C# .NET: Descending comparison of a SortedDictionary?

Question

I'm want a IDictionary<float, foo> that returns the larges values of the key first.

private IDictionary<float, foo> layers = new SortedDictionary<float, foo>(new DescendingComparer<float>());

class DescendingComparer<T> : IComparer<T> where T : IComparable<T>
{
    public int Compare(T x, T y)
    {
        return -y.CompareTo(x);
    }
}

However, this returns values in order of the smallest first. I feel like I'm making a stupid mistake here.

Just to see what would happen, I removed the - sign from the comparator:

    public int Compare(T x, T y)
    {
        return y.CompareTo(x);
    }

But I got the same result. This reinforces my intuition that I'm making a stupid error.

This is the code that accesses the dictionary:

foreach (KeyValuePair<float, foo> kv in sortedLayers)
{
    // ...
}

UPDATE: This works, but is too slow to call as frequently as I need to call this method:

IOrderedEnumerable<KeyValuePair<float, foo>> sortedLayers = layers.OrderByDescending(kv => kv.Key);
foreach (KeyValuePair<float, ICollection<IGameObjectController>> kv in sortedLayers) { 
    // ...
}

UPDATE: I put a break point in the comparator that never gets hit as I add and remove kv pairs from the dictionary. What could this mean?

I think you are confusing ascending with descending. Ascending is 1 2 3 4 5 (smallest value first), descending is 5 4 3 2 1 (largest value first). — dtb, Apr 23 '10 at 22:41
`y.CompareTo(x)` should work for descending order. Why would it not? :o If you give few samples and output it will be easier to know. See [reverse-sorted-dictionary-in-net](http://stackoverflow.com/questions/931891/reverse-sorted-dictionary-in-net) — nawfal, Jun 15 '14 at 10:28

score 4 · Accepted Answer · answered Apr 23 '10 at 22:36

4

For descending order (largest values first), you would do -x.CompareTo(y)

answered Apr 23 '10 at 22:36

Michael Petito

12,891
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@Rosarch: It definitely should work, I know I've used it before. If you debug your code, do you find that your Compare method is actually called? – Michael Petito Apr 23 '10 at 22:46
Looks like it isn't, actually. Am I creating the `SortedDictionary` incorrectly? – Nick Heiner Apr 23 '10 at 22:48
1

@Rosarch: No, you are creating the dictionary correctly. Check to make sure that you are using the same dictionary you've constructed above later on in your code when adding/removing elements and enumerating. IE. Find all references to `layers` and make sure there isn't another piece of code assigning a different `SortedDictionary`. – Michael Petito Apr 23 '10 at 22:59
2

Negating can be a bad idea. If the `int` is the most negative value possible, `-2147483648`, then the negation of that is the same number, `-2147483648`. So it is better to swap `x` and `y`, as in `y.CompareTo(x)`, than to prepend a minus, as `-x.CompareTo(y)`. If you do not know if one of the comparands is `null`, you can use `Comparer.Default.Compare(y, x)`. – Jeppe Stig Nielsen Aug 09 '16 at 20:53

score 0 · Answer 2 · answered Apr 23 '10 at 22:35

0

try:

public int Compare(T x, T y)
{
    return x.CompareTo(y);
}

answered Apr 23 '10 at 22:35

derek

4,826
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score 0 · Answer 3 · answered Apr 23 '10 at 23:59

0

In this line, switch x and y:

return -y.CompareTo(x);

Make it

return -x.CompareTo(y);

answered Apr 23 '10 at 23:59

Jacob O'Reilly

136
2

C# .NET: Descending comparison of a SortedDictionary?

3 Answers3