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For loop slows down on row 25 of 3D matrix and does not advance and original speed. What may be the issue?

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mashtine on 24 Jun 2015
Commented: mashtine on 24 Jun 2015
Hi everyone,
I am using the following code to calculate a given parameter. I have included a progress bar for testing purposes and I noted that it always sticks at 19%. Upon further snooping around, I realize the for loop does not stop but greatly slows down after 19% even though the progress bar says only 2 min 33 secs left.
I am not running out of memory (checking with top command in the terminal) and there seems to be nothing odd about the data/anything different from surrounding data.
testcase1 = single(zeros(length(lon),length(lat),length(time)));
testcase2 = single(zeros(length(lon),length(lat),length(time)));
for i = 1:length(lon);
for j = 1:length(lat);
for t = 1:length(time);
if C(i,j,t) >= -500 && C(i,j,t) <= -12
testcase1(i,j,t) = A(i,j,t)*(0.35*((-(B(i,j,t)/(K*C(i,j,t))))^(2/3)) + (2 -(10/B(i,j,t))))^(1/2);
testcase2(i,j,t) = 2*A(i,j,t)*((1 -(10/B(i,j,t)))^(1/2));
progressbar(i/121) % Update progress bar
Any ideas why this would stick/slow down here?

Accepted Answer

Guillaume on 24 Jun 2015
If that's the above code that you're running, I don't see any reason for it to hang.
However, there are many ways to speed it up. For a start, passing the output class (single) to zeros instead of generating a double matrix and then converting it to single would be faster:
testcase1 = zeros(numel(lon), numel(lat), numel(time), 'single');
testcase2 = zeros(numel(lon), numel(lat), numel(time), 'single');
Secondly, the loop is completely unnecessary (and is a performance killer). Use vectorised operations:
tf = C >= -500 & C <= -12;
testcase1(tf) = A(tf).*(0.35*((-(B(tf)./(K*C(tf)))).^(2/3)) + (2 -(10./B(tf)))).^(1/2);
testcase2(~tf) = 2*A(~tf).*((1 -(10./B(~tf))).^(1/2));
Just three lines!

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