Array indices must be positive integers or logical values.

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Ali Deniz
Ali Deniz 2021년 10월 16일
답변: Image Analyst 2021년 10월 18일
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)

채택된 답변

KSSV
KSSV 2021년 10월 16일
%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)
  댓글 수: 3
Ali Deniz
Ali Deniz 2021년 10월 16일
Okay, thank you.

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추가 답변(3개)

dpb
dpb 2021년 10월 16일
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...
  댓글 수: 1
Ali Deniz
Ali Deniz 2021년 10월 16일
I want to get a graph so I wanted to store values in V(i). For example V(0)=9.81, V(2)=19.62 ... when t=0, t=2 ....

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John D'Errico
John D'Errico 2021년 10월 16일
편집: John D'Errico 2021년 10월 16일
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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