# How to vectorize this simple for-loop

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Felix Lauwaert on 19 Aug 2015
Commented: Felix Lauwaert on 19 Aug 2015
Hello,
I'm trying to vectorize something like this:
z=zeros(3); x=[1 2 3]; y=x;
for i=1:length(x) %or y, doesn't matter
z( x(i),y(i) )=z( x(i),y(i) ) + 1;
end
So
z= [1 0 0 ; 0 1 0 ; 0 0 1]
What I really want is to do it for any x and y vectors in a 1000by1000 zero matrix.
I tried to do:
z(x,y)=z(x,y)+1;
But I get all values of z =1. I also tried:
z(x.,y.)=z(x.,y.)+1;
But an error message appears.
Thanks for your help.

Matt J on 19 Aug 2015
Edited: Matt J on 19 Aug 2015
idx=sub2ind(size(z),x,y);
z(idx)=z(idx)+1;
##### 2 CommentsShowHide 1 older comment
Felix Lauwaert on 19 Aug 2015
idx=sub2ind([length(x),length(x)],x,y); z(idx)=z(idx)+1;
Is working. Thanks!

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