ERROR: Array indices must be positive integers or logical values.

"Why am I getting the error 'Array indices must be positive integers or logical values' in MATLAB when using a for loop?"

% grupo 20
Km=1.5;
Ra=8;
La=0.03;
J=0.08;
Kg=10;
Kt=0.20;
Kv=40;
T=0.02;
syms k;
%F2 y F3
A=[0 1 0; 0 0 1; -Kg*Km^2 -Ra*J -La*J];
B=[0; 0; Km];
C=[1 0 0];
sys=ss(A,B,C,0)

sys =
 
  A = 
            x1       x2       x3
   x1        0        1        0
   x2        0        0        1
   x3    -22.5    -0.64  -0.0024
 
  B = 
        u1
   x1    0
   x2    0
   x3  1.5
 
  C = 
       x1  x2  x3
   y1   1   0   0
 
  D = 
       u1
   y1   0
 
Continuous-time state-space model.

sys1=c2d(sys,T,'zoh')

sys1 =
 
  A = 
            x1       x2       x3
   x1        1     0.02   0.0002
   x2  -0.0045   0.9998     0.02
   x3    -0.45  -0.0173   0.9998
 
  B = 
           u1
   x1   2e-06
   x2  0.0003
   x3    0.03
 
  C = 
       x1  x2  x3
   y1   1   0   0
 
  D = 
       u1
   y1   0
 
Sample time: 0.02 seconds
Discrete-time state-space model.

[V,D]=eig(sys1.A)

V = 
  -0.0618 - 0.0965i  -0.0618 + 0.0965i  -0.1235 + 0.0000i
   0.1573 - 0.2876i   0.1573 + 0.2876i   0.3393 + 0.0000i
   0.9378 + 0.0000i   0.9378 + 0.0000i  -0.9325 + 0.0000i

D = 
   1.0265 + 0.0516i   0.0000 + 0.0000i   0.0000 + 0.0000i
   0.0000 + 0.0000i   1.0265 - 0.0516i   0.0000 + 0.0000i
   0.0000 + 0.0000i   0.0000 + 0.0000i   0.9465 + 0.0000i

V1=inv(V)

V1 = 
  -1.4158 + 2.4544i   0.4749 + 0.9156i   0.3602 + 0.0082i
  -1.4158 - 2.4544i   0.4749 - 0.9156i   0.3602 - 0.0082i
  -2.8476 + 0.0000i   0.9551 + 0.0000i  -0.3478 + 0.0000i

Dk=D^k;
Af=V*Dk*V1

Af = 

X = zeros(3, 1);
y = zeros(25, 1);
for k=1:25
    for j=1:K_sym
        X(k)=Af(k)*X(0)+j*Af(K_sym)*B
        y(k)=C*X(k)
    end
end

Unrecognized function or variable 'K_sym'.

[Dyuman Joshi]

I am not able to reproduce the same error you obtained.

However, there is another error, as you can see below, which occurs because you are trying to use a variable you have not defined.

% grupo 20
Km=1.5;
Ra=8;
La=0.03;
J=0.08;
Kg=10;
Kt=0.20;
Kv=40;
T=0.02;
syms k;
%F2 y F3
A=[0 1 0; 0 0 1; -Kg*Km^2 -Ra*J -La*J];
B=[0; 0; Km];
C=[1 0 0];
sys=ss(A,B,C,0);
sys1=c2d(sys,T,'zoh');
[V,D]=eig(sys1.A);
V1=inv(V);
Dk=D^k;
Af=V*Dk*V1;
X = zeros(3, 1);
y = zeros(25, 1);
for k=1:25
    for j=1:K_sym
        X(k)=Af(k)*X(0)+j*Af(K_sym)*B;
        y(k)=C*X(k)
    end
end

Unrecognized function or variable 'K_sym'.

[Guillermo Pourreau Pons]

I'm sorry, the correct code is this one:

for k=1:25

for j=1:k-1

X(k)=Af(k)*X(0)+j*Af(K-1)*B

y(k)=C*X(k)

end

end

I continue getting the same error, and I dont know how to avoid X(0). Have you any idea?

Answer 1

[Rik]

Actually, the result of your code is a missing variable. Once you fix that, you will have to edit your code to avoid X(0).

[Guillermo Pourreau Pons]

True, I'm sorry

The correct code is this one:

for k=1:25

for j=1:k-1

X(k)=Af(k)*X(0)+j*Af(K-1)*B

y(k)=C*X(k)

end

end

I continue getting the same error, and I dont know how to avoid X(0). Have you any idea?

[Rik]

You can find an explanation for the layout tools here. It is the toolbar you see when you edit your question.

Since you didn't write any comments and only use single/two letter variable names, I have no clue what you want to do. So I have no idea what you should change. Currently you are trying to retrieve the zeroth element of X. Since that is before the first element, it doesn't exist.