Matrix indexing using a vector

Let's say I have a vector b where each row correspond to an index for each row in the matrix A. I want to assign 1 to these elements, like in the matrix C, without loops. Thank you

A = zeros(4,3);
b = [2;1;2;3];
C = [0 1 0; 1 0 0; 0 1 0; 0 0 1]

Answer 1

[Stephen23]

You can use sub2ind for this:

>> A = zeros(4,3);
>> col = [2;1;2;3];
>> row = 1:numel(col);
>> A(sub2ind(size(A),row(:),col(:))) = 1
A =
   0   1   0
   1   0   0
   0   1   0
   0   0   1

Answer 2

[Andrei Bobrov]

C = accumarray([(1:numel(b))' b],1)