Comparing a pair of values to an array of pairs of values C#

Question

I have a pair of values int x and int y and would like to see if this pair of values appear in an 2D array of pairs of values.

Typically, what I have now is:

if ((x == 5 && y == 2) || (x == 6 && y == 1) || ETC)
    return true;

Since there are quite a few possibilities that need to return true, I figured it would be easier to create an array with those pairs of values and see if x and y appear together as a pair in that array:

int[,] arrayOfPairs = new int[,]
    {
        {5, 2},
        {6, 1},
    };

I know in C++ we can use std::find to do that but don't know if it is possible in C#.

Thanks for your help!

dana · Accepted Answer · 2017-01-30 16:36:14Z

3

Try using an array of Tuple<int, int>... For example:

Tuple<int, int>[] pairs = new Tuple<int, int>[]
{
    Tuple.Create(5, 2),
    Tuple.Create(6, 1),
};

Then you can determine if an element exists like so:

if (Array.IndexOf(pairs, Tuple.Create(3, 4)) != -1)
{
    // element exists.
}

There are slicker ways to do this w/ LINQ as well:

if (pairs.Any(x => x == Tuple.Create(3, 4)))
{
    // element exists
}

Per @Martheen's suggestion, here is how to implement using a HashSet.

HashSet<Tuple<int, int>> pairs = new HashSet<Tuple<int, int>>()
{
    Tuple.Create(5, 2),
    Tuple.Create(6, 1),
};

Which can be use as follows:

if (pairs.Contains(Tuple.Create(3, 4)))
{
    // element exists
}

Since you are using Unity, you could substitute a Tuple for a Vector2. Then my first method will work.

Also, if you want to get the speed benefits of using a HashSet but don't have access to that collection, then you could use a regular old Hashtable.

// initialize pairs
Hashtable pairs = new Hashtable().
pairs.Add(new Vector2(5F, 2F), true);
pairs.Add(new Vector2(6F, 1F), true);

// check for existence
if (pairs.ContainsKey(new Vector2(3F, 4F))
{
    // element exists
}

edited Jan 30, 2017 at 16:36

answered Jan 27, 2017 at 17:53

dana

18.3k7 gold badges71 silver badges91 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Martheen Over a year ago

Since Tuple also implement the Equals and GetHashCode, it can be faster to use HashSet instead of regular Array, since .Contains is O(1) instead of O(n). This assume large enough n and frequent comparison to bother.

Cyril_n Over a year ago

Thanks so much! The problem is I'm using this on Unity 3D which is stuck at .NET 2.0 which does not support Tuples. Any way I can do this without having to use Tuples?

dana Over a year ago

@Cyril_n - answer has been updated to use Unity data structures

poke · Accepted Answer · 2017-01-30 10:18:42Z

Note that int[,] arrayOfPairs is a multi-dimensional array. It is not an array of pairs, and it’s not even an array of arrays. When accessing items, you will need two indices, which will basically confuse the relationship those two numbers have as part of their pair.

Instead, you should use an actual array of arrays: int[][]. That way, e.g. the first element of that array will be an array of members of the pair. Of course, this structure will not enforce any dimensions, so you cannot actually be sure that an element of that array is a 2-element array.

int[][] arrayOfPairs = new int[][]
{
    new int[] { 5, 2 },
    new int[] { 6, 1 },
};

A better solution is to use something that will actually enforce that there is a complete pair, i.e. exactly 2 elements. You can use the Tuple type for this, or even come up with your own type for this purpose. The Tuple type already comes with a good equality comparer, so you can use it well for your purpose. You can even put them in a set for quick membership tests:

var pairs = new HashSet<Tuple<int, int>>()
{
    Tuple.Create(5, 2),
    Tuple.Create(6, 1)
};

Console.WriteLine(pairs.Contains(Tuple.Create(5, 3))); // false
Console.WriteLine(pairs.Contains(Tuple.Create(5, 2))); // true

One final note: Your original condition does not really make much sense:

if ((x == 5 && y == 2) && (x == 6 && y == 1) && ETC)

It’s impossible for x, y to be 5, 2 and 6, 1 at the same time, so that condition would be always false.

If you have to use .NET 2.0 which does not have tuples, you can simply create your own type for this purpose:

public struct Pair
{
    public int X { get; set; }
    public int Y { get; set; }

    public Pair(int x, int y)
    {
        X = x;
        Y = y;
    }

    public override bool Equals(object other)
    {
        var otherPair = other as Pair?;
        return otherPair != null && otherPair.Value.X == X && otherPair.Value.Y == Y;
    }

    public override int GetHashCode()
    {
        return (17 + X.GetHashCode()) * 23 + Y.GetHashCode();
    }
}

And then your example would look like this:

var pairs = new HashSet<Pair>()
{
    new Pair(5, 2),
    new Pair(6, 1)
};

Console.WriteLine(pairs.Contains(new Pair(5, 3))); // false
Console.WriteLine(pairs.Contains(new Pair(5, 2))); // true

If you want to keep it as an array of arrays, you can also write your own Enumerable.Contains method (LINQ came with .NET 3.5):

public bool Contains(int[][] array, int[] item)
{
    foreach (int[] elem in array)
    {
        if (elem.Length != 2)
            throw new ArgumentException("Array element length should be exactly 2", "array");

        if (elem[0] == item[0] && elem[1] == item[1])
            return true;
    }

    return false;
}

Used like this:

Console.WriteLine(Contains(arrayOfPairs, new int[] { 5, 2 })); // true
Console.WriteLine(Contains(arrayOfPairs, new int[] { 5, 1 })); // false

Thanks, the first solution seems perfect since I cannot use Tuple with Unity3D. I'm however wondering how to test that my coordinate appear in that array of arrays.
@Cyril_n Added an example using a custom type. If you do a lot membership tests, you should do that instead of using an array of arrays, since that’s a lot more efficient then.

Avan · Accepted Answer · 2017-01-27 18:10:58Z

0

Try doing it using Objects .

class pair
{
     int x;
     int y;
}

Store all of your pair objects in List. And when you are comparing against any value . Just iterate over the List of objects and see if any object has the same x and y coordinate.

Perhaps you may have to overload Equals() and GetHashCode() both if you are equating objects.

answered Jan 27, 2017 at 18:10

Avan

3963 silver badges17 bronze badges

Comments

Kip Morgan · Accepted Answer · 2017-01-30 15:42:55Z

-2

Some people think LINQ query syntax is easiest to read:

    static void Main(string[] args)
    {
        int[][] arrayOfPairs = new int[][]
            {
                new int[] { 5, 2 },
                new int[] { 6, 1 },
            };

        System.Console.WriteLine(IsMatch(1, 3, arrayOfPairs));
        System.Console.WriteLine(IsMatch(5, 2, arrayOfPairs));
        System.Console.ReadKey();
    }

    static bool IsMatch(int x, int y, int[][] pairs)
    {
        var results = 
            from pair in pairs
            where pair[0] == x && pair[1] == y
            select pair;

        return results.Any();
    }

Outputs: False True

edited Jan 30, 2017 at 15:42

answered Jan 27, 2017 at 18:10

Kip Morgan

7383 silver badges12 bronze badges

3 Comments

David Libido Over a year ago

This method will always iterate the entire collection, which can be a performance issue.

Kip Morgan Over a year ago

I agree, and we should always consider performance. However, the question is not asking for the fastest way. Sometimes more readable code is better than the most optimized solution.

Kip Morgan Over a year ago

The fastest C# solution is likely the one using the HashSet. theburningmonk.com/2011/03/hashset-vs-list-vs-dictionary

Collectives™ on Stack Overflow

Comparing a pair of values to an array of pairs of values C#

4 Answers 4

3 Comments

3 Comments

Comments

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

3 Comments

3 Comments

Comments

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related