# Speeding up matrix comparison within a loop (logical indexing???)

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Andriy Boubriak on 27 Jan 2015
Commented: Andriy Boubriak on 28 Jan 2015
At the moment as part of a larger programme I have the following code, however it takes a 10s of minutes to run. I've seen people on these forums reference something called "logical indexing" that removes the need for the loop however googling logical indexing I couldn't figure out quite what it is or how it works.
for i=2:12
sizeOfIm=size(squeeze(imlab50(i,:,:,:)));
outputIm=zeros(sizeOfIm(1),sizeOfIm(2),sizeOfIm(3));
outputIm(:,:,:)=squeeze(imlab50(i,:,:,:));
outputIm=uint8(outputIm);
for loopy = 1 : sizeOfIm(1)
for loopx = 1:sizeOfIm(2)
if (squeeze((backGrndColour(i,:)).'-mindif(:))>squeeze(imlab50(i,loopy,loopx,:)))
if (squeeze(backGrndColour(i,:)).'-maxdif(:) <squeeze(imlab50(i,loopy,loopx,:)))
outputIm(loopy,loopx,:)=0;
end
end
end
end
In short how do I do the following comparison for every value in the matrices, without having three serperate nested loops that vary i, loopy, and loopx?
if (squeeze((backGrndColour(i,:)).'-mindif(:))>squeeze(imlab50(i,loopy,loopx,:)))

Sara on 27 Jan 2015
Without having any data to test it, I'd say you could do:
for i=2:12
x = squeeze(imlab50(i,:,:,:));
outputIm=uint8(x);
[j,k] = find(squeeze((backGrndColour(i,:)).'-mindif(:)) > x);
outputIm(j,k,:) = 0;
end
In the find, check if it should be x or outputIm.
Andriy Boubriak on 27 Jan 2015
I got the followng error
"Error using > Number of array dimensions must match for binary array op.
Error in locator (line 32) [j,k] = find(squeeze((backGrndColour(i,:)).'-mindif(:)) > x);"

Image Analyst on 27 Jan 2015
What is imlab50? Is it a uint8 video?
And this code:
outputIm=zeros(sizeOfIm(1),sizeOfIm(2),sizeOfIm(3));
outputIm(:,:,:)=squeeze(imlab50(i,:,:,:));
outputIm=uint8(outputIm);
can most likely be replaced with this to save some time possibly.
outputIm = uint8(squeeze(imlab50(i,:,:,:)));
Another thing - you made a very common error, getting x and y wrong. You're calling the first index x and the second one y - that's wrong. The first index is the row index and thus should be y not x, and the second index is the column and thus should be called x, not y.
Andriy Boubriak on 28 Jan 2015
it's an array which holds 12 cie-lab images.