Why do some lists of tuples are sorted and not others?

Question

I need to sort 28 Tuple. Chars are in A-Z, '+', '*'.

The ASCII order is: *, +, A...Z and I want to get this order: A...Z, +, *.

To do that I use this Comparer:

public class AlphabeticPaginationComparer : IComparer<Tuple<char, int>>
{
    public int Compare(Tuple<char, int> a, Tuple<char, int> b)
    {
        // '*' is greater than all letters and '+',
        if (a.Item1 == '*')
        {
            return 1;
        }

        if (b.Item1 == '*')
        {
            return -1;
        }

        // '+' is greater than all letters
        if (a.Item1 == '+')
        {
            return 1;
        }

        if (b.Item1 == '+')
        {
            return -1;
        }

        // Finally we compare letters normally
        return a.Item1 - b.Item1;
    }
}

Here is my code:

IOrderedEnumerable<Tuple<char, int>> orderedList = 
    origList.OrderBy(c => c, new AlphabeticPaginationComparer());

List<Tuple<char, int>> orderedList2 = orderedList.ToList();

This list is sorted

List<Tuple<char, int>> origList = new List<Tuple<char, int>>
{
    Tuple.Create('L', 15),
    Tuple.Create('R', 12),
    Tuple.Create('E', 3),
    Tuple.Create('P', 9),
    Tuple.Create('B', 30),
    Tuple.Create('C', 13),
    Tuple.Create('M', 11),
    Tuple.Create('A', 5),
    Tuple.Create('V', 5),
    Tuple.Create('H', 6),
    Tuple.Create('S', 5),
    Tuple.Create('G', 15),
    Tuple.Create('T', 8),
    Tuple.Create('D', 15),
    Tuple.Create('K', 4),
    Tuple.Create('I', 2),
    Tuple.Create('F', 7),
    Tuple.Create('W', 3),
    Tuple.Create('+', 6),
    Tuple.Create('O', 1),
    Tuple.Create('Z', 1),
    Tuple.Create('J', 0),
    Tuple.Create('N', 0),
    Tuple.Create('Q', 0),
    Tuple.Create('U', 0),
    Tuple.Create('X', 0),
    Tuple.Create('Y', 0),
    Tuple.Create('*', 176)
};

but not this one, the program hangs on .ToList().

List<Tuple<char, int>> origList = new List<Tuple<char, int>>
{
    Tuple.Create('A', 70),
    Tuple.Create('C', 119),
    Tuple.Create('B', 27),
    Tuple.Create('G', 39),
    Tuple.Create('D', 22),
    Tuple.Create('E', 21),
    Tuple.Create('F', 20),
    Tuple.Create('I', 25),
    Tuple.Create('H', 14),
    Tuple.Create('J', 4),
    Tuple.Create('K', 9),
    Tuple.Create('L', 24),
    Tuple.Create('M', 50),
    Tuple.Create('N', 5),
    Tuple.Create('O', 15),
    Tuple.Create('P', 17),
    Tuple.Create('R', 12),
    Tuple.Create('S', 68),
    Tuple.Create('T', 18),
    Tuple.Create('U', 12),
    Tuple.Create('V', 19),
    Tuple.Create('W', 5),
    Tuple.Create('X', 1),
    Tuple.Create('Y', 1),
    Tuple.Create('+', 1),
    Tuple.Create('Q', 0),
    Tuple.Create('Z', 0),
    Tuple.Create('*', 618)
};

I tried with random lists, and this problem seems also random.

Any help will be welcome.

You aren't following the rules of IComparer in that the order must be deterministic. Your implementation isn't deterministic because your logic for handling `+` and `*` doesn't handle when both items are `+` or `*`. This non-determinism can cause an infinite loop in the .NET sorting code. — Ed T, Feb 11 '16 at 16:38

Yacoub Massad · Accepted Answer · 2016-02-11T16:30:31.900

1

What happens when a.Item1 is + and b.Item1 is also +? You need to return 0, not 1 because they are equal.

Here is how you can fix this:

public class AlphabeticPaginationComparer : IComparer<Tuple<char, int>>
{
    public int Compare(Tuple<char, int> a, Tuple<char, int> b)
    {
        if (a.Item1 == b.Item1)
            return 0;

        //...
    }
}

edited Feb 11 '16 at 16:30

answered Feb 11 '16 at 16:26

Yacoub Massad

27,509
2
36
62

Ideally, you are right but these lists come from a GroupBy and there can not be a duplicate on the char. Oups, you are right. I don't understand why my program runs only sometimes. – D. Lucazeau Feb 11 '16 at 16:28
@D.Lucazeau, for some reason, LINQ is trying to compare `('+', 1)` with itself (I am not sure why) and it is getting a 1 where it should have got a 0. – Yacoub Massad Feb 11 '16 at 16:33
You have fixed my problem. Thanks – D. Lucazeau Feb 11 '16 at 16:34
@YacoubMassad the linq sort routines will occasionally compare an item to itself, so even if the collection has unique items you still need to return 0 if the two values are "equal" as you do. – D Stanley Feb 11 '16 at 20:35

Why do some lists of tuples are sorted and not others?

1 Answers1