CODEWITHSUNDEEP
matlabrandom
ximu
ximu

Reputation: 39

Choose k vectors from n vectors in Matlab

Suppose there are a matrix of three row vectors

A = [1,2;
     1,3;
     2,3]

I would like to create a new matrix B which draws two vectors from A with repetitions, and there are 3^2 possible combinations. Some simple implementation is as follows:

For i = 1:3
    c = A(i,:);
    for j=1:3
        d = A(j,:);
        B = [c;d];
    end
end

But, in general, if I need to choose k vectors from n vectors, what is the more general way to write such loop? It's difficult to continue write loop using i, j, ... I guess. Thanks!

Upvotes: 1

Views: 55

Answers (3)

ximu
ximu

Reputation: 39

Thanks for all previous suggestions. In the end I figure out what I want is called permutations with repetitions. The matlab function permun from file exchange solves my problem.

Upvotes: 0

Dan
Dan

Reputation: 45741

You can just use randi for this to pick k uniformly distributed numbers in the range 1:n (with replacement)

k = 2;
n = size(A,1);
rowIdx = randi(n,k)
B = A(rowIdx,:)

Upvotes: 1

Shai
Shai

Reputation: 114786

For sampling at random, matlab has randsample:

rowIdx = randsample( size(A,1), k, true );
B = A(rowIdx,:);

Upvotes: 1

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