Efficiently unpack a vector into binary matrix Octave

Use a sparse matrix (which also saves a lot of memory) which can be used in further calculations as usual:

y = [1; 2; 4; 1; 3]
y01 = sparse (1:rows (y), y, 1)

if you really want a full matrix then use "full":

full (y01)
ans =
1   0   0   0
0   1   0   0
0   0   0   1
1   0   0   0
0   0   1   0