# How to use "imregionalmax" without using for loop

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moh mor on 12 Jan 2024
Edited: Matt J on 15 Jan 2024
Hello, I have an 51×20×20 Array and I wish to find the second biggest peak in each 20×20 array (I find peaks by performing "imregionalmax" over each 20×20 arrays). As the dimension of my array represents, I have 51 of these 20×20 arrays. I want to do this without using for loop to iterate over all these 51 arrays and also, increase my speed. Can anyone help me with this?
UPDATE
This is my original time-consuming part of code:
z = 0;
for i=1:size(AF_Nuni,1),
peaks = sort(AF_Nuni(i,imregionalmax(squeeze(AF_Nuni(i,:,:)))));
peaksi = peaks(end - 1);
z = z + peaksi;
end
"AF_Nuni" is a 1156*34*34 gpuArray. it takes about 8.2 seconds which is not acceptable for me.
Catalytic on 15 Jan 2024
Are the values of AF_Nuni non-negative?
moh mor on 15 Jan 2024
yes it is nonnegative and something between 0 and 1.

Matt J on 13 Jan 2024
Edited: Matt J on 14 Jan 2024
Perhaps as follows,
AF_Nuni = permute(AF_Nuni,[2,3,1]); %Avoid this permutation by forming AF_Nuni
%in an appropriate order
AF_Nuni( ~imregionalmaxSices(AF_Nuni) ) = nan;
peaks=maxk( reshape( AF_nuni, [],size(AF_Nuni,3) ) ,2,1);
z=sum(peaks(2,:));
function D=imregionalmaxSices(A)
B=padarray(A,[1,1],-inf); %A is the input array
C=reshape( imregionalmax(B(:,:)) ,size(B));
D=C(2:end-1,2:end-1,:); %the result
end
moh mor on 15 Jan 2024
unfortunatley, it gives the following error:
Error using movmax
Invalid data type. First input must be numeric or logical.
Matt J on 15 Jan 2024
Edited: Matt J on 15 Jan 2024
First input must be numeric or logical.
And indeed it should be! Surely if you are giving movmax non-numeric input, it is inadvertent, and something you can easily debug.

Image Analyst on 13 Jan 2024
peaks = sort(AF_Nuni(i,imregionalmax(squeeze(AF_Nuni(i,:,:)))));
The way you're indexing could be slowing you down. Normally you loop over the last index, not the first one. If you could rotate your volumetric image in advance and then iterate over the last index. You could access memory much more efficiently and you would not need to use squeeze.
moh mor on 13 Jan 2024
Edited: moh mor on 13 Jan 2024
thank you @Image Analyst,
Sorry , but I do not why when I reshape AF_Nuni to a 34*34*1156 and want to do whatever you said, I face with this error in the 1st iteration, i.e. i=1 (I have checked AF_Nuni demention and size already). can you help me with this?
peaks = sort(AF_Nuni(imregionalmax(AF_Nuni(:,:,i)),i));
here the error:
Error using gpuArray/subsref
Index exceeds matrix dimension.
Image Analyst on 14 Jan 2024
for i=1:size(AF_Nuni,1),
you need
for i=1:size(AF_Nuni,3)

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