MATLAB suddenly behaving very slowly...?

Hi!

I was working on some code to barrel distort an image, then I saved the code, took a lunch break, and when I came back (not making any changes whatsoever), the code was running orders of magnitudes slower.

After doing some playing around with the tic and toc functions, I have obtained the following very strange result

(I have tried restarting my computer/MATLAB several times already):

%I, m2, n2, and A are all m x n x 1 arrays
%i=j=1

tic;

I(m2(i, j), n2(i, j)) = A(i, j);

toc;

Elapsed time is 0.000003 seconds.

tic;

for i = 1:1

      for j = 1:1

          I(m2(i, j), n2(i, j)) = A(i, j);

      end

end

toc;

Elapsed time is 0.001875 seconds.

(This code was run in a script, so there's no delay from not pressing enter or something like that).

What do you make of this? This is a 625-fold increase in time!

My first reaction was there's some overhead in calling the 2 for loops, however this is not the case, as I can demonstrate with the following example:

tic;
A(1, 1) = 1;
toc;
Elapsed time is 0.000002 seconds.

tic;
for i = 1:1
    for j = 1:1
        A(1, 1) = 1;
    end
end
toc;
Elapsed time is 0.000001 seconds.

It's quite baffling to me...

[Walter Roberson]

In the first case, i and j are going to have their default values as being sqrt(-1)

[Anish]

To clarify - i and j had been set to 1 (i = 1; j = 1;) before that.