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How to optimize this loop

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Israel Campiotti
Israel Campiotti on 16 Aug 2017
Closed: MATLAB Answer Bot on 20 Aug 2021
I want to do a loop like this
for i = 1:n
A(i,B(i,1)) = 1;
end
but my matrix has size one million. Is there a way to do it faster?

Answers (2)

Michelangelo Ricciulli
Michelangelo Ricciulli on 16 Aug 2017
I think that this should save some time, since it avoids the for-loop:
i = 1:n;
A(i,B(i,1)) = 1;
  1 Comment
Image Analyst
Image Analyst on 16 Aug 2017
In my testing that brought it from 0.01 seconds to 24 minutes.

Image Analyst
Image Analyst on 16 Aug 2017
On my computer, it takes only 9 milliseconds for a million elements:
rows = 1000000;
columns = 5;
A = rand(rows, columns);
B = randi(columns, rows, 1);
tic
for k = 1 : rows
A(k, B(k)) = 1;
end
toc
Why do you need it faster?
  1 Comment
Israel Campiotti
Israel Campiotti on 16 Aug 2017
Mine is taking more than 20minutes

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