MATLAB: Nchoosek – non consecutive numbers

#nchoosekalgorithmcombinationspermutations

If have a set of 12 numbers (eg. 5, 7, 13, 14, 17, 18, 21, 24, 27, 35, 37, 39) and I would like to calculate a list of all the possible combinations of those numbers using only 7 of them. How would I do that? I don't care to know all the possible permutations of 7 numbers (that's the next step), just the combinations. Thank you.

Using C = m!/n!(m-n)! where m = 12 and n = 7 I know the result should yield a 7 X 792 matrix.

Best Answer

https://www.mathworks.com/help/stats/combnk.html

https://www.mathworks.com/help/matlab/ref/nchoosek.html

Notice the syntax involving v, a vector of specific values.

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MATLAB: Nchoosek help

Hi Sean, 4 people in a line is just factorial(4). nchoosek assumes you are interested in how many ways you can choose a number of objects out of set where order is not important. Note that arrangement in a circle is different.

Wayne

MATLAB: Nchoosek with multiple arrays

Simply generate a matrix of indices:

idx = nchoosek(1:33,3);

and then for each row in that matrix perform your calculations, where M is your matrix:

for k = 1:size(idx,1)
    ix = idx(k,:);
    v1 = M(ix,1); % column 1
    v2 = M(ix,2); % column 2
    v3 = M(ix,3); % column 3
    ... do whatever with those vectors
end

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MATLAB: Nchoosek help

MATLAB: Nchoosek with multiple arrays

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