Motivation: Sometimes certain items in a list don't count towards your totals. For example, counting plane passengers in rows, where babies sit on a parent's laps.

Challenge: write a program to split a list of items into chunks. Each chunk (except possibly the last) is the same size, where size is defined as the number of items passing a predicate function.


  1. Your program must take
    • a list of items
    • a positive integer chunk size
    • a predicate function (takes an item, and returns true or false)
  2. You must return the input list split into chunks
  3. Each chunk is a list of items
  4. Overall the items must remain in the same order, with none disgarded
  5. The number of items passing predicate in each chunk (except possibly the last) should match the input chunk size.
  6. items failing the predicate should not count toward this size
  7. Items failing the predicate are
    1. still included in the output chunks
    2. allocated to the earliest chunk, in the case that a chunk is "full" but the next items are ones failing the predicate
      • thus the final chunk may not consist solely of items failing the predicate
  8. The final chunk may be of size less than the chunk size because all the items have been accounted for.

Non exhaustive examples:

The simplest example is to consider 1s and 0s, where the predicate function is x ==> x > 0. In this case, the sum of each chunk must match the chunk size.

  • items: [], size: 2, predicate: x > 0 --> either [] or [[]]
  • items: [0, 0, 0, 0, 0, 0], size: 2, predicate: x > 0 --> [[0, 0, 0, 0, 0, 0]]
  • items: [0, 1, 1, 0], size: 2, predicate: x > 0 --> [[0, 1, 1, 0]]
  • items: [0, 1, 1, 0, 1, 0, 0], size: 2, predicate: x > 0 --> [[0, 1, 1, 0], [1, 0, 0]]
  • items: [0, 1, 0, 0, 1, 0, 1, 1, 0], size: 2, predicate: x > 0 --> [[0, 1, 0, 0, 1, 0], [1, 1, 0]]

And let's finish with the plane passengers where babies sit on a parent's lap example. A for adult, b for baby, plane row is 3 seats wide, adult always listed before their baby:

  • items: [A, b, A, b, A, A, A, b, A, b, A, A, b], size: 3, predicate: x => x == A --> [[A, b, A, b, A], [A, A, b, A, b], [A, A, b]]
  • 6
    \$\begingroup\$ This looks like a good question, but currently it's lacking a winning criterion. I suggest using code-golf. \$\endgroup\$
    – Laikoni
    Commented Jul 29, 2018 at 22:56
  • 3
    \$\begingroup\$ Can we assume the list items are single characters? Or, say, numbers? \$\endgroup\$
    – xnor
    Commented Jul 29, 2018 at 23:04
  • \$\begingroup\$ chunking sounds interesting, though could maybe be replaced by set-partitions. \$\endgroup\$ Commented Jul 29, 2018 at 23:28
  • \$\begingroup\$ @Laikoni tagged code-golf \$\endgroup\$
    – Tom Viner
    Commented Jul 30, 2018 at 0:32
  • 1
    \$\begingroup\$ @Ourous I've added "because all the items have been accounted for", i.e. the last chunk doesn't get a chance to get "full up", because that's just the end of the input items. \$\endgroup\$
    – Tom Viner
    Commented Jul 30, 2018 at 1:09

15 Answers 15


Brachylog, 37 bytes


Try it online!

I was pleasantly surprised to find that this - pretty much a restatement of the question - successfully terminates and produces correct output.

Assumes the predicate is present as predicate 2 below this code. Outputs a list of lists ("chunks"), or false for an empty input.


hW&               % First input is W, the expected "weight" of each chunk
                  %  (i.e. the number of items passing predicate in each chunk)

t                 % Take the second input, the list of items
 ~c.              % Output is a partition of this list
    k{    }ᵐ      % For each partition (chunk) except the last, 
      ↰₂ˢ         %   Select the items in the chunk that pass the predicate
         l        %   Get the length of that
                  % (So now we have the list of the "weights" of each chunk)
            ;Wx   % Remove the input expected weight from this list, and 
               Ė  %  the result of this should be empty.
                  %  This verifies that the list of weights is either 
                  %  composed of all W-values, or is empty (when input is [0 0 0] for eg.)

    ∧.bhᵐ↰₂ᵐ      % And, the first element of each chunk (except the first) should
                  %  pass the predicate. This is another way of saying:
                  %  "Items failing the predicate are allocated to the earliest chunk"

    ∧.t↰₂ˢl≤W     % And, the final chunk (which we haven't constrained so far)
                  %  should have weight ≤ the input expected weight
                  %  This disallows putting everything in the final chunk and calling it a day!

    ∧             % (no further constraints on output)

Apl (Dyalog Unicode) 17 16 bytes (SBCS)

Thanks to Adám for saving me 1 byte.


Try it online! for explaination purposes I will leave up the 17 byte solution.


⍺⍺¨⍵ aplies the predicate to the list returning a boolean vector
+\ generates a running total
1⌈ replaces leading 0s with 1s
⌈⍺÷⍨ divides each element by the chunk size and rounds up
⍵⊆⍨ partitions the original vector by this

  • 2
    \$\begingroup\$ That's impressive! And I like the output display, appropriate visual for the problem. \$\endgroup\$
    – Sundar R
    Commented Jul 30, 2018 at 12:41
  • \$\begingroup\$ Save a byte by converting to program (tradfn body): w⊆⍨⌈⎕÷⍨1⌈+\⎕¨w←⎕ \$\endgroup\$
    – Adám
    Commented Aug 2, 2018 at 17:54

Clean, 96 92 bytes

Uses a named function f :: a -> Bool allowed according to meta consensus.

import StdEnv,StdLib
$l n|l>[]=last[[i: $t n]\\i<-inits l&t<-tails l|n>=sum[1\\e<-i|f e]]=[]

Try it online!

Expanded (with default highlighting to make comments show up):

$ l n // define function $ on `l` and `n`
 | l > [] // if `l` is not the empty list
  = last [ // the last element of ...
                   \\ i <- inits l // prefixes of `l`
                    & t <- tails l // matching suffixes of `l`
                    | n >= // where n is greater than or equal to
                           sum [1 \\ e <- i | f e] // the number of matching elements in the prefix
          [i: $t n] // prepend that prefix to the application of $ to the rest of the list
 = [] // if `l` is empty, return empty

Haskell, 72 bytes

p#s|let l@(h:t)!a|sum[1|e<-h:a,p e]>s=a:l![]|n<-a++[h]=t!n;_!a=[a]=(![])

Try it online!

p#s     = (![])         -- main function that takes a predicate function 'p',
                        -- a size 's' and a input list without a name that is
                        -- directly passed as the first argument to function '!'
  let  l@(h:t)!a        -- function '!' is a local function (so that 'p' and 's'
                        -- are in scope). Takes a list 'l' of at least one element
                        -- 'h' (and 't' being the rest of the list) and an
                        -- accumulator 'a'
   |sum[1|e<-h:a,p e]>s -- if 'p' holds for more than 's' elements in 'h' and 'a'
     =a:l![]            --   put 'a' in front of a recursive call with the same list
                        --   'l' and an empty accumulator
   |n<-a++[h]           -- else bind 'n' to 'h' appended to 'a' and
     =t!n               --   call '!' again with 't' and 'n'
  _!a=[a]               -- base case for '!'. If the input list is empty, return
                        --   a singleton list with 'a' 

R, 58 bytes


Try it online!

  • -5 bytes thanks to @Giuseppe
  • 1
    \$\begingroup\$ 58 bytes \$\endgroup\$
    – Giuseppe
    Commented Jul 30, 2018 at 20:43

Java 10, 207 186 159 148 145 bytes

a->n->{var r=new java.util.Stack();for(int l=a.size(),c=0,j=0,i=-1;i++<l;)if(i==l||f(a.get(i))&&++c>n&i>0)r.add(a.subList(j,j=i*=c=1));return r;}

Java is definitely not the right language for this challenge (or any codegolf-challenge of course..)

-21 bytes thanks to @O.O.Balance
-3 bytes thanks to @ceilingcat

Try it online.


a->n->{                   // Method with List & int parameters and List of Lists return
  var r=new java.util.Stack();
                          //  Result-list, starting empty
  for(int l=a.size(),     //  Size of the input-List
          c=0,            //  Count-integer, starting at 0
          j=0,            //  Range-integer, starting at 0
          i=-1;++i<l;)    //  Loop `i` in the range (-1, `l`]:
    if(i==l||             //   If this is the last iteration
       f(a.get(i))        //   Or if the black-box function is true for the current item
       &&++c              //    Increase the counter by 1
         >n&              //    If the counter is now larger than the size
         &i>0)            //    and it's not the first item of the List
      a.subList(j,j=i     //     Add a sub-List to the result from range [`j`, `i`)
                          //     And set `j` to `i` at the same time
                  *=c=1));//     And reset `c` to 1 at the same time as well
  return r;}              //  Return the List of Lists as result

Black box input format:

Assumes a named function boolean f(Object i) is present, which is allowed according to this meta answer.

I have an abstract class Test containing the default function f(i), as well as the lambda above:

abstract class Test{
  boolean f(Object i){
    return true;

  public java.util.function.Function<java.util.List, java.util.function.Function<Integer, java.util.List<java.util.List>>> c =
    a->n->{var r=new java.util.Stack();for(int l=a.size(),c=0,j=0,i=-1;i++<l;)if(i==l||f(a.get(i))&&++c>n&i>0)r.add(a.subList(j,j=i*=c=1));return r;}

For the test cases, I overwrite this function f. For example, the last test case is called like this:

System.out.println(new Test(){
  boolean f(Object i){
    return (char)i == 'A';
}.c.apply(new java.util.ArrayList(java.util.Arrays.asList('A','b','A','b','A','A','A','b','A','b','A','A','b'))).apply(3));
  • 1
    \$\begingroup\$ "(or any codegolf-challenge of course..)" ehh I dunno, you've beaten out my Clean answers in at least a few cases. Anyway, I always look forward to your answers. \$\endgroup\$
    – Οurous
    Commented Jul 30, 2018 at 8:27
  • 2
    \$\begingroup\$ @Οurous It's more of a meme that Java is not suitable for codegolf in any way, which I guess applies to Clean as well if we compare it to actual golf languages like Jelly, 05AB1E, etc. I still enjoy solving all these codegolf-challenges in Java though (and you in Clean as well I assume). And once in a (long) while, Java is able to beat Python. ;) And I was once a leading answer with Java, until bash ruined it (and R golfed further). xD \$\endgroup\$ Commented Jul 30, 2018 at 8:30
  • 1
    \$\begingroup\$ 186 bytes if you return a List of arrays: bit.ly/2mSjCIc \$\endgroup\$ Commented Jul 30, 2018 at 11:02
  • \$\begingroup\$ @O.O.Balance Thanks! Smart use of Arrays.copyOfRange! \$\endgroup\$ Commented Jul 30, 2018 at 11:43
  • \$\begingroup\$ @O.O.Balance been able to golf a bit more by using taking the input as List and using .sublist. Your functionality remains the same apart from that, but it saves a lot of bytes and removes the import. (And now it also works for the test-case with characters instead of integers.) \$\endgroup\$ Commented Jul 30, 2018 at 11:59

Jelly, 10 bytes


A full program taking the monadic black-box function as the first optional argument, the list as the second optional argument and the chunk size as the third optional argument which prints a Python representation of the resulting list of lists (to avoid Jelly's implicit smashing of lists containing characters).

Try it online! (note that a list of characters is passed to a Jelly program by formatting it as a Python quoted string)


vⱮTm⁵ḊœṖŒṘ - Main Link: B, L, S
 Ɱ         - map across second argument with (i.e. for X in L):
v          -   evaluate as a monad with input (i.e. B(X))
  T        - truthy indices (e.g. [0,0,1,0,1,1,1,0,0,0,1,0,0]T -> [3,5,6,7,10])
    ⁵      - 3rd optional argument = S
   m       - modulo slice   (e.g. [3,4,7,9,12,15,16,18,19,20]m3 -> [[3,4,7],[9,12,15],[16,18,19],[20]]
     Ḋ     - dequeue        (e.g. [[3,4,7],[9,12,15],[16,18,19],[20]]Ḋ -> [[9,12,15],[16,18,19],[20]]
      œṖ   - partition right (L) at the indices in that
        ŒṘ - print Python representaion

C (gcc), 70 66 bytes

I use a structure to note the start of a sub-list, as C doesn't know about such things.

Thanks to ceilingcat for the suggestions.


Try it online!


MATL, 19 bytes


Based on jslip's excellent APL answer.

MATL doesn't really have user-defined functions as such, but it does have a way to call out to the environment it's running on (MATLAB/Octave), so this uses that for the predicate function. Usage would be something like this, but that functionality is disabled online for security reasons, so here's a version that uses a hardcoded isodd predicate function instead: Try it on MATL Online.

H    % Push the function name to be called, assumed to be 
     %  stored in the H clipboard
y    % Take the first input, push copies of it both below and above the 
     %  function name
X$   % Call the function (say 'isupper') with the input as argument
     %  returns a boolean vector
Ys   % Cumulative sum
i/   % Take the chunk size and divide each element by it
Xk   % ceil
1y>+ % Turn the (initial) 0s to 1s
!    % Transpose. Now we have a column vector specifying which chunk 
     %  each input element should go into
w    % Bring the other copy of the input on top 
7    % Code for this function: '@(x){x.'}'
     %  i.e. place each chunk in a row vector and enclose it in a cell
XQ   % 'accumarray' - split the input into chunks based on
     %   the given column vector, apply the given function to each chunk
     %   (which here just wraps it up as a cell), and accumulate the results
     %   in an array.
     % implicit output

JavaScript (ES6), 69 bytes

Saved 3 bytes thanks to @tsh

Takes input in currying syntax (size)(predicate)(array).


Try it online!


Ruby, 57 bytes


Try it online!

Anonymous lambda accepting input array a, chunk size n, and predicate g. Maintains a counter c of items matching predicate and groups items by the number of chunks already used up. Unfortunately, the initial value -1/n doesn't round up to 0, so we have to spend a few bytes to fix it.


R, 100 bytes

function(L,K,P,s=sapply(L,P),y=cut(cumsum(s),seq(0,sum(s),K),,T))for(l in levels(y))cat(L[y==l],"

Try it online!

outgolfed handily by digEmAll

  • \$\begingroup\$ I don't know if named lists as output are ok (and if I missed some edge cases...): 65 bytes \$\endgroup\$
    – digEmAll
    Commented Jul 30, 2018 at 19:09
  • \$\begingroup\$ Hmm well I'd post that as a separate answer! \$\endgroup\$
    – Giuseppe
    Commented Jul 30, 2018 at 19:31

Python 2, 92 bytes

lambda a,c,p:reduce(lambda r,v:[r[:-1]+[r[-1]+[v]],r+[[v]]][sum(map(p,r[-1]+[v]))>c],a,[[]])

Try it online!


JavaScript (Node.js), 90 bytes

(s,p,r=[l=[]],n=s+1)=>g=a=>0 in a?g(a,(n=(n||s)-p(v=a.shift()))||r.push(l=[]),l.push(v)):r

Try it online!

Invoke as F(2, x => x > 0)([0, 1, 1, 0])


Mathematica, 82 bytes



f[l_,s_,p_] :=                (* define a function that takes three    *)
                              (* arguments                             *)

  Last@Reap[                  (* collect and return results which were *)
                              (* gathered by the procedural loop below *)

    i=t=-1;                   (* initialize two counters               *)

    Do[                       (* start iterating over the list         *)

      If[p@e,                 (* if element passes predicate           *)
        If[                   (* then if preincremented i is 0 modulo  *)
          ++i~Mod~s==0&&i>0,  (* chunk size and i > 0                  *)
          t++                 (* increment t                           *)
      ];e~Sow~t,              (* finally sow the element tagged with   *)
                              (* the current value of t                *)

    {e,l}]                    (* e references current element of list  *)
                              (* l is the list itself                  *)

l is the input list; s is chunk size; p is an unnamed/anonymous/pure/lambda function that returns true/false operating on elements of the list.

Last@Reap[...] returns a list of lists of every element that was Sow-n inside of .... They are grouped into sublists by the second argument of e~Sow~t which is shorthand for Sow[e, t].

I had to initialize counters to -1 to handle a chunk size of 1, otherwise I would need to check Mod[i, s] (i~Mod~s) equal to 1, which could never happen.

The rest of the code is explained in the ungolfed block.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.