Vectorize my code in azimuth calculation.

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Mani Ahmadian 2014-9-23

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评论： Mani Ahmadian 2014-9-25

Hi I have lots of points(in Latitude & Longitude) and I want to calculate the azimuth of each pairs of them. I use code as attached file 'AzimuthCalc.m'. when I run my function, it's so time consuming and it's elapsed time is 109.371341 seconds. I'm sure the problem caused by my non-vectorize code.

So, would you please check my function and help me to improve it?

Thanks a lot Mani

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Adam 2014-9-23

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Run:

profile on
your code
profile off
profile viewer

and find out where your code is slow. No point in guessing when you can use facts!

Mani Ahmadian 2014-9-23

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Dear Adam,

Thanks so much. I guess the problem is here(nested for):

for ii=1:length(lat)
    for jj=ii+1:length(lat)
        AzimuthPoints(ii,jj)=azimuth('rh',lat(ii,1),long(ii,1),lat(jj,1),long(jj,1));
    end
end

If I can change my simple code to vectorize one, problem will solve. So, I'm trying to find a way to do that, so plz help me.

Thanks

Mnai

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Answer 1

Guillaume 2014-9-23

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I don't have the mapping toolbox, so I'm just relying on the documentation to write my answer.

For a start, you can replace the inner loop with:

AzimuthPoints(ii,ii+1:end)=azimuth('rh',lat(ii),long(ii),lat(ii+1:end),long(ii+1:end));

I believe the following may work to replace both loops (and the AzimuthPoints declaration)

[lat1, lat2] = meshgrid(lat);
[long1, long2] = meshgrid(long);
AzimuthPoints = azimuth('rh', lat1, long1, lat2, long2);

The downside of this method is that it's going to calculate the whole AzimuthPoints matrix, not just the upper triangle, so it may actually be slower.

You could do:

lat1 = triu(lat1, 1);
%same with lat2, long1 and long2

to replace the lower triangle and main diagonal with zero, on the assumption that calculating

azimuth('rh', 0, 0, 0, 0)

is faster.

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Guillaume 2014-9-24

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TrueAzimuth should be 0s and 1s but of type logical. If you used my code to generate it, it shoul already be logical, but if not:

TrueAzimuth = logical(TrueAzimuth);

Mani Ahmadian 2014-9-25

Thanks, It works correctly.

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Answer 2

Roger Stafford 2014-9-24

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You could try this. It calls on 'azimuth' in the same sequence as in your for-loop code, but it is vectorized. Whether it is faster depends on whether the 'tril' and 'repmat' calls take less time than the overhead in the for-loops and individual 'azimuth' calls in your code.

   n = length(lat);
   p = tril(repmat(1:n,n,1),-1);
   p = p(p>0);
   q = tril(repmat((1:n)',1,n),-1);
   q = q(q>0);
   AzimuthPoints = zeros(n);
   AzimuthPoints(p+n*(q-1)) = azimuth('rh',lat(p),long(p),lat(q),long(q));