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Vectorization and a strange result

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Maria on 14 Oct 2020
Closed: MATLAB Answer Bot on 20 Aug 2021
I am struggling since few hours with a vectorized operation.
I have some vector quantities, and I wanted to vectorize. I put in the attachment a .mat file.
For testing, I compute the fx1 and fx_1 in this way
fx1 = const_1 .* exp(-1j.*beta(1:2)'.*const_2)./const_2 .* t;
fx_1 = const_1 .* exp(-1j.*beta(1)*const_2)./const_2 .* t;
where beta is a vector. I would expect that the first row of fx1 and fx_1 coincide, but they don't. I am very confused what I am missing here. Ideally, I would like to do this
fx1 = const_1 .* exp(-1j.*beta'.*const_2)./const_2 .* t;
instead of multiplying in a for loop for each beta(i).

Answers (1)

Star Strider
Star Strider on 14 Oct 2020
Your ‘mytest.mat’ file contains:
fx_1: [1×150 double]
beta: [1×25 double]
t: [1×150 double]
const_1: [1×150 double]
const_2: [1×150 double]
fx1: [2×150 double]
It would help to know what you want to do with those vectors and the ‘fx1’ matrix. That is not obvious from the code you posted.
Star Strider
Star Strider on 15 Oct 2020
You are computing ‘fx’ differently in the looop than in the vectorised calculation.
If you plot them:
for i = 1 : length(beta)
fx(i,:) = const_1 .* exp(-1j.*beta(i).*const_2)./const_2 .* t;
set(gca, 'ZScale','log')
ylabel('\beta (index)') % ‘beta’ Is Complex, So Plotting The Index Values
you will see that they are essentially just mirror-images of each other with respect to the z-axis. The magnitudes are of courrse different, the vectorised approach goes from to , and the loop goes from to . Chioose whatever version makes sense in the context of what you want to do. I have no idea what that is.
I would use the fully-vectorised approach (in my code), since to me that makes more sense.




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