Newton's Method Help

I have a code that works for Newton's method, but I am trying to make it into an algorithm that takes an imput from the interval [a, b] instead of x0. In other words I guess the initial point is calculated inside of the cody using x0=(a+b)/2. Another thing I would like to do is insert an if statement into the main loop that breaks out of the loop if the x is outside of [a, b]. This is the original that works, but uses an input of x0.

function x=mynewton(f,df,x0,tol,maxiter)
x=x0;
xold=x
for i=1:maxiter
    x=x-f(x)/df(x)
  err(1)=abs(x-xold)
   xold=x
   if err(1)<tol
       break
   end
end

And then this is what I tried to do but there is an error

function x=mynewton2(f,df,x0,tol,maxiter)
x=(a+b)/2;
xold=x
for i=1:maxiter
    x=x-f(x)/df(x)
  err(1)=abs(x-xold)
   xold=x
   if err(1)<tol
       break
   end
       if x<a
           break
       end
           if x>b
               break
           end
   end
end

I am really sorry if this is a stupid question, I am just very new to MATLAB and am not the best with technology. Any help would be much appreciated!

Answer 1

[Alan Stevens]

Assuming you make

x0 = [a, b];

before you pass it to the function, then replace

x = (a+b)/2;

by

x = (x0(1) + x0(2))/2;

within mynewton2.

[Kaeden Beaty]

ohh thank you so much! so x0(1) refers to the x value of x0. So if I wrote x0=[a,b,c] then x0(3) would refer to the value of c right?

[Alan Stevens]

Yes. (though x0(1) refers to the first value of x0).