How to create a specific diagonal Matrix

2018 4 20
2 个回答
更新时间：2018 4 20
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Hi everyone! I'm trying to find the way to write a generic code to obtain a matrix like the one in this example:
if true
A=[1 1 1 0 0 0 0 0 0;0 0 0 1 1 1 0 0 0;0 0 0 0 0 0 1 1 1]
end
To be clearer, i need a code to fill my matrix, which dimension is (T,T^2). In the example i have T=3. I need the general code because i need a matrix for each T i set, and it should respect the criteria of the example. This means that i have, for each row, a ones(1,T) vector that starts when the ones vector of the previous row ends. The first row should start with a ones(1,T) vector, as in the example. It is a sort of diagonal matrix i guess. I need it because it is part of the unequality constraints matrix in a linear program i'm developing. Hope someone could help me! thanks a lot in advance!

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回答（2 个）

Birdman
Birdman 2018-4-20

0 个投票

Try this:

T=3;
for i=1:T
    A(i,:)=circshift([ones(1,T) zeros(1,T^2-T)],T*(i-1));
end
Stephen23
Stephen23 2018-4-20
编辑：Stephen23 2018-4-20

0 个投票

That is exactly what blkdiag is for:

>> V = [1,1,1];
>> blkdiag(V,V,V)
ans =
   1   1   1   0   0   0   0   0   0
   0   0   0   1   1   1   0   0   0
   0   0   0   0   0   0   1   1   1

This is much simpler and much more efficient than trying to write your own code using loops. Remember that MATLAB is a high-level language, so loops are often not the best solution.

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Stefano Di Vito
Stefano Di Vito 2018-4-20
Thank you, it worked too. But if i had an high T, how would i obtain the right matrix order without typing (V,V,V,....) T-times? forgive my ignorance of course!
Sean de Wolski
Sean de Wolski 2018-4-20 
x = repmat({[ones(1, 3)]}, 1, 3)
blkdiag(x{:})
Stephen23
Stephen23 2018-4-20

@Stefano Di Vito: using the method that Sean de Wolski showed. It uses a comma-separated list to provide as many input arguments as you want:

https://www.mathworks.com/help/matlab/matlab_prog/comma-separated-lists.html

https://www.mathworks.com/matlabcentral/answers/320713-how-to-operate-on-comma-separated-lists

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