Error in 'find' function when value is out of range. How to get around?

Question

Yun Inn 2012-8-22

0
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https://ww2.mathworks.cn/matlabcentral/answers/46477-error-in-find-function-when-value-is-out-of-range-how-to-get-around

I have eight thicknesses, Th=0,5,10,15,20,25,30,35,40. For each of the Th, I have matrix, ratioAB.

Then, I have Z that is a 2D matrix consisting of pixels with different thicknesses. For each of the pixel, I need to find the correct ratioAB depending on the thickness. The result, RF, should be a matrix of ratioAB.

Because the Th range from 0 to 40, when I have negative value or value more than 40 in the Z matrix, I get the error 'Subscript indices must either be real positive integers or logicals.'

How do I fix the code?

m=1024;
n=1024;
Th = [0,5,10,15,20,25,30,35,40];
i=1:length(Th)
ratioAB(:,:,i)=A./B   % A and B are matrices for each Th
RF=ones(m,n);
for i=1:m
    for j=1:n
        Z2=Z(i,round(j));
col = find(Th > Z2,1,'first')-[1 0]
RF(i,j)= diff(cat(3,ratioAB(i,j,col)),[],3)/diff(Th(col))*(Z2 Th(col(1)))+ratioAB(i,j,col(1));
    end
end