ram issue while generating subsets

alright, I wrote a code in matlab which in a part of that it needs to generate all subsets of a vector and process all of them one by one. For example having vector X, I first used the function combntns(X, k) to generate the subsets but there is a problem that X can be in a very high dimension so to save the all subsets of X it requires massive amount of ram! And I always run out of ram memory while running the code. That is so unnecessary to save all the subsets since the algorithm will process them one by one. I wonder is there any function to do this for me or I have to write a code manually to do so?

[Matt J]

I think the path forward is to explain why you think you need ALL subsets, so that we can assess whether that is true or not. It sounds like a brute force approach that's probably meant to be done differently, maybe with a few compromises.

[sina]

ok, I don't need ALL subsets I have a limit for subset size for example ALL subsets with length 3

Answer 1

[sina]

I just used this code I wrote, in case anyone faced this problem in future.

function nextSubset = getNextSubset(set, currentSubset)
  % this function returns indexes of next subset
  % subsets will return with their length order
  % returns NaN if end of subsets reached

    if exist('currentSubset', 'var') == 0
        nextSubset = [];
        return
    end
    if isempty(currentSubset)
        nextSubset = 1;
        return
    end

    n = length(set);
    m = length(currentSubset);

    if currentSubset == n-m+1:n
        if m == n
            nextSubset = NaN;
            return
        end

        nextSubset = 1:m+1;
        return
    end

    nextSubset = currentSubset;
    nextSubset(m) = nextSubset(m) + 1;

    if nextSubset(m) == n+1
        c = m - 1;
        while nextSubset(c) == n-m+c
            c = c - 1;

            if c == 0
                nextSubset = NaN;
                return
            end
        end

        nextSubset(c:m) = nextSubset(c)+1:m-c+nextSubset(c)+1;
    end
end

Answer 2

[Matt J]

ok, I don't need ALL subsets I have a limit for subset size for example ALL subsets with length 3

If the length of the subset is small (typically around 3 if that's what you're saying), then nchoosek should give you what you're looking for pretty easily and without too much strain in memory,

 Xsubsets=nchoosek(X,3);

[sina]

nop, it has the the previous problem too. I just didn't want to save the subset all at once to prevent occupying memory. The code I wrote manually does exactly what is needed to be done.