Getting pair-set using LINQ

Question

When i have a list

IList<int> list = new List<int>();
list.Add(100);
list.Add(200);
list.Add(300);
list.Add(400);
list.Add(500);

What is the way to extract a pairs

Example : List elements {100,200,300,400,500}

Expected Pair : { {100,200} ,{200,300} ,{300,400} ,{400,500} }

Solution 1

This will give you an array of anonymous "pair" objects with A and B properties corresponding to the pair elements.

var pairs = list.Where( (e,i) => i < list.Count - 1 )
                .Select( (e,i) => new { A = e, B = list[i+1] }  );

Solution 2

The most elegant way with LINQ: list.Zip(list.Skip(1), Tuple.Create)

A real-life example: This extension method takes a collection of points (Vector2) and produces a collection of lines (PathSegment) needed to 'join the dots'.

static IEnumerable<PathSegment> JoinTheDots(this IEnumerable<Vector2> dots)
{
    var segments = dots.Zip(dots.Skip(1), (a,b) => new PathSegment(a, b));
    return segments;
}

Solution 3

You can use a for loop:

var pairs = new List<int[]>();
for(int i = 0; i < list.Length - 1; i++)
    pairs.Add(new [] {list[i], list[i + 1]);

You can also use LINQ, but it's uglier:

var pairs = list.Take(list.Count - 1).Select((n, i) => new [] { n, list[i + 1] });

EDIT: You can even do it on a raw IEnumerable, but it's much uglier:

var count = list.Count();
var pairs = list
    .SelectMany((n, i) => new [] { new { Index = i - 1, Value = n }, new { Index = i, Value = n } })
    .Where(ivp => ivp.Index >= 0 && ivp.Index < count - 1)    //We only want one copy of the first and last value
    .GroupBy(ivp => ivp.Index, (i, ivps) => ivps.Select(ivp => ivp.Value));

Solution 4

More general would be:

    public static IEnumerable<TResult> Pairwise<TSource, TResult>(this IEnumerable<TSource> values, int count, Func<TSource[], TResult> pairCreator)
    {
        if (count < 1) throw new ArgumentOutOfRangeException("count");
        if (values == null) throw new ArgumentNullException("values");
        if (pairCreator == null) throw new ArgumentNullException("pairCreator");
        int c = 0;
        var data = new TSource[count];
        foreach (var item in values)
        {
            if (c < count)
                data[c++] = item;
            if (c == count)
            {
                yield return pairCreator(data);
                c = 0;
            }
        }
    }

Solution 5

Following solution uses zip method. Zip originalList and originalList.Skip(1) so that one gets desired result.

    var adjacents =
            originalList.Zip(originalList.Skip(1),
                             (a,b) => new {N1 = a, N2 = b});

View more solutions

Comments

[leppie]

Please specify your problem better. What you have now can mean one of any number of things...

[SLaks]

Do you want to operate on a raw IEnumerable or only an IList<T>? If so, see my edit.

[GameKyuubi]

Can someone explain what this does? It's quite dense.

[tvanfosson]

@GameKyuubi the (e,i) notation represents the signature that takes both the element and the index of the element. The Where clause allows all but the last element in the sequence. The Select clause creates a new anonymous object with the A element assigned the original element in the sequence and the B element assigned the following element in the sequence. That is, if e represents the ith element at each iteration, then B gets the i+1th element.