Code running very slow

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Darlington Mensah
Darlington Mensah am 7 Jun. 2016
My code seems to run very slowly and I can't think of any way to make it faster. All my arrays have been preallocated. S is a large number of element (say 10000 element, for example). I know my code runs slowly because of the "for k=1:S" but i cant think of another way to perform this loop at a relatively fast speed. Can i please get help because it takes hours to run.
[M,~] = size(Sample2000_X);
[N,~] = size(Sample2000_Y);
[S,~] = size(Prediction_Point);
% Speed Preallocation
Distance = zeros(M,N);
Distance_Prediction = zeros(M,1);
for k=1:S
for i=1:M
for j=1:N
Distance(i,j) = sqrt(power((Sample2000_X(i)-Sample2000_X(j)),2)+power((Sample2000_Y(i)-Sample2000_Y(j)),2));
end
Distance_Prediction(i,1) = sqrt(power((Prediction_Point(k,1)-Sample2000_X(i)),2)+power((Prediction_Point(k,2)-Sample2000_Y(i)),2));
end
end
  2 Kommentare
Adam
Adam am 7 Jun. 2016
I'm sure there are ways to vectorise this which I don't have time to suggest myself, but at a glance:
Distance(i,j) = sqrt(power((Sample2000_X(i)-Sample2000_X(j)),2)+power((Sample2000_Y(i)-Sample2000_Y(j)),2));
does not depend on k so you don't need to calculate it inside the k loop. Pre-calculate it outside once (well, once for each i, j pair) and save S-1 calculations of it.
Darlington Mensah
Darlington Mensah am 7 Jun. 2016
You are right. I realized it after posting the question and have made changes. Thanks @Adam

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Darlington Mensah
Darlington Mensah am 7 Jun. 2016
I realized the major problem was organization of my code. I was performing calculation in a loop where it was absolutely unnecessary. So i seperated the code in two blocks and it Works much faster.
for i=1:M
for j=1:N
Distance(i,j) = sqrt(power((Sample2000_X(i)-Sample2000_X(j)),2)+power((Sample2000_Y(i)-Sample2000_Y(j)),2));
end
end
for k=1:S
for i=1:M
Distance_Prediction(i,1) = sqrt(power((Prediction_Point(k,1)-Sample2000_X(i)),2)+power((Prediction_Point(k,2)-Sample2000_Y(i)),2));
end
end
Thanks to the community for the help.

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