Array indices must be positive integers or logical values.
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Ali Deniz
le 16 Oct 2021
Réponse apportée : Image Analyst
le 18 Oct 2021
I am trying to solve an equation by using Runge-Kutta Euler Method. Why do I get "Array indices must be positive integers or logical
values." error?
%Euler Method
%parameters
g=9.81;
rho=1.2;
s=0.00011;
m=0.023;
Cd=0.9;
%Initial Condition
V0=0;
V=0;
%I choose dt as
dt=2;
%time interval
t0=0;
ts=10;
t=0;
i=0;
while dt<ts;
d1=g-0.5.*rho.*V0.^2*(s/m).*Cd;
phiAvg=d1;
V(i)=V0+dt.*phiAvg;
V0=V;
t(i)=t;
i=i+1;
t=t+dt;
end
plot(V,t)
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KSSV
le 16 Oct 2021
%Euler Method
%parameters
g=9.81;
rho=1.2;
s=0.00011;
m=0.023;
Cd=0.9;
%Initial Condition
V0=0;
V=zeros([],1);
%I choose dt as
dt=0.2;
%time interval
t0=0;
ts=10;
t=0;
i=0;
while t<ts
i=i+1;
d1=g-0.5.*rho.*V0.^2*(s/m).*Cd;
phiAvg=d1;
V(i)=V0+dt.*phiAvg ;
V0=V(i);
% t(i)=t;
t=t+dt;
end
plot(V)
3 commentaires
dpb
le 16 Oct 2021
" I must get V(0)=9.81..."
We can't always get what we want. MATLAB arrays are unalterably and inviolatably 1-based, NOT zero-based.
You must create a secondary variable to plot against to have a zero origin graph.
Plus de réponses (3)
dpb
le 16 Oct 2021
In
...
i=0;
while dt<ts;
d1=g-0.5.*rho.*V0.^2*(s/m).*Cd;
phiAvg=d1;
V(i)=V0+dt.*phiAvg;
V0=V;
t(i)=t;
...
what is i first time through the loop?
Using the debugger is very helpful in such cases...
John D'Errico
le 16 Oct 2021
Modifié(e) : John D'Errico
le 16 Oct 2021
It does not matter that you WANT V(0) to be something. MATLAB does not support zero based indexing. PERIOD.
However, nothing stops you from starting the vector at V(1). And then when you index into the vector, just use V(ind + 1). Now when ind == 0, there is no problem.
If you want to do a plot?
t = 0:1:10; % or whatever it should be
plot(t,V(t+1))
Image Analyst
le 18 Oct 2021
This most FAQQY of FAQs is thoroughly explained in the FAQ:
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