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

Question

Benson Gou 2021-5-23

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/837758-how-to-quickly-assign-the-values-of-a-matrix-using-a-given-array

评论： DGM 2021-5-24

采纳的回答： DGM

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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

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Answer 1

DGM 2021-5-23

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编辑：DGM 2021-5-23

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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

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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 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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Answer 2

Walter Roberson 2021-5-23

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https://ww2.mathworks.cn/matlabcentral/answers/837758-how-to-quickly-assign-the-values-of-a-matrix-using-a-given-array#answer_706668

在 MATLAB Online 中打开

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