Array indices must be positive integers or logical values.
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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)
0 Comments
Accepted Answer
KSSV
on 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 Comments
dpb
on 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.
More Answers (3)
dpb
on 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
on 16 Oct 2021
Edited: John D'Errico
on 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))
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