ram issue while generating subsets

2013 7 10
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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?

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Matt J
Matt J 2013-7-10
编辑：Matt J 2013-7-10
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
sina 2013-7-10
ok, I don't need ALL subsets I have a limit for subset size for example ALL subsets with length 3

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sina
sina 2013-7-10
编辑：sina 2013-7-10

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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

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Matt J
Matt J 2013-7-10
编辑：Matt J 2013-7-10

0 个投票

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);

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sina
sina 2013-7-10
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.

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