How can i use elements with equal indexes?

Question

Azime Beyza Ari 2022-4-10

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https://ww2.mathworks.cn/matlabcentral/answers/1693260-how-can-i-use-elements-with-equal-indexes

评论： Azime Beyza Ari 2022-4-10

Hello everyone, I am working on an constraint where i will only use the elements of an matrix if their indexes are equal like;

for i=1:36
    for j=1:36 
        if  i=j
            M*x(i,j)-y(j)>=0
        else 
        end 
    end 
end

my matrix has 36x36 dimensions so need to go from 1 to 36.

this is what i did for y ;

y(:,1)

y is 36x1 so this works for y.

i i can not use any loops or conditions i am trying to write this only in one line! if there is a little tip like

x(do something here);

that is what i need.

Hope i can make what i need clear.

Thank you in advance!

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Answer 1

Matt J 2022-4-10

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https://ww2.mathworks.cn/matlabcentral/answers/1693260-how-can-i-use-elements-with-equal-indexes#answer_939565

在 MATLAB Online 中打开

problem.Constraints.diagcon=diag(x)*M-y>=0;

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Azime Beyza Ari 2022-4-10

Thank you this was what i needed!

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Answer 2

Torsten 2022-4-10

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编辑：Torsten 2022-4-10

在 MATLAB Online 中打开

Put "-M" in the matrix of inequality constraints where x_jj is addressed and "+1" where y(j) is addressed.

Then you have set the constraint

-M*x_jj + y_j <= 0

or

M*x_jj - y_j >= 0

This will result in 36 new rows in the constraint matrix A and 36 new zeros in the vector b of the right-hand side.

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Torsten 2022-4-10

在 MATLAB Online 中打开

And

for i=1:36
    constr(i) = M*x(i,i)-y(i) >= 0
end

does not work ?

Steven Lord 2022-4-10

在 MATLAB Online 中打开

So you only want to operate on diagonal elements of x?

x = magic(4)
x = 4×4
    16     2     3    13
     5    11    10     8
     9     7     6    12
     4    14    15     1
y = [1; 2; 3; 4]
y = 4×1
     1
     2
     3
     4
result = 2*diag(x) - y
result = 4×1
    31
    20
     9
    -2
n = 3;
spotcheck = [2*x(n, n)-y(n), result(n)] % These should match 
check = 1×2
     9     9