CODEWITHSUNDEEP
matlabmatrix
motaha
motaha

Reputation: 403

How to compute the outer product of two binary vectors

I am generating a random binary matrix with a specific number of ones in each row. Now, I want to take each row in the matrix and multiply it by its transpose (i.e row1'*row1).

So, I am using row1=rnd_mat(1,:) to get the first row. However, in the multiplication step I get this error

"Both logical inputs must be scalar. To compute elementwise TIMES, use TIMES (.*) instead."

Knowing that I don't want to compute element-wise, I want to generate a matrix using the outer product. I tried to write row1 manually using [0 0 1 ...], and tried to find the outer product. I managed to get the matrix I wanted.

So, does anyone have some ideas on how I can do this?

Upvotes: 2

Views: 1231

Answers (1)

rayryeng
rayryeng

Reputation: 104464

Matrix multiplication of logical matrices or vectors is not supported in MATLAB. That is the reason why you are getting that error. You need to convert your matrix into double or another valid numeric input before attempting to do that operation. Therefore, do something like this:

rnd_mat = double(rnd_mat); %// Cast to double
row1 = rnd_mat(1,:);
result = row1.'*row1;

What you are essentially computing is the outer product of two vectors. If you want to avoid casting to double, consider using bsxfun to do the job for you instead:

result = bsxfun(@times, row1.', row1);

This way, you don't need to cast your matrix before doing the outer product. Remember, the outer product of two vectors is simply an element-wise multiplication of two matrices where one matrix is consists of a row vector where each row is a copy of the row vector while the other matrix is a column vector, where each column is a copy of the column vector.

bsxfun automatically broadcasts each row vector and column vector so that we produce two matrices of compatible dimensions, and performs an element by element multiplication, thus producing the outer product.

Upvotes: 3

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