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How to vectorize matrix assignment process below

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jyp
jyp on 25 Nov 2020
Commented: jyp on 25 Nov 2020
I try to vectorized the following code in the following way to speed up the process. But it doesn't work.
I try to use this code to assign some part of the matrix by values as some functions of the index i (a(i), b(i), .etc).
N=100;
A=zeros(2*N,2*N);
for i=2:N-1
A(2*i-1,2*i-1)=a(i);
A(2*i-1,2*i)=b(i);
A(2*i-1,2*i+1)=c(i);
A(2*i,2*i-3)=d(i);
A(2*i,2*i-1)=e(i);
A(2*i,2*i)=f(i);
end
_______________
vectorized code:
N=100;
A=zeros(2*N,2*N);
i=2:N-1;
A(2*i-1,2*i-1)=a(i);
A(2*i-1,2*i)=b(i);
A(2*i-1,2*i+1)=c(i);
A(2*i,2*i-3)=d(i);
A(2*i,2*i-1)=e(i);
A(2*i,2*i)=f(i);
But matlab says the assignment can be carried out because the dimensions doesn't match. So is there any proper way to do this?

  2 Comments

Stephan
Stephan on 25 Nov 2020
All values from a(i) are overrided by e(i). Same for b(i) and f(i).
jyp
jyp on 25 Nov 2020
Sorry for the typos. I've corrected them.

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Accepted Answer

KSSV
KSSV on 25 Nov 2020
Read about sub2ind. Convert your sub indices into global indices and use.
idx = sub2ind(size(A),2*i-1,2*i-1) ;
A(idx) = a(i) ;
You try to follow something like above.

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