speed up this matlab code?

Question

Is there a good way to speed up this block of matlab code (n in particular, can be large) using matrix operations, or anything? Over 1/4 of my execution time is in this small block of code.

% Get the bin indexes that we will place the network in
bins = [];
for n=low_freq:0.5:high_freq;
    bins = [bins, (n-spec_start)/spec_bin_size+1];
end

Test code:

spec_start=2400
spec_bin_size=0.5
low_freq = 2437
high_freq=2438

bins = [];
for n=low_freq:0.5:high_freq;
    bins = [bins, (n-spec_start)/spec_bin_size+1];
end

bins  % 75 76 77

bins = [];
bins = (low_freq:0.5:high_freq - spec_start)./spec_bin_size + 1;

bins  % empty?

Jonas · Accepted Answer · 2011-11-16 21:03:35Z

6

You can skip the loop:

bins = ((low_freq:0.5:high_freq) - spec_start)./spec_bin_size + 1;

In situations where you can't do vectorized calculations as above, you should at least preallocate the output array

edited Nov 16, 2011 at 21:03

answered Nov 16, 2011 at 20:37

Jonas

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4 Comments

Jonas Over a year ago

@strictlyrude27: Sorry for making you waste your time.

gnychis Over a year ago

hmmmm, I think something is slightly off. I think the result of this script should be 75,76,77 .. but I get an empty matrix from your code. Test code included in edit'ed question. Thanks so much for the help!

gnychis Over a year ago

a-ha, just a little adjustment to parens: ((low_freq:0.5:high_freq) - spec_start)./spec_bin_size + 1

Dusty Campbell Over a year ago

missing square brackets. how about this:([low_freq:0.5:high_freq] - spec_start)./spec_bin_size + 1

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