How to iterate lists with different lengths to find all permutations?

Question

This one should not be too hard but my mind seems to be having a stack overflow (huehue). I have a series of Lists and I want to find all permutations they can be ordered in

Answer 1

Nested loops:

List<int> listA = (whatever), listB = (whatever);
var answers = new List<Tuple<int,int>>;
for(int a in listA)
    for(int b in listB)
        answers.add(Tuple.create(a,b));
// do whatever with answers

Answer 2

Try this:

Func<IEnumerable<string>, IEnumerable<string>> combine = null;
combine = xs =>
    xs.Skip(1).Any()
    ? xs.First().SelectMany(x => combine(xs.Skip(1)), (x, y) => String.Format("{0}{1}", x, y))
    : xs.First().Select(x => x.ToString());

var strings = new [] { "AB", "12", "$%" };

foreach (var x in combine(strings))
{
    Console.WriteLine(x);
}

That gives me:

A1$
A1%
A2$
A2%
B1$
B1%
B2$
B2%

Answer 3

I can't say if the following is the easiest way, but IMO it's the most efficient way. It's basically a generalized version of the my answer to the Looking at each combination in jagged array:

public static class Algorithms
{
    public static IEnumerable<T[]> GenerateCombinations<T>(this IReadOnlyList<IReadOnlyList<T>> input)
    {
        var result = new T[input.Count];
        var indices = new int[input.Count];
        for (int pos = 0, index = 0; ;)
        {
            for (; pos < result.Length; pos++, index = 0)
            {
                indices[pos] = index;
                result[pos] = input[pos][index];
            }
            yield return result;
            do
            {
                if (pos == 0) yield break;
                index = indices[--pos] + 1;
            }
            while (index >= input[pos].Count);
        }
    }
}

You can see the explanation in the linked answer (shortly it's emulating nested loops). Also since for performace reasons it yields the internal buffer w/o cloning it, you need to clone it if you want store the result for later processing.

Sample usage:

var list1 = new List<int> { 1 };
var list2 = new List<int> { 1, 2 };
var lists = new[] { list1, list2 };

// Non caching usage
foreach (var combination in lists.GenerateCombinations())
{
    // do something with the combination
}

// Caching usage
var combinations = lists.GenerateCombinations().Select(c => c.ToList()).ToList();

UPDATE: The GenerateCombinations is a standard C# iterator method, and the implementation basically emulates N nested loops (where N is the input.Count) like this (in pseudo code):

for (int i₀ = 0; i₀ < input[0].Count; i₀++)
for (int i₁ = 0; i₁ < input[1].Count; i₁++)
for (int i₂ = 0; i₂ < input[2].Count; i₂++)
...
for (int i_N-1 = 0; i_N-1 < input[N-1].Count; i_N-1++)
yield { input[0][i₀], input[1][i₁], input[2][i₂], ..., input[N-1][i_N-1] }

or showing it differently:

for (indices[0] = 0; indices[0] < input[0].Count; indices[0]++)
{
    result[0] = input[0][indices[0]];
    for (indices[1] = 0; indices[1] < input[1].Count; indices[1]++)
    {
        result[1] = input[1][indices[1]];
        // ...
        for (indices[N-1] = 0; indices[N-1] < input[N-1].Count; indices[N-1]++)
        {
            result[N-1] = input[N-1][indices[N-1]];
            yield return result;
        }
    }
}