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

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

Answer 1

[DGM]

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

[Benson Gou]

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]

I really have no familiarity with handling sparse matrices or how to optimize for them. I'm sure someone else can though.

Answer 2

[Walter Roberson]

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