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How to quickly assign the values of a matrix using a given array?

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Dear All,
I have an array A which contains two columns of integers. I want to build a matrix B in the following way.
For example,
A = [2 9; 1 5; 8 3]
A = 3×2
2 9 1 5 8 3
The matrix B should be suilt as follows:
B = [0 1 0 0 0 0 0 0 -1 0; 1 0 0 0 -1 0 0 0 0 0; 0 0 -1 0 0 0 0 1 0 0]
B = 3×10
0 1 0 0 0 0 0 0 -1 0 1 0 0 0 -1 0 0 0 0 0 0 0 -1 0 0 0 0 1 0 0
How can I create matrix B for a goven A?
Thanks.
Benson

Risposta accettata

DGM
DGM il 23 Mag 2021
Modificato: DGM il 23 Mag 2021
Here is one way:
A = [2 9; 1 5; 8 3]
A = 3×2
2 9 1 5 8 3
s = [size(A,1) 10]; % you're going to have to specify the output width
B = zeros(s);
B(sub2ind(s,(1:s(1)).',A(:,1))) = 1;
B(sub2ind(s,(1:s(1)).',A(:,2))) = -1;
B % show the result
B = 3×10
0 1 0 0 0 0 0 0 -1 0 1 0 0 0 -1 0 0 0 0 0 0 0 -1 0 0 0 0 1 0 0
  2 Commenti
Benson Gou
Benson Gou il 23 Mag 2021
Hi, DGM,
Thanks for your quick reply.
I run your code. it took 0.0107 second, much less than original 0.15 second. But when I tried to use sparse to reduce the CPU time futher, but it took 0.13 second. I want the final matrix B is sparse.
Thanks.
Benson
DGM
DGM il 24 Mag 2021
I really have no familiarity with handling sparse matrices or how to optimize for them. I'm sure someone else can though.

Accedi per commentare.

Più risposte (1)

Walter Roberson
Walter Roberson il 23 Mag 2021
S = 10;
A = [2 9; 1 5; 8 3]
A = 3×2
2 9 1 5 8 3
(A(:,1) == (1:S))-(A(:,2) == (1:S))
ans = 3×10
0 1 0 0 0 0 0 0 -1 0 1 0 0 0 -1 0 0 0 0 0 0 0 -1 0 0 0 0 1 0 0

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