How can I solve the max iterations exceeded?

I'm trying to solve root finding problem by using Newton method when I run the code I got this message. How can I avoid this message? Is there a problem with my code?

max_iter=10; err=1; tol=10^-6; n_iter=0;

while err>tol
    n_iter=n_iter+1;
    delta=-F(r)/dF(r);
    r=r+delta;
    err=abs(delta/r);
    if n_iter>max_iter
        warning('max iterations exceeded')
        break
    end
end 
end 

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function out=F(r)
K=9*10^9; e= 1.6*10^-19; p=0.33*10^-10; Alpha=1.74637*10^-16;
out=-K*(e^2./r)+ Alpha*exp(-r./p); 
end

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function out=dF(r)
K=9*10^9; e= 1.6*10^-19; p=0.33*10^-10; Alpha=1.74637*10^-16;
out=K*(e^2./r^2)- (Alpha/p)*exp(-r./p);
end

[James Tursa]

Probably a problem with your code (e.g., recursion without proper convergence criteria, etc), but impossible to say without seeing your code. Can you post it?

[John Chilleri]

Providing your code would be useful, but without it, I can only suggest that you look into changing the MaxIter option, as the default may be too low for you.

Answer 1

[James Tursa]

Had to hunt for awhile to find a place where the function switches signs, but try this for a starting point:

r = 0.1e-9

Answer 2

[Steven Lord]

It's impossible to say for certain without seeing a sample of your code, but more likely than not you're accidentally doing something like this (question 3.18 in this FAQ, if that link doesn't work.) If you call a function from within itself, you need to ensure that you have a base case that does not call the function itself, to end the recursion.

Answer 3

[fs]

Thank you very much. As I go smaller and smaller for r, I got more n_iter. Thank you again.