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

possible duplicate of [How can I change the values of multiple points in a matrix?](http://stackoverflow.com/questions/6850368/how-can-i-change-the-values-of-multiple-points-in-a-matrix) — gnovice, Mar 30 '12 at 17:26

Hans Then · Answer 1 · 2012-09-20T22:15:27.173

15

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.

edited Sep 20 '12 at 22:15

answered Sep 17 '12 at 09:00

Hans Then

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I don't understand the syntax for this – Dan May 09 '19 at 13:03
Play with it. Try with a small identity matrix and first run single values instead of a vector x. E.g. `eye(4)(3,:)` This will take the third row of the identity matrix. Then try with a vector `eye(4)([2,3,1], :)`. This will take the second, third and first row of the identity matrix. – Hans Then May 17 '19 at 11:48

score 3 · Accepted Answer · answered Mar 30 '12 at 17:24

3

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;

answered Mar 30 '12 at 17:24

yuk

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Charity Leschinski · Answer 3 · 2012-08-29T14:39:06.597

I had a very similar question, so I didn't want to open a new one. I wanted to convert a row vector of indices into a matrix with ones in the rows (instead of columns) of the indices. I could have used the previous answer and inverted it, but I thought this would perform better with very large matrices.

octave> x = [2 1 3 1];
octave> m = setRowsToOne(x, 3)
m =

   0   1   0   1
   1   0   0   0
   0   0   1   0

I couldn't see how to use sub2ind to accomplish this, so I calculated it myself.

function matrixResult = setRowsToOne(indexOfRows, minimumNumberOfRows)
   numRows = max([indexOfRows minimumNumberOfRows]);
   numCols = columns(indexOfRows);
   matrixResult = zeros(numRows, numCols);
   assert(indexOfRows > 0, 'Indices must be positive.');
   matrixResult(([0:numCols-1]) * numRows + indexOfRows) = 1;
end

x = [2 1 3 1];
m = setRowsToOne(x, 3)

score 0 · Answer 4 · answered Sep 28 '15 at 14:26

0

You can use accumarray which makes this very easy, like so:

accumarray([ (1:length(x))', x ], 1, [4, 3])

The 1:length(x) part specifies into which rows the ones go, and x into which columns.

answered Sep 28 '15 at 14:26

jlh

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Convert vector of indices into matrix

4 Answers4