Extension method for Enumerable.Intersperse?

Question

I learned the intersperse function from Haskell, and have been looking for an implementation in c#.

Intersperse takes 2 arguments, an IEnumerable<T> source and a T element. It returns an IEnumerable with element inserted between every element of source.

One possible use-case is to put an arbitrary integer in between a list of integers, for example:

// returns: {1, 0, 2, 0, 3}
(List<int>() {1, 2, 3}).Intersperse(0);

This is a general case of string.Join(...).

Answer 1

Something the others have missed: if you only want it in between items, and not also in front or behind, you need to do an extra check:

public static IEnumerable<T> Intersperse<T>(this IEnumerable<T> source, T element)
{
    bool first = true;
    foreach (T value in source)
    {
        if (!first) yield return element;
        yield return value;
        first = false;
    }
}

or

public static IEnumerable<T> Intersperse<T>(this IEnumerable<T> source, T element)
{
    var e = source.GetEnumerator(); 
    bool b = e.MoveNext();
    if (b) yield return e.Current;

    while (e.MoveNext())
    {
        yield return element;
        yield return e.Current;
    }
}