How to optimize this loop

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?

Answer 1

[Michelangelo Ricciulli]

I think that this should save some time, since it avoids the for-loop:

i = 1:n;
A(i,B(i,1)) = 1;

[Image Analyst]

In my testing that brought it from 0.01 seconds to 24 minutes.

Answer 2

[Image Analyst]

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?

[Israel Campiotti]

Mine is taking more than 20minutes