List.Sort（按长度）在不同的计算机上返回不同的结果

Question

I have the following code :

List<string> Words = item.Split(' ').ToList<string>();
Words.Sort((a, b) => b.Length.CompareTo(a.Length));

Which is supposed to sort a List of words from a line in a file (item) according to their size. However, if two words have the same length, they are supposed to be sorted by the order of appearence in the line.

The problem here is that if the line is, for example "abc", on my computer, the list will have three sorted items (0 - a, 1 - b, 2 - c), but on another computer, using the same .Net version (4.5), the sorted items will be (0 - c, 1 - b, 2 - a)

Is there a way to enforce the same result troughout different computers ?

Answer 1

List.Sort is an unstable sort , meaning in your case that elements of the same length can go in different order.

This implementation performs an unstable sort; that is, if two elements are equal, their order might not be preserved. In contrast, a stable sort preserves the order of elements that are equal.

Answer 2

You can force the same results using LINQ and set of OrderBy / ThenBy calls, instead of Sort .

var result = source.Select((v, i) => new { v, i })
                   .OrderBy(x => x.v.Length)
                   .ThenBy(x => x.i)
                   .Select(x => x.v)
                   .ToList();

But you should be aware, that it will create new list, instead of sorting existing one in place:

Answer 3

The method List.Sort() is an unstable sort. You cannot predict the order of duplicate keys.

There are 2 generic methods to solve this problem: use a stable sort, or force uniqueness by extending the key to include identity information.

One of the common stable sorts is an Insertion sort. I believe this would be the sort used by SortedList, but it does not allow duplicate keys. Failing that, you can write your own, either in Linq or by hand. Even a bubble sort is stable!

The preferred way is to keep the item identity. Create a list of pairs, where each pair consists of the key value and its position in the list. Sort the list of pairs, or insert the pairs directly into a sorted list, because every pair is unique. The ordering of these pairs, and the keys they contain, is guaranteed the same on all platforms.