Storing pair of ints on the list

How can I store pairs of integers in a List? I know I could make a class for them like:

class Pair  
{
    int i1,i2;
}

But if I do that I'm not able to use the Contains function to check if a given pair is in the list. How can I do that so I can easily store integers in a List and check if a pair of integers already exists? I cannot use table because it is not known how many pairs there will be.

EDIT: Forgot to add: In my program pairs (x, y) and (y, x) are to be treated as equals.

EDIT: (x,y) and (y,x) are equals while checking if Point is in the list, but x and y can not be swapped because x and y represent a connection between two points (integer is id of point, and no I can't use any reference etc...). When I'm checking if List contains a connection it is not important if it is (x,y) or (y,x) but later I would need that information.

Answers


If you're using .NET 4.0, you could use the Tuple class as in

var tuple = new Tuple<int, int>(17, 42);
var otherTuple = Tuple.Create(17, 42);

and

var list = new List<Tuple<int, int>>();

Note that if you go the route of using Tuple<int, int> then you will need to create a custom implementation of IEqualityComparer<Tuple<TFirst, TSecond>> to reflect your equality rules that (x, y) be considered equal to (y, x). You will then have to pass an instance of this comparer to List<T>.Contains(T, IEqualityComparer<T>) (here T is Tuple<int, int> for you).

class TupleAsUnorderedPairComparer : IEqualityComparer<Tuple<TFirst, TSecond>> {
    public bool Equals(Tuple<TFirst, TSecond> x, Tuple<TFirst, TSecond> y) {
        if(Object.ReferenceEquals(x, y)) {
            return true;
        }
        if(x == null || y == null) {
            return false;
        }
        return x.Item1 == y.Item1 && x.Item2 == y.Item2 ||
               x.Item1 == y.Item2 && x.Item2 == y.Item1;
    }

    public int GetHashCode(Tuple<TFirst, TSecond> x) {
        if(x == null) {
            return 0;
        }
        return x.Item1.GetHashCode() ^ x.Item2.GetHashCode();
    }
}

Otherwise, if you can't or don't want to use Tuple then you will need to implement an IEqualityComparer<Pair> for your Pair class or override Object.Equals and Object.GetHashCode.

class Pair {
    public int First { get; private set; }
    public int Second { get; private set; }
    public Pair(int first, int second) {
        this.First = first;
        this.Second = second;
    }

    public override bool Equals(object obj) {
        if(Object.ReferenceEquals(this, obj)) {
            return true;
        }
        Pair instance = obj as Pair;
        if(instance == null) {
            return false;
        }
        return this.First == instance.First && this.Second == instance.Second ||
               this.First == instance.Second && this.Second == instance.First;
    }

    public override int GetHashCode() {
        return this.First.GetHashCode() ^ this.Second.GetHashCode();
    }
}

and

class PairEqualityComparer : IEqualityComparer<Pair> {
    // details elided 
}

If you use

list.Contains(pair);

then it will use Equals and GetHashCode but if you use

list.Contains(pair, new PairEqualityComparer);

then it will use PairEqualityComparer.Equals and PairEqualityComparer.GetHashCode. Note that these could be different than your implementations of Object.Equals and Object.GetHashCode.

Finally, if testing for containment is something that you'll be doing often then a List is not your best bet; you should use a class designed for that purpose like a HashSet.


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