Why is my List.Sort method in C# reversing the order of my list?

Question

I have a list of items in a generic list:

The comparator on them takes the form:

this.sortIndex.CompareTo(other.sortIndex)

When I do a List.Sort() on the list of items, I get the following order out:

It has obviously worked in the sense that the sort indexes are in the right order, but I really don't want it to be re-ordering the 'B' items.

Is there any tweak I can make to my comparator to fix this?

possible duplicate of [Why does List.Sort method reorder equal IComparable elements?](http://stackoverflow.com/questions/800007/why-does-listt-sort-method-reorder-equal-icomparablet-elements) — nawfal, Jun 13 '14 at 12:02

score 5 · Answer 1 · answered Apr 20 '10 at 10:30

5

you need to use a "stable sort" algorithm if you don't want items that are equal to change position.

Check out "merge sort" for an example of a stable sort algorithm. Here's an implementation of it in C#.

answered Apr 20 '10 at 10:30

Mark Heath

score 5 · Accepted Answer · answered Apr 20 '10 at 10:34

5

OrderBy preserves order for equal items:

myList = myList.OrderBy(item => item.SortIndex).ToList();

answered Apr 20 '10 at 10:34

David Hedlund

score 2 · Answer 3 · edited May 23 '17 at 11:53

2

StableSort() extension method for List<T> is here

edited May 23 '17 at 11:53

Community

answered May 04 '12 at 13:36

xhafan

score 1 · Answer 4 · answered Apr 20 '10 at 10:33

1

You can change your comparator to do a secondary sort on the value:

if (this.sortIndex.CompareTo(other.sortIndex) == 0) // same sortIndex
{
   return this.Value.CompareTo(other.Value);
}
return 0;

answered Apr 20 '10 at 10:33

Oded

Is that not doing exactly the same thing as the example I posted? – Fiona - myaccessible.website Apr 20 '10 at 10:35
@Fiona: No, the code uses the value in a secondary comparison, but you have to do some corrections if you want to use it, as it messes up the primary comparison by returning zero instead of the result. – Guffa Apr 20 '10 at 10:42
Ah I see. I don't have a secondary comparison to use really (although I could add an artifical one and this would work fine), the list order I want to preserve is based on the output of a pretty complex recursive function. – Fiona - myaccessible.website Apr 20 '10 at 10:58

digEmAll · Answer 5 · 2010-04-20T10:48:02.337

1

Sort uses QuickSort, and it doesn't assure original sequence in case of comparison equality.

If you still want to use List.Sort you could add a second comparison with the original index like:

int c = this.sortIndex.CompareTo(other.sortIndex);
if (c == 0)
  c = this.originalIndex.CompareTo(other.originalIndex);
return c;

otherwise you can sort with other "stable" algorithms (e.g. LINQ OrderBy).

edited Apr 20 '10 at 10:48

answered Apr 20 '10 at 10:36

digEmAll

5 Answers5