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How do I use a vector as a set of indices?

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So, I have a 5000x1 vector y, the entries of y are all integer values between 1 and 10.
I want to create a new matrix, yrec, that is 10x5000, such that if y(1)=10 then yrec(10,1)=1, if y(2)=8 then yrec(8,2)=1, if y(3)=4 then yrec(4,3)=1 etc.
and all other entries in that row are equal to zero. I can do this through a for loop quite easily as follows
m=5000;
num_labels=10;
yrec=zeros(num_labels,m); %recoded y
for i=1:m
yrec(y(i),i)=1;
end
which works without issue. However, I was wondering if there is a vectorized way to do this? hopefully more computationally efficient than a for loop?
I tried
yrec(y,1:m)=1;
but this just sets every entry in yrec equal to 1 for some reason.

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Accepted Answer

Alex Mcaulley
Alex Mcaulley on 3 Feb 2020
Try this:
m = 5000;
num_labels = 10;
y = randi(num_labels,m,1);
yrec = zeros(num_labels,m);
yrec(sub2ind(size(yrec),y',1:m)) = 1;

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