count of times if condition is met

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Tareq Khreim 2020-9-21

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评论： Tareq Khreim 2020-9-28

I have a simple if statement to count how many times certain values in a matrix are within a certain range. ptarget is a 100x3 matrix representing ijk vectors, in which I want to assess if the i and j componenets are within -0.5 and 0.5. If both are within this range, I count this. For some reason, the counting variable hits just stays at 1. How can I fix this?

N = 1e2 % Iterations
% ...
    
for i=1:N
hits = 0;
    if ptarget(i,1)>=-0.5 && ptarget(i,1)<=0.5 && ptarget(i,2)>=-0.5 && ptarget(i,2)<=0.5
        hits = hits + 1
    end
end

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Stephen23 2020-9-22

编辑：Stephen23 2020-9-22

"...I have a simple if statement to count how many times certain values in a matrix are within a certain range..."

Rather than unnecessary nested loops, the simpler MATLAB approach would be like this:

idx = ptarget(:,1)>=-0.5 && ptarget(:,1)<=0.5 && ptarget(:,2)>=-0.5 && ptarget(:,2)<=0.5
hits = nnz(idx)

https://www.mathworks.com/help/matlab/matlab_prog/vectorization.html

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Answer 1

Jeff Miller 2020-9-22

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put

hits = 0;

before the 'for' loop. You are resetting hits to 0 each time you check a new ptarget

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Tareq Khreim 2020-9-28

Thank you!

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Answer 2

Image Analyst 2020-9-22

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Try this (no for loop needed):

rowsInRange = ptarget(:,1) >= -0.5 & ptarget(:,1) <= 0.5 & ptarget(:,2) >= -0.5 & ptarget(:,2) <= 0.5; % A logical vector.
hits = nnz(rowsInRange); % Count # of "true" values.