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 using recursion? Below is a simple example of what im trying to convert into a recursive call.

for(int a= 0; a < 10; a++)
{
    for(int b = 0; b < 20; b++)
    {
        for(int c = 0; c < 10; c++)
        {
            int[] indexes = new int[3]{a,b,c}
            collection.add(indexes);
        }
    }
}

EDIT: The solution needs to be able to be adjusted at runtime, such that a user can select how many levels are required.

Answer 1

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

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

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

// Per http://stackoverflow.com/q/1577822
public static IEnumerable<T> Yield<T>(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()));