How can i increase speed of for loop in matlab? [closed] - matlab

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I want to read the value of pixcels in image result to compare this value with some
I use to for loop
function GrdImg= GrdLbp(VarImg,mapping,LbpImg)
tic
p=mapping.samples;
[Ysize,Xsize]=size(result);
GImg=zeros(Ysize,Xsize);
temp=[];
cnt=1;
for n=0:p-1
temp(cnt)=2^n;
temp(cnt+1)=(2^p)-1-(2^n);
cnt=cnt+2;
end
for i=1:Ysize
i
for j=1:Xsize
if isempty(find(result(i,j)==temp(:,:)))==1
GImg(i,j)=sqrtm(Vresult(i,j));
end
end
end
but it works too slow, Could you help me what can I use instead of for loop?
Thanks a lot

Answer 1

You didn't really give enough information to answer your question - since, as was stated in the comments, you aren't doing anything with the values in the loop right now. So let me give you a few ideas:
1) To compare all the pixels with a fixed value, and return the index of all pixels greater than 90% of the maximum:
threshold = 0.9 * max(myImage(:));
prettyBigPixels = find(myImage > threshold);
2) To set all pixels < 5% of max to zero:
threshold = 0.05 * max(myImage(:));
myImage(myImage < threshold) = 0;
In the first case, the find command returns all the indices (note - you can access a 2D matrix of MxN with a single index that goes from 1 to M*N). You can use ind2sub to convert to the individual i, j coefficients if you want to.
In the second case, putting (myImage < threshold) as the index of the matrix is called logical indexing - it is very fast, and will access only those elements that meet the criterion.
If you let us know what you're actually doing with the values found we can speed things up more; because right now, the net result of your code is that when the loop is finished, your value Temp is equal to the last element - and since you did nothing in the loop we can rewrite the whole thing as
Temp = pixel(end);
EDIT Now that you show what you are doing in your inner loop, we can optimize more. Behzad already showed how to speed up the computation of the vector temp - nothing to add there, it's the right way to do it. As for the two nested loops, which are likely the place where most time is spent, you can find all the pixels you are interested in with a single line:
pixelsOfInterest = find(~ismember(result(:), temp(:)));
This will find the index of pixels in result that do not occur in temp. You can then do
GImg(pixelsOfInterest) = sqrt(result(pixelsOfInterest));
These two lines together should replace the functionality of everything in your code from for i=1:Ysize to the last end. Note - your variables seem to be uninitialized, and change names - sometimes it's result, sometimes it's Vresult. I am not trying to debug that; just giving you a fast implementation of your inner loop.

Answer 2

As of question completely edited I answer new rather than edit my former answer, by the way.
You can improve your code in some ways:
1. instead of :
for n=0:p-1
temp(cnt)=2^n;
temp(cnt+1)=(2^p)-1-(2^n);
cnt=cnt+2;
end
use this one:
temp=zeros(1,2*p);
n=0:p-1;
temp(1:2:2*p)=2.^n; %//for odd elements
temp(2:2:2*p)=2^p-1-2.^n; %//for even elements (i supposed p>1)
2.when code is ready for calculating and not for debugging or other times, NEVER make some variables to print on screen because it makes too long time (in cpu cycles) to run. In your code there are some variables like i that prints on screen. remove them or end up them by ;.
3.You can use temp(:) in last rows because temp is one-dimensional
4.Different functions are for different types of variables. in this code you can use sqrt() instead of sqrtm(). it may be slightly faster.
5. The big problem in this code is in your last comparison, if non elemnt of temp matrix is not equal with result specific element then do something! its hard to make this part improved unless knowing the real aim of the code! you may be solve the problem in other algorithm that has completely different code. But if there is no way, so use it in this way (nested loops) Good Luck!

Answer 3

It seems your image is grayscle or monocolor , because Temp=pixel(i,j) gives a number not 3-numbers by the way.
Your question has not more explanation so I think in three type of numbers that you are comparison with.
compare with a constant number
compare with a series of numbers
compare with a two dimensional matrix of numbers
If first or third one is your need, solution is very easy (absolutely in third one, size of matrix must be equal to pixel size)
Comparison with a number (c is number or two-dimensional array)
comp=pixel - c;
But if second one is your need, you can first reshape pixel to one-dimensional matrix then compare it with the series of number s (absolutely length of this serie must be equal to product of pixel rows number and columns number; you can re-reshape pixel matrix after comparison to primary two dimensional matrix.
Comparison with a number serie s
pixel_temp = reshape(pixel,1,[]);
comp = pixel_temp - s;
pixel_compared = reshape(pixel_temp,size(pixel,1),size(pixel,2)); % to re-reshape to primary size