Finding all subsets of specified size

Question

I've been scratching my head about this for two days now and I cannot come up with a solution. What I'm looking for is a function f(s, n) such that it returns a set containing all subsets of s where the length of each subset is n.

Demo:

s={a, b, c, d}

f(s, 4)
{{a, b, c, d}}

f(s, 3) 
{{a, b, c}, {a, b, d}, {a, c, d}, {b, c, d}}

f(s, 2)
{{a, b}, {a, c}, {a, d}, {b, c}, {b, d}, {c, d}}

f(s, 1)
{{a}, {b}, {c}, {d}}

I have a feeling that recursion is the way to go here. I've been fiddling with something like

f(S, n):
   for s in S:
       t = f( S-{s}, n-1 ) 
       ...

But this does not seem to do the trick. I did notice that len(f(s,n)) seems to be the binomial coefficient bin(len(s), n). I guess this could be utilized somehow.

Can you help me please?

Answer 1

Let us call n the size of the array and k the number of elements to be out in a subarray.

Let us consider the first element A[0] of the array A.
If this element is put in the subset, the problem becomes a (n-1, k-1) similar problem.
If not, it becomes a (n-1, k) problem.

This can be simply implemented in a recursive function.

We just have to pay attention to deal with the extreme cases k == 0 or k > n.

During the process, we also have to keep trace of:

• n: the number of remaining elements of A to consider

• k: the number of elements that remain to be put in the current subset

• index: the index of the next element of A to consider

• The current_subset array that memorizes the elements already selected.

  Here is a simple code in c++ to illustrate the algorithm

Output

For 5 elements and subsets of size 3:

3 4 5
2 4 5
2 3 5
2 3 4
1 4 5
1 3 5
1 3 4
1 2 5
1 2 4
1 2 3

#include    <iostream>
#include    <vector>

void print (const std::vector<std::vector<int>>& subsets) {
    for (auto &v: subsets) {
        for (auto &x: v) {
            std::cout << x << " ";
        }
        std::cout << "\n";
    }
}
//  n: number of remaining elements of A to consider
//  k: number of elements that remain to be put in the current subset
//  index: index of next element of A to consider

void Get_subset_rec (std::vector<std::vector<int>>& subsets, int n, int k, int index, std::vector<int>& A, std::vector<int>& current_subset) {
    if (n < k) return;   
    if (k == 0) {
        subsets.push_back (current_subset);
        return;
    }  
    Get_subset_rec (subsets, n-1, k, index+1, A, current_subset);
    current_subset.push_back(A[index]);
    Get_subset_rec (subsets, n-1, k-1, index+1, A, current_subset);
    current_subset.pop_back();         // remove last element
    return;
}

void Get_subset (std::vector<std::vector<int>>& subsets, int subset_length, std::vector<int>& A) {
    std::vector<int> current_subset;
    Get_subset_rec (subsets, A.size(), subset_length, 0, A, current_subset);
}

int main () {
    int subset_length = 3;     // subset size
    std::vector A = {1, 2, 3, 4, 5};
    int size = A.size();
    std::vector<std::vector<int>> subsets;

    Get_subset (subsets, subset_length, A);
    std::cout << subsets.size() << "\n";
    print (subsets);
}

Live demo

Answer 2

One way to solve this is by backtracking. Here's a possible algorithm in pseudo code:

def backtrack(input_set, idx, partial_res, res, n):
  if len(partial_res == n):
    res.append(partial_res[:])
    return
  
  for i in range(idx, len(input_set)):
    partial_res.append(input_set[i])
    backtrack(input_set, idx+1, partial_res, res, n) # path with input_set[i]
    partial_res.pop()
    backtrack(input_set, idx+1, partial_res, res, n) # path without input_set[i]

Time complexity of this approach is O(2^len(input_set)) since we make 2 branches at each element of input_set, regardless of whether the path leads to a valid result or not. The space complexity is O(len(input_set) choose n) since this is the number of valid subsets you get, as you correctly pointed out in your question.

Now, there is a way to optimize the above algorithm to reduce the time complexity to O(len(input_set) choose n) by pruning the recursive tree to paths that can lead to valid results only.

If n - len(partial_res) < len(input_set) - idx + 1, we are sure that even if we took every remaining element in input_set[idx:] we are still short at least one to reach n. So we can employ this as a base case and return and prune.

Also, if n - len(partial_res) == len(input_set) - idx + 1, this means that we need each and every element in input_set[idx:] to get the required n length result. Thus, we can't skip any elements and so the second branch of our recursive call becomes redundant.

backtrack(input_set, idx+1, partial_res, res, n) # path without input_set[i]

We can skip this branch with a conditional check. Implementing these base cases correctly, reduces the time complexity of the algorithm to O(len(input_set) choose k), which is a hard limit because that's the number of subsets that there are.