C# & LINQ, Select two (consecutive) items at once [duplicate]

Question

Using LINQ on an ordered set (array, list), is there a way to select or otherwise use two consecutive items? I am imagining the syntax:

list.SelectTwo((x, y) => ...)

Where x and y are the items at index i and i + 1 in the list/array. There may be no way to do this, which I accept as a possibility, but I would at least like to say I tried to find an answer.

I am aware that I could use something other and LINQ to achieve this.

Thank you in advance.

Answer 1

System.Linq.Enumerable.Zip combines two IEnumerables by pairing up the i-th element for each i. So you just need to Zip your list with a shifted version of it.

As a nice extension method:

using System.Collections.Generic;
using System.Linq;

static class ExtMethods
{
    public static IEnumerable<TResult> SelectTwo<TSource, TResult>(this IEnumerable<TSource> source,
                                                                        Func<TSource, TSource, TResult> selector)
    {
        return Enumerable.Zip(source, source.Skip(1), selector);
    }
}

Example:

Enumerable.Range(1,5).SelectTwo((a,b) => $"({a},{b})");

Results in:

(1,2) (2,3) (3,4) (4,5)

Answer 2

Another answer presents a nice and clean solution using LINQ's Skip and Zip.

It is absolutely correct, but I'd like to point out that it enumerates the source twice. That may or may not matter, depending on each individual use case. If it matters for your case, here's a longer alternative that is functionally equivalent but enumerates the source once:

static class EnumerableUtilities
{
    public static IEnumerable<TResult> SelectTwo<TSource, TResult>(this IEnumerable<TSource> source,
                                                                   Func<TSource, TSource, TResult> selector)
    {
        if (source == null) throw new ArgumentNullException(nameof(source));
        if (selector == null) throw new ArgumentNullException(nameof(selector));

        return SelectTwoImpl(source, selector);
    }

    private static IEnumerable<TResult> SelectTwoImpl<TSource, TResult>(this IEnumerable<TSource> source,
                                                                        Func<TSource, TSource, TResult> selector)
    {
        using (var iterator = source.GetEnumerator())
        {
            var item2 = default(TSource);
            var i = 0;
            while (iterator.MoveNext())
            {
                var item1 = item2;
                item2 = iterator.Current;
                i++;

                if (i >= 2)
                {
                    yield return selector(item1, item2);
                }
            }
        }
    }
}

Example:

var seq = new[] {"A", "B", "C", "D"}.SelectTwo((a, b) => a + b);

The resulting sequence contains "AB", "BC", "CD".