How can i use elements with equal indexes?

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!

Answer 1

[Matt J]

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

[Azime Beyza Ari]

Thank you this was what i needed!

Answer 2

[Torsten]

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.

[Azime Beyza Ari]

I dont see how this is related to what i am asking. but tried and it did not worked. I want to do this ;

it should go like;

9,9

10,10

....

36,36

i did this for y and it worked ;

y(:,1)

[Torsten]

And

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

does not work ?

[Steven Lord]

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