MATLAB error in "for" loop: Array indices must be positive integers or logical values [duplicate]

Question

in the following example, I get error when I want to simulate.

R1=(2e-3)*[0 3 0
    0 2 0
    0 1 0
    0 0 0
    0 -1 0
    0 -2 0
    0 -3 0];

R2=(5e-3)*[0 3 0
    0 2 0
    0 1 0
    0 0 0
    0 -1 0
    0 -2 0
    0 -3 0];

R3=(5e-3)*[0 3+0.3*randn() 0
    0 2+0.3*randn() 0
    0 1+0.3*randn() 0
    0 0+0.3*randn() 0
    0 -1+0.3*randn() 0
    0 -2+0.3*randn() 0
    0 -3+0.3*randn() 0];
hold on
phases=[0 0 0 0 0 0 0];
phi=90;
AF_tot=zeros(1,1802);
for theta=-90:0.1:90
    AF_tot(theta*10+901)=Array_beam_cal(R3,phases,theta,phi);
end
global_theta=-90:0.1:90;
plot(global_theta,AF_tot,'linewidth',1.5);
xlabel('theta(degree)');
ylabel('array factor');

Error is:

Array indices must be positive integers or logical values.

Noticing my example code, all indexes in "AF_tot(theta*10+901)" are positive. If I change part of previous code to following one:

AF_tot=zeros(1,181);
    for theta=-90:90
        AF_tot(theta+91)=Array_beam_cal(R3,phases,theta,phi);
    end

error does not occur.

Answer 1

Instead of using theta as the loop variable and index with with, which you can’t do because it’s not an integer sequence, use a separate index:

theta = -90:0.1:90;
AF_tot = zeros(size(theta));
for ii = 1:numel(theta)
    AF_tot(ii) = … theta(ii) …;
end
plot(global_theta,AF_tot,…);

Note a few simplifications with this approach: I didn’t need to figure out the number 1802, and I didn’t have to figure out how to map theta values to indices. I also won’t need to update that value and that mapping if I change the theta values. Simpler code is less likely to have bugs, is easier to read, and is easier to maintain.

Answer 2

floating point maths isn't always accurate, it may result in -89.000001 which causes this error.

just round the numbers to avoid this.

AF_tot(round(theta*10+901))=Array_beam_cal(R3,phases,theta,phi);

side note: AF_tot should be 1x1801 not 1x1802.