MATLAB: How count the number of occurence beyond a threshoold value in a matrix

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Suppose there is a matrix of a = 512*512 elements. What is the count of the elements having more than 255 in the matrix? Kindly suggest.?

Best Answer

sum(a(:)>255)

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MATLAB: Efficient number occurence count

In this situation, accumarray will be faster than histcounts, but still not as fast as the for-loop,

tic;
hc=accumarray(num(:),1,[mx,1]).';
toc
Elapsed time is 0.172890 seconds. %for-loop

Elapsed time is 0.236013 seconds. %accumarray

unless the values are pre-sorted,

num=sort(num(:));
tic;
hc=accumarray(num(:),1,[mx,1]).';
toc
Elapsed time is 0.168976 seconds. %for-loop
Elapsed time is 0.075965 seconds. %accumarray

I think this is simply one of those situations where Matlab's for-loop optimization has caught up to vectorized code.

MATLAB: Count number of times value occurs

Method for large a and b

[u,~,j] = unique(b(:));
[b, i] = ismember(a(:),u);
c = accumarray(i(b),1,size(u));
c = c(j)'

Best Answer

Related Solutions

MATLAB: Efficient number occurence count

MATLAB: Count number of times value occurs

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