Using elements of a vector to set elements of a matrix

Question

I have a vector whose elements identify the indices (per column) that I need to set in a different matrix. Specifically, I have:

A = 7
    1
    2

and I need to create a matrix B with some number of rows of zeros, except for the elements identified by A. In other words, I want B:

B = zeros(10, 3); % number of rows is known; num columns = size(A)
B(A(1), 1) = 1
B(A(2), 2) = 1
B(A(3), 3) = 1

I would like to do this without having to write a loop.

Any pointers would be appreciated.

Thanks.

Answer 1

Use linear indexing:

B = zeros(10, 3);
B(A(:).'+ (0:numel(A)-1)*size(B,1)) = 1;

The second line can be written equivalently with sub2ind (may be a little slower):

B(sub2ind(size(B), A(:).', 1:numel(A))) = 1;