Is there an equivalent to Python's enumerate() for .NET IEnumerable

Question

I could not find a related question.

In python you can easily loop through a sequence (list, generator etc) and collect the index of the iteration at the same time thanks to enumerate(seq) like this :

>>> for (i,item) in enumerate(["toto","titi","tutu"]):
...     print i, item
...
0 toto
1 titi
2 tutu

Is there something similar for IEnumerable, that would, for instance, transform a IEnumerable<T> in a IEnumerable<Tuple<Int32,T>> ?

(I know it would be easily done thanks to the correct function in Select() .. but if it exists, I'd rather use it :) )

UPDATE FYI, I am curious about this kind of possibility to be able to do something like : "give me the index of the last item that fulfils this condition", which would then be accomplished through :

myEnumeration.First(t => some condition on t.Item2 ... ).Item1;

Answer 1

C# 7 finally allows you to do this in an elegant way:

static class Extensions
{
    public static IEnumerable<(int, T)> Enumerate<T>(
        this IEnumerable<T> input,
        int start = 0
    )
    {
        int i = start;
        foreach (var t in input)
            yield return (i++, t);
    }
}

class Program
{
    static void Main(string[] args)
    {
        var s = new string[]
        {
            "Alpha",
            "Bravo",
            "Charlie",
            "Delta"
        };

        foreach (var (i, o) in s.Enumerate())
            Console.WriteLine($"{i}: {o}");
    }
}

Answer 2

Instead of using a Tuple<,> (which is a class) you can use a KeyValuePair<,> which is a struct. This will avoid memory allocations when enumerated (not that they are very expensive, but still).

public static IEnumerable<KeyValuePair<int, T>> Enumerate<T>(this IEnumerable<T> items) {
    return items.Select((item, key) => new KeyValuePair(key, item));
}