Convert vector of indices into matrix

Question

I want to convert a vector of indices into a matrix with ones in the columns of the indices.

x = [2;1;3;1];
m = someFunc(x,3)
% m =
%
%   0   1   0
%   1   0   0
%   0   0   1
%   1   0   0

Answer 1

I tested the sub2ind function, but on the coursera Machine Learning forum I was pointed to this beauty.

m = eye(num_cols)(x,:);

It uses the identity matrix to select the appropriate column based on the value in x.

Answer 2

One way is to use SUB2IND function:

colN = 3;
assert(max(x)<=colN,'Not enough columns') %# check that you have enough columns
%# other checks that x is valid indices

m = zeros(numel(x),colN);
m(sub2ind(size(m),1:numel(x),x')) = 1;