# Calculation using matrix indexing for elements with a certain thresholds

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Leon on 20 Aug 2021
Commented: Leon on 21 Aug 2021
I need to calculate the percentage differences of all elements that are larger than 5 in two matrices (A and B). Both A and B have a size of 1000 x 500. I want my results C to be in the same size of 1000 x 500 as well, so that I could plot them on a contour.
Below are my code:
Ind = A>5 & B>5;
C = (A(Ind)-B(Ind))./B(Ind)*100;
Here is the problem. Even though both my matrix "A" and the index variable "Ind" have a size of 1000 x 500, A(Ind) becomes a column data. Therefore my C is a column data, instead of retaining its original set up of 1000 x 500. In this case, how could I do the calculation to ensure my results will be on the grid and with a size of 1000 x 500?
Many thanks.

Sulaymon Eshkabilov on 20 Aug 2021
Edited: Sulaymon Eshkabilov on 20 Aug 2021
There is a couple of small errs in your code and here is a corrected code:
...
A2 = (A>5 & B>5).*A; % Picks up all greater than 5 elements of A and preserves
B2 = (B>5 & B>5).*B; % Picks up all greater than 5 elements of B and preserves
C = (A2-B2)./B2*100;
OPTIONAL: Now you may get rid off or substitute NANs, with 0:
C(isnan(C))=0;
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Leon on 21 Aug 2021
It works. Many thanks!

R2021a

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