Matrix integral over layers

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Lev Mihailov
Lev Mihailov 2020-7-17
回答： Bjorn Gustavsson 2020-7-17
Hello! I have a matrix in it the number of steps per time. Is it possible to integrate each column of the matrix layer by layer (layer sizes may differ)
MatrixHelpStep=rand(30,40);
layer=(1:5:30); % for example, I took layer 5 (but it can change)
% trying to create a loop for layers
for i=1:length(layer)-1
Imhs=trapz(MatrixHelpStep(layer(i):layer(i)+1))
end
it gives me only one value
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Walter Roberson
Walter Roberson 2020-7-17
Could you confirm that you want overlap? trapz(x(1:5,1)) uses 5 elements to form the output, but trapz(x(5:10,1)) uses 6 elements to form the output. Are you wanting 1:5, 6:10, 11:15, 16:20 and so on?
Lev Mihailov
Lev Mihailov 2020-7-17
yes, I want such overlaps

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Bjorn Gustavsson
Bjorn Gustavsson 2020-7-17
For this I would try cumtrapz - it calculates the cumulative (sp?) trapezoidal integral in the same manner as cumsum calculates the cumulative sum along an array. Doing that would give you the cumulative integrals of your matrix with the same size as your matrix. That way you can easily calculate every following integral between 2 boundaries from that one just by taking the difference between the corresponding boundary-elements. Something like this:
I = peaks(40);
I = I(1:30,:);
ciI = cumtrapz(I);
i10to17ofI = ciI(17,:) - ciI(10,:);
HTH

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