How to create matrix of all combinations?

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Mr M.
Mr M. 2018-7-27
评论： Matt J 2018-8-23
Suppose I have variables x1, x2, x3. x1 can be any element of the set {1,2,3}, x2 can be {1,2,3,4,5} and x3 can be {10,11,12,13,14,15} for example. How to create the matrix of all combinations, in which each row is a possible combination: [3,4,12] for example. Is it possible to do it without a for cycle with some tricks?

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回答（2 个）

Patrick Zwierzynski
编辑：Patrick Zwierzynski 2018-7-27
If you change x1, x2, and x3 to be vectors instead of sets, you can use the "combvec" function and simply call "combvec(x1, x2, x3)". You can then use the column indices of the resulting matrix to select the different combinations.
Here's the link to the documentation: https://www.mathworks.com/help/nnet/ref/combvec.html
Bruno Luong
Bruno Luong 2018-7-27
c={[1 2 3],[1 2 3 4 5],[10 11 12 13 14 15]}
[c{:}]=ndgrid(c{:});
n=length(c);
c = reshape(cat(n+1,c{:}),[],n)
  2 个评论
Mr M.
Mr M. 2018-7-27
[c{:}]=ndgrid(c{:}); is very very very slow! :(
Matt J
Matt J 2018-8-23
Not slower than it should be. If you have many combinations, then naturally it will take a long time.

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