Array indices must be positive integers or logical values.

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)

Answer 1

[KSSV]

%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)

[Ali Deniz]

On this way, I get V(1)=19.62. But I must get V(0)=9.81 and then V(2)=19.62. Thank you.

[dpb]

" 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.

[Ali Deniz]

Okay, thank you.

Answer 2

[dpb]

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...

[Ali Deniz]

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 ....

Answer 3

[John D'Errico]

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))

[Ali Deniz]

Okay, thank you.

Answer 4

[Image Analyst]

This most FAQQY of FAQs is thoroughly explained in the FAQ:

https://matlab.fandom.com/wiki/FAQ#.22Subscript_indices_must_either_be_real_positive_integers_or_logicals..22