Recursion in place of multiple nested for loops?

Question

Im having some issues with trying to update a nested for loop to use recursion instead. Is it possible to access the a,b and c variables from the earlier for loops when usin

Answer 1

Here's a recursive solution (using a functional programming style):

public static IEnumerable> GetCombinations(IEnumerable limits)
{
    if (limits.Any() == false)
    {
        // Base case.
        yield return Enumerable.Empty();
    }
    else
    {
        int first = limits.First();
        IEnumerable remaining = limits.Skip(1);
        IEnumerable> tails = GetCombinations(remaining);

        for (int i = 0; i < first; ++i)
            foreach (IEnumerable tail in tails)
                yield return Yield(i).Concat(tail);
    }
}

// Per http://stackoverflow.com/q/1577822
public static IEnumerable Yield(T item)
{
    yield return item;
}

Sample use:

var sequences = GetCombinations(new [] { 5, 3, 2, 4 /* ... */ });
foreach (var sequence in sequences)
    Console.WriteLine(string.Join(", ", sequence));

/* Output:
0, 0, 0, 0
0, 0, 0, 1
0, 0, 0, 2
0, 0, 0, 3
0, 0, 1, 0
0, 0, 1, 1
0, 0, 1, 2
0, 0, 1, 3
0, 1, 0, 0
0, 1, 0, 1
0, 1, 0, 2
... */

For OP's specific scenario (adding arrays to collection):

var sequences = GetCombinations(new [] { 10, 20, 10 });
collection.AddRange(sequences.Select(s => s.ToArray()));