How can i solve for this issue with the logical flow

n=length(q);
coef=k*dt/dr;
q_new=zeros(n,1);
q(0)=0;

% for i=1:n+1
%     q_new(i)=q(i)+(coef*(q(i+1)-(2*q(i))+q(i-1)));
% end
for i=1:n+1
    
    term1=(q(i+1)*((r(i+1)+dr)/r(i)*dr));
    term2=2*q(i)*(1/dr);
    term3=q(i-1)*((r(i)-dr)/r(i)*dr);
    
    qnew(i)=q(i)+(coef*(term1-term2+term3));
    q_new=qnew(i);
   
end

Can someone please help me with this i cant seem to make it work, what can i do to make the the error of logical go away?

Array indices must be positive integers or logical values: %This is the error show

Answer 1

[Chad Greene]

I think the error is here:

term3=q(i-1)*((r(i)-dr)/r(i)*dr);

The first time through the loop, i=1, meaning i-1=0. There is no zeroth element of q. Which element of q are you trying to access?

[Mark Loui]

I see, is there any method i can do to solve this issue

i declare that q(0)=0 inside the loop? the problem is still there. Any method i can do to solve this issue?

[Chad Greene]

If q(0) = 0, then q(i-1)*((r(i)-dr)/r(i)*dr) will equal zero whenever i=1. Therefore I suggest

if i==1
    term3=0; 
else 
    term3=q(i-1)*((r(i)-dr)/r(i)*dr);
end

[Mark Loui]

Hmm sounds good, i try and see thanks