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

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Benson Gou
Benson Gou 2021-5-23
评论： DGM 2021-5-24
Ran in:
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

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采纳的回答

DGM
DGM 2021-5-23
编辑：DGM 2021-5-23
Ran in:
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 个评论
Benson Gou
Benson Gou 2021-5-23
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 2021-5-24
I really have no familiarity with handling sparse matrices or how to optimize for them. I'm sure someone else can though.

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更多回答（1 个）

Walter Roberson
Walter Roberson 2021-5-23
Ran in:
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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