How do I use a Lambda expression to sort INTEGERS inside a object?

Question

I have a collection of objects and I know that I can sort by NAME (string type) by saying

collEquipment.Sort((x, y) => string.Compare(x.ItemName, y.ItemName));

that WORKS.

But I want to sort by a ID (integer type) and there is no such thing as Int32.Compare

So how do I do this? This doesn't work

collEquipment.Sort((x, y) => (x.ID < y.ID));  //error

I know the answer is going to be really simple. Lambda expressions confuse me.

score 16 · Answer 1 · answered May 04 '10 at 15:42

16

collEquipment.Sort((x, y) => y.ID.CompareTo(x.ID));

answered May 04 '10 at 15:42

JWL_

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The short explanation of *why* this works is that Sort does not take a less-than operator - it takes a "comparator" operator which returns <0 if x0 if x>y, or 0 if x==y. So it always expects an integer result, not a boolean. The CompareTo() method on int returns <0, 0, or >0 in this pattern, so you can use it to solve your problem succinctly, although using if-else would work as well (if (x.IDy.ID) return 1; else return 0;). – Chiara Coetzee Oct 06 '12 at 03:30

score 8 · Answer 2 · answered May 04 '10 at 15:57

Here you go, sort a list against any property that implements IComparable[<T>] (which int does):

public static class ListExtensions {
    public static void Sort<TSource, TValue>(
        this List<TSource> list,
        Func<TSource, TValue> selector) {
        list.Sort((x,y) => Comparer<TValue>.Default
            .Compare(selector(x),selector(y)));
    }
}

Now:

collEquipment.Sort(x => x.ItemName);

or

collEquipment.Sort(x => x.ID);

SysAdmin · Accepted Answer · 2010-05-04T15:42:29.123

7

try this

collEquipment.Sort((x, y) => y.ID - x.ID);

edited May 04 '10 at 15:42

answered May 04 '10 at 15:37

SysAdmin

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possibly has some wrap-around issues with very large numbers – Marc Gravell May 04 '10 at 16:01
great works thanks! its just IDs for items in a DB.. so won't be too large – punkouter May 04 '10 at 16:07
@Marc Gravell - I guess this would be the same case for any method right? – SysAdmin May 04 '10 at 16:07
2

You shouldn't use `-` to implement CompareTo - it does not work (overflow problems etc.) – BlueRaja - Danny Pflughoeft May 04 '10 at 16:08
Still don't get how "y.ID - x.ID" = "Sort these integers" Have to study more. – punkouter May 04 '10 at 16:08
1

@punkouter: He is using subtraction to implement CompareTo, which is flawed. See [this post](http://stackoverflow.com/questions/2728793/java-integer-what-is-faster-comparison-or-subtraction). – BlueRaja - Danny Pflughoeft May 04 '10 at 16:14
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@punkouter: The sort func needs to return a 0, >0, or a <0 result. Subtracting the compare values will do that (more or less), but it would be better to use the Int32.CompareTo() method. So replace y.ID - x.ID with y.ID.CompareTo(x.ID), as JWL_ pointed out below. – code4life May 04 '10 at 17:35

How do I use a Lambda expression to sort INTEGERS inside a object?

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