Array indices must be positive integers or logical values.

z=0; 
z1=0;
aa=zeros(3,3);

for i=1:k
    z=z+t(i);
    z1=z1+t(i-1);
    aa(1,1)= aa(1,1) + (Qbar(1,1)*((-(T*0.5)+z)-(-(T*0.5)+z1)));
    
end

Answer 1

[Karim]

The following part of you code can produce an error: i starts at 1, hence you are trying to acces t(0) which is not possible. In matlab the indexing starts at 1, hence the first element in the array t will be t(1)

for i = 1:k
    ...
        z1 = z1+t(i-1);
    ...
end

Answer 2

[Dhritishman]

z1=z1+t(i-1);

In your code, the value of i starts at 1, so t(i-1) will be trying to access t(0) which MATLAB doesn't allow as indexing should start from 1. Hence, it throws an error. The first element in the array t will be t(1), not t(0).