Question

I’m trying to get a binary one-of-K coding of a integer vector in Octave. I’ve got a vector y, say

y = [1 ; 2 ; 3 ; 1 ; 3]

and I want a matrix

Y = [1 0 0
     0 1 0
     0 0 1
     1 0 0
     0 0 1]

I can construct the one-of-K matrix by hand with

Y = [];
Y = [Y y == 1];
Y = [Y y == 2];
Y = [Y y == 3];

But when I try to construct it with a for loop,

Y = [];
for i = unique(y),
    Y = [Y y == i];
endfor

something goes wrong:

error: mx_el_eq: nonconformant arguments (op1 is 5x1, op2 is 3x1)

I don’t even understand the error message. Where’s my mistake?

Answer 1

Ok, found it. I wish the tutorial had told me this.

Y = [];
for i = unique(y)',
%                ^
%  -------------/
    Y = [Y y == i];
end

Apparently, for loops through the columns of a vector from left to right and unique returns a column vector, so the “nonconformant arguments” are y (5×1) and unique(y) (3×1).