How do I split or chunk a list into equal parts, with Dart?

Question

Assume I have a list like:

var letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'];

I would like a list of lists of 2 elements each:

var chunks = [['a', 'b'], ['c', 'd'], ['e', 'f'], ['g', 'h']];

What's a good way to do this with Dart?

Solution 1

Here is another way:

  var chunks = [];
  int chunkSize = 2;
  for (var i = 0; i < letters.length; i += chunkSize) {
    chunks.add(letters.sublist(i, i+chunkSize > letters.length ? letters.length : i + chunkSize)); 
  }
  return chunks;

Run it on dartpad

Solution 2

Quiver (version >= 0.18) supplies partition() as part of its iterables library (import 'package:quiver/iterables.dart'). The implementation returns lazily-computed Iterable, making it pretty efficient. Use as:

var letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'];
var pairs = partition(letters, 2);

The returned pairs will be an Iterable<List> that looks like:

[['a', 'b'], ['c', 'd'], ['e', 'f'], ['g', 'h']]

Solution 3

A slight improvement on Seth's answer to make it work with any list or chunk size:

var len = letters.length;
var size = 2;
var chunks = [];

for(var i = 0; i< len; i+= size)
{    
    var end = (i+size<len)?i+size:len;
    chunks.add(letters.sublist(i,end));
}

Solution 4

  pairs(list) => list.isEmpty ? list : ([list.take(2)]..addAll(pairs(list.skip(2))));

Solution 5

another solution;

List chunk(List list, int chunkSize) {
  List chunks = [];
  int len = list.length;
  for (var i = 0; i < len; i += chunkSize) {
    int size = i+chunkSize;
    chunks.add(list.sublist(i, size > len ? len : size));
  }
  return chunks;
}

List nums = [1,2,3,4,5];

print(chunk(nums, 2));

// [[1,2], [3,4], [5]]

View more solutions

Comments

[Seth Ladd]

Is there a better way?

[Alan Knight]

For efficiency I suspect the boring for loop with an index is the best, but it's fun to try and be terse.

[Florian Loitsch]

This solution is in O(n^2). I like terse code, but I'm not sure it's worth the performance hit.

[Rusty Rob]

One thing I like about python is the zip function which allows you to go pairs_iter = zip(it, it)

[Rusty Rob]

dart could probably do with a zip function. columns = zip(rows) or rows = zip(columns) is fairly common.

[Rusty Rob]

also the general case would be zip(repeat(it, n)) to get chunks of length n.

[cbracken]

You can find a zip function in Quiver's iterables library. Implementation is here.

[Hasan A Yousef]

what is the ~

[Günter Zöchbauer]

It's for integer division ~/ | Divide, returning an integer result dartlang.org/guides/language/language-tour

[Pom12]

This answer does not work when the list does not contain a pair number of elements. You should add a check of whether you reached the end before calling sublist, otherwise you will get an error like RangeError (end): Invalid value: Not in range 14..15, inclusive: 16

[Galeen]

chunks.add(letters.sublist(i, i+2 > letters.length ? letters.length : i + 2));

[Suragch]

Oops, I missed that Seth Ladd's answer already used sublist. I'll keep this as a more generalized example, though.

[Jebiel]

What about using ListComprehension? [for (int i = 0; i < list.length; i += chunkSize) list.sublist(i, min(i + chunkSize, list.length))] By using min(), you don't need to do any ternary check, and you are sure to get the remaining elements as a sublist, even though there are not enough to satisfy chunkSize. The result of above listComprehension looks very much like the result of quiver's partition function, as described by @cbracken.

[genericUser]

@Pom12 I have edited the answer to support different sizes of chunks and added a dartpad link. Enjoy!