Creating Indicator matrix based on vector with group IDs

Question

I have a vector of group IDs:

groups = [ 1 ; 1; 2; 2; 3];

which I want to use to create a matrix consisting of 1's in case the i-th and the j-th element are in the same group, and 0 otherwise. Currently I do this as follows:

n = size(groups, 1);
indMatrix = zeros(n,n);
for i = 1:n
    for j = 1:n
        indMatrix(i,j) = groups(i) == groups(j);
    end  
end
indMatrix

indMatrix =

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

Is there a better solution avoiding the nasty double for-loop? Thanks!

score 2 · Accepted Answer · answered Oct 07 '19 at 09:08

2

This can be done quite easily using implicit singleton expansion, for R2016b or later:

indMatrix = groups==groups.';

For MATLAB versions before R2016b you need bsxfun to achieve singleton expansion:

indMatrix = bsxfun(@eq, groups, groups.');

answered Oct 07 '19 at 09:08

Luis Mendo

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What's the role of adding the extra '.' after groups? Seems to work also without. – bonifaz Oct 07 '19 at 09:14
1

@bonifaz Good question! See [here](https://stackoverflow.com/q/25150027/2586922) – Luis Mendo Oct 07 '19 at 09:23

Creating Indicator matrix based on vector with group IDs

1 Answers1