A Useful Linq Extension Method: Intersperse

Have you ever had a case where you want to insert a constant item in-between each element of a list of items? For example, if I’ve got a list of numbers:

1,2,3,4,5

And I want to insert 0 between each one to get:

1,0,2,0,3,0,4,0,5

Meet my friend Intersperse (stolen from Haskell, if you hadn’t guessed):

var items = new[] {1, 2, 3, 4, 5}.Intersperse(0).ToArray();

foreach (var item in items)
{
    Console.Write(item);
}

This code outputs:

102030405

And here is the Intersperse extension method:

public static IEnumerable<T> Intersperse<T>(this IEnumerable<T> items, T separator)
{
    var first = true;
    foreach (var item in items)
    {
        if (first) first = false;
        else
        {
            yield return separator;
        }
        yield return item;
    }
}

Some interesting uses. System.String is, of course, an array of char, so you can do this:

var doubleSpaced = new string("hello world".Intersperse(' ').ToArray());
Console.WriteLine(doubleSpaced);
// outputs 'h e l l o   w o r l d'

Or how about this perennial problem, creating a comma separated list:

var strings = new[] {"one", "two", "three"};
var commaSeprated = new string(strings.Intersperse(",").Concat().ToArray());
Console.WriteLine(commaSeprated);
// outputs one,two,three