Filling matrix with ones depending on vector

Question

In Octave, I have a vector with indexes e.g. a = [ 1 2 3 1 2 3]. I now want a matrix m = zeros(size(a,2), max(a)) to have ones depending on vector a:

How do I do that?

I tried this, but it didn't work: m(a,:) = 1;

possible duplicate of [Creating an m by n matrix of 0s and 1s from m-sized vector of column indexes](http://stackoverflow.com/questions/10665179/creating-an-m-by-n-matrix-of-0s-and-1s-from-m-sized-vector-of-column-indexes) — Stewie Griffin, Jan 07 '15 at 16:22

Dan · Accepted Answer · 2015-01-08T06:05:26.920

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Assuming:

a = [1 2 3 1 2 3];
sz = [numel(a), max(a)];

using sub2ind:

m = zeros(sz);
ind = sub2ind(sz, 1:sz(1), a);
m(ind) = 1;

using sparse

m = full(sparse(1:sz(1), a, 1));

edited Jan 08 '15 at 06:05

answered Jan 07 '15 at 13:50

Dan

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Nice answer. IMO, the following is a bit simpler `full(sparse(1:numel(a), a, 1))`. – Stewie Griffin Jan 07 '15 at 16:20
@user1406177 Look at the last answer to this duplicate question: http://stackoverflow.com/questions/10665179/creating-an-m-by-n-matrix-of-0s-and-1s-from-m-sized-vector-of-column-indexes it makes use of broadcasting in Octave whih Matlab doesn't have and is much simpler. However, I would recommend against using this method as you will remove compatibility with Matlab which, even if it seems irrelevant to you now, is probably something worth preserving as it makes you code portable. – Dan Jan 08 '15 at 06:04

score 0 · Answer 2 · edited Mar 02 '15 at 03:16

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You can also index into an identity matrix like so.

a = [ 1 2 3 1 2 3];

I = eye(max(a));
m = I(a, :);

edited Mar 02 '15 at 03:16

Nathan Tuggy

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answered Mar 02 '15 at 02:49

simlist

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Filling matrix with ones depending on vector

2 Answers2