ODE error Array indices must be positive integers or logical values.

Question

Luis Montenegro 2020-11-1

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https://ww2.mathworks.cn/matlabcentral/answers/632689-ode-error-array-indices-must-be-positive-integers-or-logical-values

评论： Ameer Hamza 2020-11-1

I've been trying to get this working, but every time I run the script I get an error saying "Array indices must be positive integers or logical values and points to the dxdt(2) for the error. This is the code:

t = (0:0.1:5);
condIni = [0;0;0;0;0];
[T,Y] = ode45(@Grua,t,condIni);
x1 = Y(:,1);
x2 = Y(:,2);
x3 = Y(:,3);
x4 = Y(:,4);
plot(t,x1,'-b','linewidth',2)
xlabel('tiempo (s)');
ylabel('desplazamiento (m)');
figure()
plot(t,x2,'-b','linewidth',2)
xlabel('tiempo (s)');
ylabel('Velocidad (m/s)');
figure()
plot(t,x3,'-b','linewidth',2)
xlabel('tiempo (s)');
ylabel('desplazamiento angular (rad)');
figure()
plot(t,x4,'-b','linewidth',2)
xlabel('tiempo (s)');
ylabel('Velocidad angular (rad/s)');
function dxdt = Grua(t,x)
L=3;
m=100;
M=30;
r1=0.05;
r2=0.1;
R=0.15;
ct=0.01;
cp=0.1;
c=1;
K=10;
Kb=9;
Lb=1e-2;
Rb=6.2;
J=1e-3;
E=100;
N=r1/r2;
g=9.81;
dxdt(1) = x(2);
dxdt(2) = (K*L*x(5)-L*ct*x(2)+N*R*cp*x(4)-L*N^2*R^2*c*x(2)+L*R*N*m*g*x(3))/(L(M*N^2*R^2+J));
dxdt(3) = x(4);
dxdt(4) = ((-cp*x(4)-m*g*L*x(3))/L^2) -((m*R*L*N)/L^2)*((K*L*x(5)-L*ct*x(2)+N*R*cp*x(4)-L*N^2*R^2*c*x(2)+L*N*R*g*m*x(3))/(L(M*N^2*R^2+J)));
dxdt(5) = (E-Kb*x(2)-x(5)*Rb)/Lb;
dxdt = dxdt';
end

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Answer 1

Ameer Hamza 2020-11-1

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You missed the multiplication operator in the denominator

dxdt(2) = ../(L*(M*N^2*R^2+J));
              %^ this is missing
dxdt(4) = ../(L*(M*N^2*R^2+J)));
              %^ this is missing

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Luis Montenegro 2020-11-1

Oh gosh... There is a lot of stuff I completely missed that. Thank you so much

Ameer Hamza 2020-11-1

I am glad to be of help!

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Answer 2

KSSV 2020-11-1

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在 MATLAB Online 中打开

The indices for an array in MATLAB should be posititve integers or logicals. If you give fractions, 0, negative as index it will throw error.

Example:

A = rand(1,10) ;  
A(1)   % no error 
A(10)   % no error 
A(0)   % error as index cannot be zero 
A(-1)  % error, as index cannot be negative 
A(0.5)  % error, as index cannot be fraction 
idx = A<0.5 ;
A(idx)    % idx has 0, 1 they are logicals, so allowed