Basically, I'd like a set of sets that contains from (0..9), then (0, 1..9), (1, 2..9)..(8,9), and so on and so forth up until (0,1,2,3,4,5,6,7,8,9). I know this can be accomplished by nesting for loops in the manner below, but I'm wondering if there's a neater way of doing it?
Preferably something that could be accomplished within C#, but I'm interested in any algorithm.
for (int i = 0; i < max; i++) {
yield {i};
for (int j = i + 1; j < max; j++) {
yield {i, j};
for (int k = j + 1; k < max; k++) {
yield {i, j, k};
for (int l = k + 1; l < max; l++) {
yield {i, j, k, l};
for (int m = l + 1; m < max; m++) {
yield {i, j, k, l, m};
// And so on and so forth
}
}
}
}
}
I wrote this a while ago. It uses a Stack. It's generic, so it can be used for other sequences as well.
static IEnumerable<T[]> CombinationsAnyLength<T>(params T[] values)
{
Stack<int> stack = new Stack<int>(values.Length);
int i = 0;
while (stack.Count > 0 || i < values.Length) {
if (i < values.Length) {
stack.Push(i++);
int c = stack.Count;
T[] result = new T[c];
foreach (var index in stack) result[--c] = values[index];
yield return result;
} else {
i = stack.Pop() + 1;
if (stack.Count > 0) i = stack.Pop() + 1;
}
}
}
CombinationsAnyLength(1, 2, 3, 4) outputs:
1 12 123 1234 124 13 134 14 2 23 234 24 3 34 4
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