Sorting ascending and descending in Dart?

Question

I have this

var nlist = [4,2,1,5];
var compare = (a, b) => a.compareTo(b);
nlist.sort(compare);
print(nlist);        // [1,2,4,5]

and here (where I changed the (b, a) to (a, b))

var nlist = [4,2,1,5]
var compare = (b, a) => a.compareTo(b);
nlist.sort(compare);
print(nlist);        // [5,4,2,1]

Why does this little modification change from ascending to descending order?

score 83 · Answer 1 · answered Oct 02 '19 at 16:15

83

For ascending

var nlist = [1, 6, 8, 2, 16, 0]
nlist.sort((a, b) => a.compareTo(b));

For descending

var nlist = [1, 6, 8, 2, 16, 0]
nlist.sort((b, a) => a.compareTo(b));

answered Oct 02 '19 at 16:15

Oluwafemi Tairu

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2

Simple Solution. Thx – sh0umik Oct 28 '19 at 18:26

Günter Zöchbauer · Accepted Answer · 2015-01-12T08:54:00.710

33

Would be interesting what you actually expected from happening after the change.

Compare returns +1, 0, -1 depending on whether the 2nd argument is greater than the 1st or 0 if they are equal. When you swap the two arguments +1 becomes -1 and vice versa this leads to a descending order instead of an ascending one.

1.compareTo(2)

returns -1 and

print(nlist.sort((a, b) => a.compareTo(b)));

prints the list elements in ascending order, so yes, ascending is default.

edited Jan 12 '15 at 08:54

answered Jan 12 '15 at 08:38

Günter Zöchbauer

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Thanks @Günter Zöchbauer. But so, what's the "default" order it follow, +1 or -1? I mean in case (a, b) if it return 1 then I know a > b then it will put a before b or b before a – user3071121 Jan 12 '15 at 08:52
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The default order is `(a, b)=>a.compareTo(b)`. This is the order you get if you provide no compare function to sort - it defaults to using the elements' own `compareTo` functions in this order. This is the default order for the types themselves, which is numeric order on numbers and lexicograpical order on strings. If you need to sort elements that are not `Comparable`, you can't fall back on `compareTo`, and have to write a function that returns negative if the first element is smaller than the second, positive if it's the other way around, and zero if the elements are equal. – lrn Jan 13 '15 at 08:27

score 18 · Answer 3 · answered Oct 01 '19 at 18:26

18

For Ascending:

var nlist = [4,2,1,5]
var compare = (b, a) => a.compareTo(b);

For Descending:

var compare = (b, a) => -a.compareTo(b);

answered Oct 01 '19 at 18:26

arc

339
3
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This is a perfect answer! – Kavinda Jayakody Nov 15 '19 at 10:30
4

@arc The answer is upside down. First one will give values in **descending** order and second one will give in **ascending** order. – Intkhab Ahmed Feb 13 '20 at 16:04

score 10 · Answer 4 · answered Nov 27 '18 at 16:26

10

Another way to sort

  var nlist = [4,2,1,5];
  var ascending = nlist..sort();
  var descending = ascending.reversed;
  print(ascending);  // [1, 2, 4, 5]
  print(descending);  // [5, 4, 2, 1]

answered Nov 27 '18 at 16:26

Szczerski

839
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`nlist..sort();` looks like a typo? – Wayne Phipps Nov 27 '18 at 16:48
2

The .. is a cascade operator in Dart, see https://stackoverflow.com/questions/49447736/list-use-of-double-dot-in-dart – jumpfightgo Jan 04 '19 at 20:29
1

`cascade` operation was really helpful, it allowed to store sorted array to a variable. Without cascade `sort` returns `void` – Saahithyan Vigneswaran Apr 17 '20 at 08:43

score 8 · Answer 5 · answered Jun 23 '21 at 19:47

For ascending

There is no need to write list.sort((a, b) => a.compareTo(b));. This works:

list.sort();

For descending

list.sort((a, b) => b.compareTo(a));

Or, as of 2021, if you use https://pub.dev/packages/fast_immutable_collections (FIC package) this works:

list.sortReversed();

Notes:

(list..sort()).reversed does NOT reverse the list, in place. Instead, it sorts the original list in ascending order, and then returns a descending iterable, not a List (i.e., Iterable<int>, not List<int>). Maybe that's what you want. If not, you have to do list = (list..sort()).reversed.toList();. However, for large lists list.sortReversed() is much faster.
In the beforementioned FIC package there is also a reversedView() method, which returns a descending VIEW of the list. A "view" means if you later add items to the original list, the reversed one will also have those items, and vice-versa. So if your list is already sorted in ascending order and you want to have access to the descending list without wasting time doing the reversal, just do:
```
list.sort();
List<int> listDescending = list.reversedView; // Very fast.
```

Disclaimer: I am the author of the FIC package.

score -1 · Answer 6 · edited Dec 21 '21 at 18:12

-1

var nlist = [1, 6, 8, 2, 16, 0];
nlist.sort((a, b) => a - b);

edited Dec 21 '21 at 18:12

Tomerikoo

18,379
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answered Dec 16 '21 at 10:53

Jisan Mahmud

1
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It is better to add some explanations to accompany your code, especially to such old question with many existing answers. Explanations will help readers not only solve their immediate problem by copy-pasting your code, but also to ***understand*** the code and be able to solve similar problems as well – Tomerikoo Dec 21 '21 at 18:11

Sorting ascending and descending in Dart?

6 Answers6

For ascending

For descending

For ascending

For descending

Notes:

Linked