one where each of its characters validly point to another of its characters.

|  |
\--/ = CLOSED

|  |


Your task, as said above, is to write a program that says if a line is closed.

Test cases coming soon.

  • \$\begingroup\$ Your -/ connections violate your own specification. I think the clearest specification would be to enumerate all possible legal pairs of connected line segments. \$\endgroup\$
    – user62131
    May 8, 2017 at 13:47
  • \$\begingroup\$ @ais523 My mistake, I will fix both this and the other. \$\endgroup\$
    – sporkl
    May 8, 2017 at 14:14

Determine if a relation is an equivalence relation

(Content to be added.)



Uncompress a permutation

I have a number of permutations, of which this is an example:

[0] 1 2 3 15 14 26 27 28 29 30 31 32 33 5 4 16 17 18 19 20 21 22 23 35 34 6 7 8 9 10 11 12 13 25 24 36 37 38 39 [40]

(0 is used for 0-indexed permutations and 40 for 1-indexed permutations.) In order to save space, I want to compress the permutation by reducing each ascending run of consecutive numbers to its first element, giving:

0/1 15 14 26 5 4 16 35 34 6 25 24 36

This compression method is invertible, because each missing element from the compressed form must follow its predecessor among the integers.

Your challenge is to write a program or function that takes a compressed permutation as input and returns an uncompressed permutation. Your input will consist of an array or list or other reasonable type representing the compressed permutation, plus an integer number of elements in the uncompressed permutation. Examples:

[0], 2 -> [0, 1] (0-indexed)
[1], 2 -> [1, 2] (1-indexed)
[6, 2, 0], 8 -> [6, 7, 2, 3, 4, 5, 0, 1] (0-indexed)
[7, 4, 1], 9 -> [7, 8, 9, 4, 5, 6, 1, 2, 3] (1-indexed)

In order to make the compression worthwhile I obviously need as short a decompression routine as possible, so this is a challenge.

  • \$\begingroup\$ I don't think there's actually a specification of the decompression routine, and the one which I would reverse engineer from the examples at the end is not compatible with the first example. \$\endgroup\$ May 8, 2017 at 14:34
  • \$\begingroup\$ I believe the specification is just "for each number that doesn't appear in the compressed permutation, place it to the right of that number minus 1". It might be worth specifying that in the question. \$\endgroup\$
    – user62131
    May 8, 2017 at 15:11
  • \$\begingroup\$ @PeterTaylor The specification is "Create a permutation of the first n integers which, when compressed by removing consecutive numbers after the first, results in the input array/list". \$\endgroup\$
    – Neil
    May 8, 2017 at 18:33
  • \$\begingroup\$ I've made an edit which I think makes that clearer. \$\endgroup\$ May 8, 2017 at 21:24

An implementation of a game called Nomic

Nomic is a game where you can change the rules. Humans are biased judges though, so let's have a computer run the rules! (Note that there are many variations of Nomic. Below is just one example.) Since you're minimalists, you want the original ruleset to be as short as possible.

Your program first accepts as input a list of players. You must be able to accept any list of 2 to 16 unique alphanumeric usernames (you may assume that the list of players names are valid according to this criteria).

Next print The name of this game is nomic. The goal is to get the last line of output of this program to be "[username] is the winner!", where [username] is your username.

Now, for each player print [username], do you want to propose a revision to the rules? If so, type the revised code below: where [username] is the player's username. (This input must be able to accept multiple lines.) If the input is empty, proceed to the next player.

If not, for each player print [username], do you want to accept or reject this change to the rules?(y/n), where [username] is the player's username. (You may assume input to this will either be y or n.) If the number of ys are more than the number of ns, execute it (in the language you are writing the implemenation). If not, proceed with the next player.

If the code executes without an error, the program ends. If not print The ruleset proposed by [username] has crashed. The game will proceed with the original ruleset. where [username] is the players username. Then proceed to the next player after them.

Note: After all the players have either proposed a rule or passed, start with the first player again.

This is . The shortest Nomic implementation wins! (You get 0 characters off your score if you show an example game you played with your friends. This bonus is doubled if all the revised rulesets are golfed.)

  • \$\begingroup\$ You say a computer has to "run the rules", so is each rule typed by a user an executable piece of code? Basically, your description of what the implementation needs to look like is extremely vague. Be specific. \$\endgroup\$
    – mbomb007
    May 12, 2017 at 16:55
  • \$\begingroup\$ Having long hardcoded strings in a challenge is generally a bad idea unless it's a compression challenge, as the question will be mostly about compressing the strings rather than about what it's meant to be about. One possibility is to not count the strings against the byte count if they appear literally in the code. \$\endgroup\$
    – user62131
    May 12, 2017 at 17:01
  • \$\begingroup\$ @mbomb007 the rule should be a piece of executable code. If it isn't, the program announces the error. \$\endgroup\$
    – PyRulez
    May 12, 2017 at 17:14
  • \$\begingroup\$ @ais523 that's a little tricky since it involves a little bit of string splicing. Is there a way around that, or should I just shorten them? \$\endgroup\$
    – PyRulez
    May 12, 2017 at 17:15
  • 3
    \$\begingroup\$ Probably best at this point to just use codes for the various situations, e.g. using just V to request someone to vote. \$\endgroup\$
    – user62131
    May 12, 2017 at 18:00

Output the Balmer Series


Write a program which outputs the following image (note, it is an svg):

It is the emission spectrum of hydrogen, caused by the Balmer series: https://en.wikipedia.org/wiki/Hydrogen_spectral_series#5

Your program must not read the image from the internet or a local file.

The image may be to an image file or a graphics window. If not a vector, the image sides must be greater than 400 pixels.

Your image must have the same colours and same ratios of spacing between each line.


Shortest code wins.

  • \$\begingroup\$ What does it mean to output the image "exactly"? It's obviously not pixel-for-pixel for two reasons: firstly, because the image isn't raster, and secondly, because the spec allows for variation in image size. \$\endgroup\$ May 12, 2017 at 22:22

Compute Area of Convex Hull of Roots of Polynomial

Given a nonzero polynomial with integer coefficients, find the area of the convex hull of the complex roots of the polynomial.


The polynomial can be given in any reasonable format, e.g. as a list of coefficients. The area must be correct to 4 significant figures. You can assume that the degree of the polynomial is no larger than 8, and that the coefficients are less than 8^8 = 16777216, (and by Rouché's theorem the roots have therefore an absolute value of less than 8^8 + 1).


The polynomial x^4 - 1 has the roots {1,-1,i,-i} which form the vertices of their convex hull, which is a square of sidelength sqrt(2) so the total area is 2.

The polynomial x^5 - x has the roots {1,-1,i,-i,0}. But the convex hull is again given by the vertices points {1,-1,i,-i}.

The constant polynomial 1 has no roots, and therefore the convex hull of its roots is empty.

The polynomial x^2 - 1 has the roots {1,-1}, so their convex hull is just a line segment, which has no area.

In the following the polynomials are given as a list of coefficient of the monomials in descending degree. (i.e. the constant comes last)

[1,2,3,4,1] 2.0382
[1,2,3,4,5] 3.5830
[1,-1,1,-1] 1.0000
[a,b] 0 (where a != 0 and b arbitrary)
  • \$\begingroup\$ For what range of inputs must the area be correct to 4s.f.? \$\endgroup\$ May 13, 2017 at 14:43
  • \$\begingroup\$ Oh I forgot to add that, thanks for mentioning! \$\endgroup\$
    – flawr
    May 13, 2017 at 14:44

Hidden Strings

This is real rough, I just wanted to get the idea down.

The strings utility that comes on many GNU/Linux distros finds the strings in executables and lists them. The goal of this challenge is to write a program that has a null-terminated string in the compiled executable (compiled languages only, sorry) but that string does not appear in the source. The scoring is the ratio of the length of the hidden string to the size of your source code.

The following constraints apply:

The string must be entirely dictionary words from todo dictionary with length greater than 2 concatenated together without spaces and without punctuation. If xyz and abc are words in the dictionary, the following is a valid string: xyzabcxyzabc, but the following is not xyabcz (assuming that it is not a concatenation of other strings in the dictionary). For those of you who care, if E is the dictionary of words of length greater than 2, then strings must be in E* where * is the Kleene star.

"Hidden" means that the string and its substrings may not appear as strings or characters in your source. This definition needs a bunch of work.

This challenge was supposed to be something like "Hey, how would you hide a funny string so that it doesn't appear obviously in the source, but does appear when someone looks at the executable". I welcome all suggestions because as I have it currently stated, I think there are a lot of problems.

  • \$\begingroup\$ I'm guessing that a minimal C program already contains a certain number of strings in the binary? Does this come down to creative you get with compiler flags, asking it to statically link as many libraries as possible? \$\endgroup\$ May 15, 2017 at 4:28
  • \$\begingroup\$ I suggest specifying a particular string that has to be hidden, in order to prevent solutions based on the default behaviour of the toolchain. (This would also simplify the question somewhat) Additionally, that string should probably be banned from thing like the filename and the current directory. \$\endgroup\$
    – user62131
    May 15, 2017 at 17:58

Write a TIO bot

Write a program/function which:

  1. Takes three strings Language, Code, and Input
  2. Do one of the following:

    1. Assume access to a browser open on https://tio.run/nexus/; click [data-id="Language"
  3. Go to https://tio.run/nexus/Language/

  4. Enters Code in the <textarea id="code">
  5. Enters Input in the <textarea id="input">
  6. Clicks the <span id="run">
  7. Waits for any of these things to happen:
    1. <textarea class="output"> changes
    2. <textarea id="debug"> changes
    3. 60 seconds pass
  8. Returns/prints the content of <textarea class="output">

Alternatively, you may use the TIO API.

  • 1
    \$\begingroup\$ I'd allow people to use the API, instead of using the HTML. \$\endgroup\$ May 15, 2017 at 15:58
  • \$\begingroup\$ @NathanMerrill Added. But good luck finding out what it is. \$\endgroup\$
    – Adám
    May 15, 2017 at 16:23
  • \$\begingroup\$ The problem with the API is that it's bound to change until it becomes stable. Incidentally, this is why I'm not documenting it at this point, as nobody should use it while it's still in flux. \$\endgroup\$
    – Dennis
    May 15, 2017 at 18:20

Convert ambiguous dates to "the correct way" (YYYY-MM-DD)

A problem I run into often with spreadsheets of data is dealing with dates of unknown ordering.

XKCD says it best

Your task is to take a series of dates that are in one of the following three formats, and output them as YYYY-MM-DD:


No other date formats are acceptable.

Since many dates are ambiguous, you must read all the dates to determine whether the set is MM/DD or DD/MM before outputting anything.

Your program must also detect the following error conditions and return an error code instead (with no other output):

  • -1: Unable to determine date ordering. (All dates provided could be DD/MM or MM/DD)
  • -2: Inconsistent date ordering. (There are dates provided that are definitely DD/MM and others that are definitely MM/DD, within the same set).
  • -3: Bad data. (There are things included that are not valid dates, as defined above, including things like 31/04/2017 and 12/1/1980).

For the purposes of this challenge, February always has 28 days, April, June, September and November have 30 days, and all others have 31 days. All years, from 0001 to 9999 are equivalent.

Sample input/output

['11/05/1988','12/12/1960'] => -1
['15/05/1988','03/18/2001'] => -2
['15/05/1988','31/06/1989'] => -3
['15/05/1988','2014-02-02', '18/03/2001'] => ['1988-05-15','2014-02-02', '2001-03-18']
['05/15/1988','03/18/2001'] => ['1988-05-15','2001-03-18']


You can receive data in whatever way is normal for your language (line-separated stdin, array of strings, whatever). You may assume that the data you receive is of the correct type (ie, your array won't contain integers, nulls etc), but may contain empty strings.

If outputting to standard output, errors can be sent to stderr, or as program termination code. If implementing a function, you can return an integer instead of an array, throw an error, or whatever seems natural.


The score for your program is L - 2Y + 20D where:

  • L is the length of your code, in bytes
  • Y is 2017 minus the year in which the first version of the language you're using was published.
  • D is the number of times the letters 'date', in any case, appear consecutively in your code.

For the purpose of Y, all versions of Lisp are 59, C is 39, C++ is 32, all EcmaScripts are 20 etc. Languages are considered the same if they have the same name (other than the version number). (No cheating by using some golf language that happens to be called C though.)


(what else?)

  • 1
    \$\begingroup\$ I think your scoring mechanism isn't going to work. It's probably best you stick to plain old shortest code. \$\endgroup\$
    – Okx
    May 16, 2017 at 11:33
  • \$\begingroup\$ Any particular reason why? \$\endgroup\$ May 16, 2017 at 22:11
  • \$\begingroup\$ Possible dup \$\endgroup\$ May 16, 2017 at 23:17
  • \$\begingroup\$ @SteveBennett If your score decreases by 20 each time 'date' is in the code, then if someone fits an infinite amount of 'date' into a comment, they win. \$\endgroup\$
    – Okx
    May 17, 2017 at 9:28
  • \$\begingroup\$ It increases, not decreases...? \$\endgroup\$ May 17, 2017 at 11:21
  • \$\begingroup\$ @DigitalTrauma They're both in the same ballpark but a lot of differences, including fundamental task (reformatting vs outputting a single probability), input formats (strict mm/dd/yyyy vs m-d-yy) etc. \$\endgroup\$ May 17, 2017 at 11:24

Be the Richest Mammut!

The Game

This game is an adaptation of a board game called Mammut, and basically an optimization problem.

Each match contains five players and five rounds, in which every player participates. Every round starts with a bunch of tiles being flipped at random on one of their sides, and put in the middle stack The tile composition will be fixed, but has not been completely decided yet There will be 30-35 of them, and each of them will have two different sides, with exactly 15 different types of tiles, and about half of those only having one single instance available. A random player is chosen as the starting player for the first round, and performance will decide the starter for the following four rounds. In order, each player that doesn't own any tiles will take an action, and this will keep going on until every player has at least one tile. The actions available are:

  1. Take an arbitrary number of tiles from the middle stack.
  2. Take all of the tiles owned by another player, and put one of them back in the middle stack.

Obviously, you can take the entire middle stack, but if there is at least one other player without tiles and you take too many, he will most likely steal your tiles right after.


Every player starts with 10 points, and if they end a scoring phase under zero, they will be given enough free points to go back to zero.

When everyone has tiles, the round is over, and we move on to scoring that round. Since this is played by bots and not humans, an infinite loop is possible, so after 1000 actions, the round will be stopped by force, and the players participating in the infinite loop who ended up empty ended will naturally end this round with a terrible score.

Scoring happens all at once, and you get (or lose) points according to the tiles you have, and the tiles the other players have. When scoring is done, we move on to the next round, or declare the winner if that was the fifth round.

The Tiles

There are different types of tiles, and you job is basically to find the best balance between them while not having enough to make your stack worth stealing.

  • Fire | The player who has the fewest fire tiles loses 5 points
  • Meat | The three players owning the most meat tiles get 8, 5 and 2 points respectively
  • Fur | Every fur tile gives you onw "fur point". During every round after the first, the player with the most fur points plays first. At the end of each round, the player with the most fur tiles gets the round number in points (1-5), while the one who has the least loses the round number in points.
  • Ivory Tile | Every Ivory tile gives you two points.
  • Axe Tile | (This one should be discussed) Every axe tile gives you one point, and every axe Tile obtained in the last round is Worth 3 points
  • Animal Tiles | There are 7 different animals (Bat, Opossum, Bunny, Buffalo, Squirrel, Beaver and Duck), and only one of each in the game. When you end the round with one, you "obtain" this animal, and getting the same one again will do nothing. At the end of the game, depending on the number of animals you have obtained, you get points. (0: 0, 1:0, 2:0, 3:4, 4:9, 5:15, 6:22, 7:30)
  • Mystery Tile | If you get a mystery tile, it will be turned around to reveal its other side during the scoring phase. The other side can't be another mystery tile or an animal tile.
  • Tie-Breaker Tile | This tile serves as a tie breaker. One side is the "clockwise" side, and the other side is the "counter-clockwise", so every round will contain a tie-breaker tile. During any and all ties during scoring, the tie-breaker tile decides who wins or loses.


At the start of the game, every player will be given a random ID between 1 and 5. The player with ID 1 will start the game. Then, for the following rounds, the most fur points decides who starts, and the ID decides the order. Example: ID 3 has the most furs, then the order will be [3, 4, 5, 1, 2].

This is considered "clockwise". The counter-clockwise order would be [3, 2, 1, 5, 4]. This is important, because when dealing with ties, the tie-breaker tile takes the order into account.

If player 4 owns the clockwise tie-breaker tiles, and has 3 meats. Player 1 and 2 also have three meats. The first, second and third place in meat then go to players ID 4, 1 and 2 respectively. If everyone has 3 fires, except player 3 and 5 who have 1, then player 3 is the one to lose points. Having the tie-breaker tiles means you are first in the tie-breaking order, and that tile's visible side decide the rest of the order.

Challenge specifications

You must make a C# class that inherits from a "Player" class. You must also override the method

List<int> ChooseTiles(Dictionary<int, List<Tile>> otherPlayers, List<Tile> center)

This method takes a dictionary of player IDs and List of tiles (could be empty), that are each player's tiles at the moment, plus a list of tiles in the middle. You must return a list of integer, which are the IDs of the tiles you want. If you try to get tiles illegally (not following the instructions for taking an action, which are either any number of tiles from the middle or all tiles except one from a player), you will be asked to choose one more time, and if you fail to follow the rules, you will be counted as "attempting to cheat" and removed from the "playing" pool during this round, so you will get scored for an empty hand. The same thing goes for throwing exceptions and infinite loops (Execution lasting more than 5 seconds). You are given one more chance then given an empty hadn for that round.

Here is the class Player you will be inheriting from:

public class Player
    public int ID;
    public int currentPoints;
    public int currentFurs;
    public List<Tile> currentTiles;
    public List<TileType> animalsGotten;

    public Player(int id)
        ID = id;
        currentPoints = 10;
        currentFurs = 0;
        currentTiles = new List<Tile>();
        animauxGotten = new List<TileType>();

    public virtual List<int> ChooseTiles(Dictionary<int, List<Tile>> otherPlayers, List<Tile> center)
        throw new Exception("Fake players don't get to play!");

    public virtual string GetName()
        return "Player";

You should also override the GetName() function, but it is not required. You are allowed to create as many private functions or attributes as you want to store data during a game, but each match will create a new instance of your player.

The Tournament

tournament will be played in a round-robin fashion. You are not garanteed a match with every single combination of four opponents available, but you are garanteed at least one match in common with every other bot, and everyone will play the same number of matches. You will also not play twice with the same 5 players. The total number of matches will depend on the number of participants. For each match, the first player will get 5 points, and the others will get respectively 3, 2, 1 and zero points depending on their points earned during the match. Furthermore, and ties in points will not be resolved, and the points of the occupied positions split between tied players. This means that if the points are (40, 37, 37, 37, 35), the points are (5, 2, 2, 2, 0)

  • \$\begingroup\$ People usually don't like KotH challenges that are restricted to one language. See some other KotH challenges for ways to get around that. \$\endgroup\$
    – Stephen
    May 16, 2017 at 13:47

Extension of MIME-Type

Your task, given this list of extension and MIME-types, is to choose exactly 20 of them and create a program that takes in either e, the extension, or m, the MIME-Type, and outputs its counterpart.

Example (if my set was [.pdf, .doc]):

Cases to handle:

f('.pdf') = application/pdf
f('application/pdf') = .pdf
f('application/msword') = .doc
f('.doc') = application/msword

Cases you don't have to touch:

f('Nonsense') = [Error, null, false, whatever]

I & O

  • Input will be a single string, so will output.
  • You must handle 20 mime-type combinations and mention which ones you are handling.
  • Trailing spaces and newlines are fine.
  • Extensions always include the preceding period.
  • When choosing a long extension like .cryptocode, if you return the MIME-type of .cryptocode for an input of .cry it will be considered an invalid submission, remember this.


  • 3
    \$\begingroup\$ kolmogorolofvlfoao-complexity??? \$\endgroup\$
    – user58826
    May 17, 2017 at 0:06
  • \$\begingroup\$ Be sure to include the list in the question itself when you post this on main. We want questions to be self-contained. \$\endgroup\$ May 17, 2017 at 3:53
  • 2
    \$\begingroup\$ "When choosing a long extension like .cryptocode, if you return the MIME-type of .cryptocode for an input of .cry it will be considered an invalid submission, remember this." - why? Do I have to detect if the input is one of my set? This will reduce my compression ratio significantly. \$\endgroup\$ May 17, 2017 at 3:56
  • \$\begingroup\$ @JanDvorak it's too big for posting here, and if pastebin goes does a lot of the internet would lose a lot of data. \$\endgroup\$ May 25, 2017 at 17:04
  • \$\begingroup\$ @programmer5000 you can post it if you want. \$\endgroup\$ Aug 27, 2017 at 2:59
  • \$\begingroup\$ Concerning the "exactly 20" requirement: Are we allowed to output anything we want if fed something not in that 20 or does it need to output a falsey value? The addition of the word "whatever" added confusion for me. \$\endgroup\$ Mar 13, 2018 at 14:22
  • \$\begingroup\$ @programmer5000 Tab-separated version for OP's benefit. \$\endgroup\$ Mar 13, 2018 at 14:31

I have a large collection of fine art. My toddler is learning to use scissors and glue; lately she has started playing with my art collection. Fortunately she is really quite good with the scissors and cuts things up into perfect squares. She then uses glue to randomly tile the cut-up squares back into a new piece of art. For example, she reinterpreted my Mona Lisa (which wikipedia kindly hosts for me) as follows:

enter image description here

The following python script simulates what my toddler has done:


import sys
import random
from PIL import Image

origname = sys.argv[1]

im = Image.open(origname)
width, height = im.size
width = (int(width + 99) / 100) * 100
height = (int(height + 99) / 100) * 100

im = im.crop((0, 0, width, height))

im2 = Image.new("RGB", (width, height), "black")

blocks = []
for x in range(width / 100):
    for y in range(height / 100):
        blocks.append(im.crop((x * 100, y * 100, (x + 1) * 100, (y + 1) * 100)))


im2.save("shuf" + origname)

Please excuse python skills - I'm still learning, but was happy to see how quick and easy it was to write this script. Polite code-reviews will be graciously accepted ;-)

It does the following:

  • loads the image whose file name was given as a command-line parameter
  • pads that image with black pixels such that the width and height are exactly divisible by 100
  • divides the image into 100x100 pixel blocks
  • randomly shuffles the blocks
  • reassembles the randomly arranged blocks back into a new image with the same size attributes as the (padded) original
  • saves the new image using the original filename prefixed with shuf

Your task is to write a program that takes the output of this script, analyses the edges of each 100x100 block and reassembles them back to the original picture.


  • an image filename. This may be passed at the commandline, via STDIN or even hard-coded, whichever is most convenient.


  • either output the a singular rearranged image to a differnent file, or display the singular rearranged output to the screen.

Input and output specs are intended to be lenient here, as filename-parsing is a non-goal of this question.

Other rules

  • The program must be able to correctly reassemble any random arrangement of the wikipedia Mona Lisa by the python script. Hard-coding of block transformations of the example image above is strictly not allowed.

  • It is understood that for some degenerate cases (e.g. chequerboard of 100x100) blocks it is impossible to correctly rearrange the image. In those cases it is acceptable to produce incorrect/undefined output. However I think in general for photo images and almost all "famous" artworks, reassembly should be possible. Works by Mark Rothko are a possible exception.

  • Common image manipulation libraries may be used (e.g. Python's PIL), but any APIs expressly designed for this purpose are strictly banned.

  • Standard “loopholes” which are no longer funny

Now my toddler got a hold of The Scream. This is the result that needs to be fixed:

enter image description here

Special thanks to Digital Trauma!

He wrote the original challenge and gave it up original post here


I need to update the images and the script to not have rotations and to not have black bars. Any ideas?

  • \$\begingroup\$ The way to make this objective is to have a large list of images (like 50 or so), and require 100% accuracy. You should also reserve the right to regenerate the test cases if anybody is hardcoding the input. \$\endgroup\$ May 16, 2017 at 16:01
  • \$\begingroup\$ Please credit the question that the inspiration (or direct copy ?!?) came from. Also, please note the comments to that question very carefully. In its current form, there significant problems (up to and including impossible to get 100% accuracy). \$\endgroup\$ May 16, 2017 at 22:52
  • \$\begingroup\$ @DigitalTrauma Sorry. I forgot to add that. It was a copy with edits based on you giving it up to gift exchange \$\endgroup\$
    – user63187
    May 17, 2017 at 0:38
  • \$\begingroup\$ Somebody suggested using lossless image formats, which would help a lot in terms of edges now being exact matches. That might be less interesting though. Nathan suggested a large list of images, which could give it the test-battery tag perhaps? My two suggestions I believe are at odds. \$\endgroup\$
    – nmjcman101
    Jun 20, 2017 at 19:25

Make some arrays

A recent post on The Daily WTF concerned the creation of four two-dimensional arrays of height n. Please write a function to create these arrays for me. Here is an example for n=4:

[[1] [[1 0 0] [[0] [[0 0 0]
 [0]  [0 1 0]  [0]  [1 0 0]
 [0]  [0 0 1]  [0]  [0 1 0]
 [0]] [0 0 0]] [1]] [0 0 1]]


  • All four arrays must be two-dimensional. Arrays which have been flattened in any way are not acceptable.
  • You can write separate functions for each array, in which case your score is the total of the scores for each function.
  • Your functions can call each other, provided you include the name in your score. Otherwise unnamed functions are acceptable.
  • Rather than returning the array(s), you can be passed them in as an input parameter (e.g. using int** in C).
  • If your language has a builtin for inputting arbitrary data structures, then you can write a full program that outputs using the input format that the builtin would recognise.
  • Alternatively, you can write a full program that outputs source code for the arrays in your language.
  • All standard loopholes are banned.
  • This is , so the shortest function wins.
  • \$\begingroup\$ Hmmm, I wonder if you can reconstruct these four arrays if you just evaluate that code block as is. If so, that might be a fun challenge too. :) \$\endgroup\$ May 17, 2017 at 12:08

Parse a 4-dimensional array

A 2D array is fairly easy to represent in ASCII art, something like this:

1  2
6 24

One approach for a 3D array is to have a number of 2D arrays, with an extra blank line between each array:

1  2
6 24

1  2
4  8

Things get a little more complicated for 4D arrays though. The best you can do is to arrange a number of 3D arrays side-by-side, but with at least two spaces between each array:

1  2   1  2
6 24   3  4

1  2   1  4
4  8  27 64

So that I don't have to ban full programs, the challenge will be to input 4 (numeric) indices along with a 4D array which will be as some sort of string in the above format, and output the (numeric) 4D array element at those indices. For example, if the 1-indexed indices were 1, 2, 1, 2 then the result would be 4. Rules:

  • You must at a minimum support integers from 0 to 255, written in decimal.
  • The array will not be ragged, but you will not be given the size of the array. Any of the dimensions may be 1, so 0 0 0 0 0 is a valid input in 0-based indexing.
  • Numbers in the array will be consistently justified (please state whether you support left or right, or decimal justification if you support floating-point numbers), but the output must not contain any whitespace except for an optional trailing newline.
  • You may choose 0 or 1-based indexing.
  • You can choose row-major format instead of column-major format, as long as both outer and inner arrays have the same format. The first two indices will however always refer to the outer array and the last two indices to the inner array indexed by the first two indices.
  • You can assume that the indices will always be valid.
  • All standard loopholes are banned.
  • This is , so the shortest program or function wins.
  • \$\begingroup\$ some sort of string in the above format so no JSON? \$\endgroup\$
    – Stephen
    May 17, 2017 at 13:24
  • \$\begingroup\$ @StephenS Correct, you won't be given an actual array, you'll be given an ASCII-art printout of the array. \$\endgroup\$
    – Neil
    May 17, 2017 at 13:26

Generating strings of Rs and spaces

If a task is to make a string of 16 characters using 2 different characters, it can be done in 2^16=65536 ways. Let's generate them all!

Write a program or function which takes a number in range [0, 65535] as input and outputs a string of 16 characters consisting of R and  (space). One number corresponds to one string and must generate the same string each time the code is run.


Suppose that a program/function takes a binary representation of the number and replaces zeros with spaces and ones with Rs to make the string. The conversion from input to output will look as follows:

0     -> "                "
1     -> "               R"
2     -> "              R "
3     -> "              RR"

Sandbox notes

  • This challenge probably needs a better name. If you have an idea, please mention it.
  • I know that this challenge will be boring if making the string from binary representation is the shortest way in all languages. I hope that this isn't the case.
  • \$\begingroup\$ The only reason I can think of that making it from the binary representation would not be the shortest way is if there's a shorter way with a Cartesian product built-in, but that's still boring. \$\endgroup\$ May 18, 2017 at 8:23


N boxes are lined up in a sequence (1 ≤ N ≤ 20). You have A red balls and B blue balls (0 ≤ A ≤ 15, 0 ≤ B ≤ 15). The red balls (and the blue ones) are exactly the same. You can place the balls in the boxes. It is allowed to put in a box, balls of the two kinds, or only from one kind. You can also leave some of the boxes empty. It's not necessary to place all the balls in the boxes.


Write a program, which finds the number of different ways to place the balls in the boxes in the described way.


Input contains one line with three integers N, A and B separated by space.


The result of your program must be an integer written on the only line of output.

Sample I/O

2 1 1

Simplify ordinal expression



Split the atom

Input: 0 <= X <= 2³² - 1

Output: List of numbers in decimal, after recursive splitting in binary format.


Example 1:

Input: 255

Current Output is 255.

Binary representation of 255 is 1111 1111. Splitting it, we get 1111 and 1111, which in decimal are 15 and 15.

Continuing the Output, we will have 255 15 15.

Now the numbers 15 and 15 will serve as inputs and these numbers are to be split.

Continuing the Output, we will have 255 15 15 3 3 3 3.

Continuing the logic, final Output will be 255 15 15 3 3 3 3 1 1 1 1 1 1 1 1 . And since 1 can no longer be split, the output stops.

Example 2:

Input: 225

Current Output is 225.

Binary representation of 225 is 1110 0001. Splitting it, we get 1110 and 0001, which in decimal are 14 and 1.

Continuing the Output, we will have 225 14 1.

Now the numbers 14 and 1 will serve as inputs and these numbers are to be split.

Since 1 is no longer split-able, the Output will be 225 14 1 3 2.

Final Output will be 225 14 1 3 2 1 1 1 0.

Example 3:

Input: 32

Output: 32 4 0 1 0.


  1. If the number of binary digits are odd, the first number will have one fewer binary digit than the next one. Example, 20 (10100) will be split as 10 and 100, with decimal output being 2 and 4.
  2. Standard loophole rules apply.
  3. 0s and 1s do not propagate further.
  4. Program crashing for trying to display too many numbers is a valid exit condition.
  • 1
    \$\begingroup\$ Surely you reversed the inequality signs. \$\endgroup\$
    – Leaky Nun
    May 18, 2017 at 11:16
  • \$\begingroup\$ s/Splitting it/Splitting it in half/ \$\endgroup\$
    – Leaky Nun
    May 18, 2017 at 11:17
  • \$\begingroup\$ @LeakyNun Yes, yes I did :D \$\endgroup\$ May 18, 2017 at 11:20
  • \$\begingroup\$ @trichoplax I updated the conditions. Is that clearer? \$\endgroup\$ May 18, 2017 at 11:34
  • \$\begingroup\$ @trichoplax I hope 3 examples clears everything up. \$\endgroup\$ May 18, 2017 at 11:53
  • \$\begingroup\$ I think this is a duplicate of an earlier question, but I haven't found it by searching for binary split. Perhaps someone else will be able to find it. \$\endgroup\$ May 18, 2017 at 11:58
  • \$\begingroup\$ @ctrl-shift-esc yes that clears up the treatment of the zeroes \$\endgroup\$ May 18, 2017 at 14:05

Pi - The Movie!


So we've had the musical version The Sound of Pi, now its time for the movie.


Your challenge is to write a full program or function that outputs an animated version of pi to 100 decimal places.

For your convenience, here's Pi to 100 decimal places


Input and Output

Input: No input required

Output: A single gif file


  • Only one digit or decimal point should be shown at a time
  • Each digit or decimal point should be shown for for between 0.25 and 1.5 seconds (your choice).
  • The font should be clearly visible (different colour to background)
  • The characters should be at least 100 pixels high and have at least a 1 pixel boarder
  • You can use built in values of pi as long as they're correct to 100 dp
  • Standard loopholes apply
  • This is , so the submission with the least amount of bytes wins!

Sandbox questions

  • Is the time constraint realistic?
  • What tags should I use?
  • \$\begingroup\$ I'd VTC this as "too broad". \$\endgroup\$ May 18, 2017 at 13:56
  • \$\begingroup\$ @EriktheOutgolfer could you explain why? a VTC on its own doesn't help me improve it :) \$\endgroup\$
    – Notts90
    May 18, 2017 at 14:01
  • \$\begingroup\$ Basically there are many ways you could make a "video file" (file formats), the font size could be invisibly small, the background color may be the same as the foreground color, digits might not fully appear (i.e. be cut) etc. \$\endgroup\$ May 18, 2017 at 14:05
  • \$\begingroup\$ i think a gif with fixed size would be better \$\endgroup\$ May 18, 2017 at 14:06
  • \$\begingroup\$ @EriktheOutgolfer added size and colour constraints. I've already specified the format should suitable for vlc or wmp. \$\endgroup\$
    – Notts90
    May 18, 2017 at 14:18
  • \$\begingroup\$ @Notts90 saying it's suitable for vlc or wmp does not say much, something that runs in my vlc may not run in yours (installed codecs and stuff) \$\endgroup\$ May 18, 2017 at 14:48
  • \$\begingroup\$ I'll go with the gif option then! \$\endgroup\$
    – Notts90
    May 18, 2017 at 16:32


Wardialing was a very interesting way to try to hack people back in the '80s and '90s. When everyone used dial-up, people would dial huge amounts of numbers to search for BBS's, computers, or fax machines. If it was answered by a human or answering machine, it hung up and forgot the number. If it was answered by a modem or a fax machine, it would make note of the number.


Your job is to make a URL wardialer. Something that tests and checks if it's a valid website from one letter of the alphabet.


  • Program must take user input. This input has to be a letter of the alphabet, no numbers. Just one letter of the alphabet and form multiple URLs that start with the letter.
  • Standard loopholes apply.
  • You must make 8 URLs from 1 letter, and test to see if it is a valid site.
  • If you hit an error (not a response code), instead of leaving it blank, go ahead and return a 404
  • If you hit a redirect (3xx), return a 200 instead.
  • You may output the results in any reasonable format, as long as it includes the website name, status codes for all the websites and the redirects
  • This is Code Golf, so shortest amount of bytes wins.

What counts as a URL for this challenge?

http://{domain-name}.{com or net or org}

For this challenge, the domain name is should only be 4 letters long, no more, no less.

What should I test?

For each 4 letter domain name, test it against three top-level domains (.com, .net, .org). Record all the response codes from each URL, remember from the constraints that any (3xx) should return 200 and be recorded as a redirect in the output and any error getting to the website should result in a 404.




| Website | .com | .net | .org | Redirects? |
| ajoe    | 200  | 200  | 200  | .com, .net |
| aqiz    | 200  | 404  | 404  | no         |
| amnx    | 200  | 503  | 404  | .com       |
| abcd    | 200  | 404  | 200  | .com       |
| ajmx    | 200  | 503  | 404  | no         |
| aole    | 200  | 200  | 200  | .com       |
| apop    | 404  | 200  | 200  | .net       |
| akkk    | 200  | 200  | 200  | .com       |
  • 2
    \$\begingroup\$ I don't like the formatting aspect; it adds a whole unnecessary layer to the meat of challenge. I also don't like the fact that you have to go through .com, .net, and .org for the same reason. But I do like the concept, and I think if you stay closer to the actual challenge ("generate a few websites and check if they exist") it could be well-received on main. \$\endgroup\$ May 22, 2017 at 19:43
  • \$\begingroup\$ but the formatting looks sooooo good \$\endgroup\$
    – kuantum
    May 22, 2017 at 19:44
  • \$\begingroup\$ Well then, maybe you should post a challenge idea to format a table like that ;-) I think we may have already had that challenge though. \$\endgroup\$ May 22, 2017 at 19:45
  • \$\begingroup\$ ehhh, nah. what should i replace it with \$\endgroup\$
    – kuantum
    May 22, 2017 at 19:47
  • \$\begingroup\$ I'd allow users to output in any reasonable format they like: an array of arrays, a newline-separated string, etc. A table like that would still be allowed, though it probably wouldn't be the shortest. \$\endgroup\$ May 22, 2017 at 19:49
  • \$\begingroup\$ URLs for websites all start with the letter h. If you want to make an argument about relative URLs, I will concede that they can also start with the non-letter /. But abusing a clearly defined term like URL to the extent to which it's abused in this draft only serves to confuse people. The task is: given an input of one letter of the alphabet, generate 8 alphabetic labels, from each of those labels to generate three domain names, and for each of those names test the response code of the URL http://{name}/ \$\endgroup\$ May 22, 2017 at 21:46
  • \$\begingroup\$ Also, in "Something has more than 4 letters in it", more than is exclusive, so this is not consistent with the example. \$\endgroup\$ May 22, 2017 at 21:47
  • \$\begingroup\$ There. I think this edit helps @PeterTaylor \$\endgroup\$
    – kuantum
    May 22, 2017 at 22:17

Program Equilibrium Iterated Prisoner's Dilemma Tournament

This is an iterated prisoners dilemma tournament. You are to make a bot that plays the prisoners dilemma against the other contestants, but with a twist- you can perfectly simulate your opponent!

Iterated Prisoner's Dilemma

The prisoner's dilemma is a two-player game in which players can choose to either "Cooperate" or "Defect" against their opponent. The payoff matrix for the prisoner's dilemma looks like this:

                           Player 2
                       C              D

                |              |               |
            C   |    (3, 3)    |    (0, 5)     |
                |              |               |
Player 1        |------------------------------|
                |              |               |
            D   |    (5, 0)    |    (1, 1)     |
                |              |               |

In the iterated prisoner's dilemma, multiple rounds are played in succession, and players can use their knowledge of the previous rounds when making a decision.

The twist

Bots are also allowed to run their opponent and examine the output before making a move. This allows for significantly more complex strategies in which bots can simulate their opponents to determine if it is safe to cooperate with them, as well as attempt to exploit less sophisticated players.


There will be a program available in the path called simulate that you can use to simulate what your opponent would do against you

simulate [instruction to your own bot] [fake history to the opponent]
simulate cooperate 00 10 01 11 01 10

All arguments are optional, and you may also use underscore _ for no argument. If you do not explicitly state the history, the real history will be used.

Example output from the simulate program:


The first number is you, the second is your opponent. 0 means defect, 1 means cooperate.

The instruction

It wouldn't be much use to simulate what your opponent would do against you, because he'd probably simulate you right back, and then you'd be stuck in an infinite loop.
The instruction is your way to simulate what the opponent would do against another agent. You could for instance have a cooperate instruction that causes your bot to cooperate mindlessly, to see what the opponent would do if you were to cooperate.


Your program will be given the following input at the beginning of each round:

Instruction (if any)
Your score
Your opponents score
Current round number
Complete history of past interactions with this bot. Space separated: the first number is you, the second is your opponent


00 10


0 for defect or 1 for cooperate, with any number of leading characters. Trailing characters (including 0 or 1's) will be ignored.



The tournament

The tournament will be round-robin elimination: Each bot will play one match against all other bots, where a match consists of 100 rounds of the prisoner's dilemma. At the end of the round-robin round, the lower-scoring half of the tournament pool will be eliminated. This process will be repeated until only one bot remains, or there is a tie.


  • Your program has to run on Linux. Please provide a brief description of how to run your program if possible
  • Your program has to give an answer within 5 seconds. Failing to do so will be interpreted as defect.
  • Your program may not perform multiple simulations at the same time
  • You may only enter a single bot, this is to prevent people from entering multiple bots that cooperate
  • You may not read or write any files. If this is required by your language, please state so in your answer.
  • You may not in any way store state between rounds. This is to prevent bots from figuring out whether they are being simulated.
  • If you exploit another loophole to figure out whether you are being simulated, you will be disqualified. Guessing (or strategical testing) is obviously allowed

About the host computer

Arch Linux 4.10.13-1
Intel(R) Core(TM) i7-7500U CPU @ 2.70GHz, 4 cores
I will disconnect from the internet while running the tournament


I got the idea as well as some of the text from this Github repository. I have not directly asked the author for permission to use the challenge, but he said that "if you want to run this tournament on your own, you are more than welcome to" in a blogpost.


I will update this periodically whenever new bots enter.


How do I best discourage/prevent players from intentionally using every bit of their time limit to sabotage other players trying to simulate them? I am thinking random time limits and/or penalties

  • 1
    \$\begingroup\$ Have you ever run a KotH? If not, I think you are about to learn how incredibly slow a round can be even if the programs have very simple logic. With the simulate instruction I anticipate taking hours to run a single round. \$\endgroup\$ May 22, 2017 at 21:50
  • \$\begingroup\$ @PeterTaylor That's good advice; I might not want to run this on my main computer \$\endgroup\$
    – BlackCap
    May 22, 2017 at 22:54

Identify the variable case

Your task is to write a function which determines which casing convention a provided variable name belongs to:

  • camelCase
  • kebab-case
  • Train-Case
  • snake_case
  • PascalCase
  • Ada_Case (not a real term, but apparently the standard casing used in Ada)

Your function should return one of the above strings, either exactly as above, or as spaced title-case: "Train case", "Screaming snake case".

Casings that include - or _ may include leading or trailing -s or _s (eg, __MY_SNAKE__ or -webkit-kebab).

Multiple consecutive upper case letters are allowed in mixed casings, as long as they follow a lower case letter (MyXMLDocument, This-CSS-Crap).

Where an input fits multiple schemes (eg, F or foo), you may return any valid classification.

If an input does not fit one of the above schemes (eg stu-pid_case), you must return something different. You may also throw an error.

The input string is guaranteed to be at least one character, and begin with a character matching /[a-zA-Z_-]/. Every other character will match /[a-zA-Z0-9$-]/

This is , so shortest length in bytes wins. Standard loopholes forbidden. Standard input and output methods.

Test cases

myFunkyVariable => camelCase
no-op => kebab-case
__c => snake_case
XML-Madness => Error
Get_Input => Ada_Case
i => camelCase (or snake_case)
char2int_factory => snake_case
  • \$\begingroup\$ You should specify exactly what each of the cases requires. (e.g. train case is separated by underscores with only the first character capital) Because this is a competition there should be no room for interpretation. \$\endgroup\$
    – Wheat Wizard Mod
    May 26, 2017 at 4:56
  • \$\begingroup\$ What ambiguity have I missed? \$\endgroup\$ May 26, 2017 at 5:19
  • \$\begingroup\$ You should define what each of the 7 cases entails. Currently you just list 7 examples, which is certainly helpful, but I don't think sufficient. \$\endgroup\$
    – Wheat Wizard Mod
    May 26, 2017 at 5:30
  • \$\begingroup\$ You really don't think they're sufficiently self-explanatory? I mean, I can add explanation, but I'm curious if you have an example of a variable name that you don't know how to classify? \$\endgroup\$ May 26, 2017 at 5:35
  • \$\begingroup\$ I think I get it based on the examples and I think any reasonable person would be able to jump to the correct conclusion, but we should strive to make questions as concrete as possible. Some people might not be aware of casing in the first place and definitions could help. It may seem like I'm making a fuss over nothing, and perhaps I am, but I think it could only benefit to add specific definitions of the terms, since the question is about identifying them. As far as examples go, I believe E_S_S is ambiguous between screaming snake and Ada. Definitions might help clear that up. \$\endgroup\$
    – Wheat Wizard Mod
    May 26, 2017 at 5:46
  • \$\begingroup\$ That one is ambiguous, which, as stated, means that either answer is ok. But I'll add an introductory paragraph about casing. \$\endgroup\$ May 26, 2017 at 5:58

21 = (1 + 3 + 5 + 7 + 9) - (1 + 3)

Sandbox question:
Is this different enough from the original challenge?


  • This is a sequel to my previous challenge, but this time with a slightly more interesting practical application (the factorization of the input).

  • It is based on 1) the fact that any odd integer n can be expressed as the difference of two squares a² - b² and 2) the well known formula:


The task

You're given a positive odd integer n. Your task is to determine how many iterations of the following algorithm need to be processed to reach n.

  • Initialization: find the highest integer k such that k² ≤ n. Build the list L = [ 1, 3, 5, ..., 2k-1 ].
  • Iteration:
    • Step #1: If the sum of the terms of L equals n, stop here.
    • Step #2: Increment k and append 2k-1 to L.
    • Step #3: While the sum of the terms of L is greater than n, remove the first term from the list. Go on with step #1.

All iterations (either full or partial) must be counted. In other words, you must return the number of times step #1 was executed.

Example #1

This is a trivial example for the perfect square n = 9:

  • Initialization: k = 3 because 3² ≤ 9 < 4², which leads to L = [ 1, 3, 5 ].
  • Step #1: 1 + 3 + 5 = 9. We're done.

We went through one iteration. So the expected result is 1.

Example #2

Here is what we get for n = 21:

  • Initialization: k = 4 because 4² ≤ 21 < 5², which leads to L = [ 1, 3, 5, 7 ].
  • Step #1: 1 + 3 + 5 + 7 = 16, which does not equal n.
  • Step #2: We increment k and we add 2k + 1 = 2 x 5 - 1 = 9 to the list: L = [ 1, 3, 5, 7, 9 ].
  • Step #3: We remove the first term from the list: L = [ 3, 5, 7, 9 ]. The sum of the terms is now 24, which is still greater than n. So we remove another term: L = [ 5, 7, 9 ].
  • Step #1 : 5 + 7 + 9 = 21. We're done.

We went through two iterations. So the expected result is 2.

Example #3

Below is a summary of all steps for a slightly more complex example with n = 145.

Initialization: k = 12 because 12² ≤ 145 < 13².

Iteration | Step | k  | L                                 | Sum
    1     |  1   | 12 | 1,3,5,7,9,11,13,15,17,19,21,23    | 144
          |  2   | 13 | 1,3,5,7,9,11,13,15,17,19,21,23,25 | 169
          |  3   | 13 | 11,13,15,17,19,21,23,25           | 144
    2     |  1   | 13 | 11,13,15,17,19,21,23,25           | 144
          |  2   | 14 | 11,13,15,17,19,21,23,25,27        | 171
          |  3   | 14 | 17,19,21,23,25,27                 | 132
    3     |  1   | 14 | 17,19,21,23,25,27                 | 132
          |  2   | 15 | 17,19,21,23,25,27,29              | 161
          |  3   | 15 | 19,21,23,25,27,29                 | 144
    4     |  1   | 15 | 19,21,23,25,27,29                 | 144
          |  2   | 16 | 19,21,23,25,27,29,31              | 175
          |  3   | 16 | 23,25,27,29,31                    | 135
    5     |  1   | 16 | 23,25,27,29,31                    | 135
          |  2   | 17 | 23,25,27,29,31,33                 | 168
          |  3   | 17 | 25,27,29,31,33                    | 145
    6     |  1   | 17 | 25,27,29,31,33                    | 145

We went through 6 iterations. So the expected result is 6.

Factorization of the input

This paragraph is for illustration purposes only. It describes how the factorization of the input can be deduced from the result of the algorithm.

Factorization of 21:

  • 21 = 5 + 7 + 9 = (1 + 3 + 5 + 7 + 9) - (1 + 3)
  • 21 = 5² - 2²
  • 21 = (5 + 2)(5 - 2)
  • 21 = 7 x 3

Rules and clarifications

  • The input is guaranteed to be a positive odd integer. It may be either a composite integer or a prime.
  • You're required to process any odd n in [ 1, ..., 65535 ] in less than one minute on mid-range hardware.
  • Given enough time, your program/function should theoretically work for any value of n that is natively supported by your language. If it doesn't, please explain why in your answer.
  • You may use any other method instead of the described one as long as it returns the correct result.
  • This is code golf, so the shortest answer in bytes wins!

Test cases

To be completed

  • \$\begingroup\$ Typo: you wrote "k²=n", which can't possibly be correct (and contradicts the rest of the post). \$\endgroup\$
    – user62131
    May 26, 2017 at 3:30
  • \$\begingroup\$ @ais523 Thanks for catching that out. (I believe all my "less than or equal to" signs where turned into equal signs when I copy/pasted through Notepad++.) \$\endgroup\$
    – Arnauld
    May 26, 2017 at 7:10
  • \$\begingroup\$ 1. There's also an extra = in the LaTeXified image. 2. You say it's a variant on an earlier question of yours, but it seems so similar that I don't see why it wouldn't be a dupe. If it isn't, I definitely think it's a dupe of codegolf.stackexchange.com/q/18349/194 : the two questions are essentially "Find the factorisation of n into two parts which are as close as possible to sqrt(n). \$\endgroup\$ May 26, 2017 at 8:12
  • \$\begingroup\$ @PeterTaylor Thanks for your feedback. Yes, I think it's a dupe indeed. \$\endgroup\$
    – Arnauld
    May 26, 2017 at 8:16
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