My Newton Raphson Script Stucks at first step

clc
a = input('Enter initial guess: ');
e = input('Tolerable error: ');
N = input('Enter maximum number of steps: ');

step = 1;

g = diff(f(x));

while abs(f(a))> e  
    if g(a) == 0
        disp('Division by zero.');
        break;
    end
     b = a - f(a)/g(a);
    fprintf('step=%d\ta=%f\tf(a)=%f\n',step,a,f(a));
    a = b;
    
    if step>N
       disp('Not convergent'); 
       break;
    end
    step = step + 1;
end

fprintf('Root is %f\n', a);

I have function file called f.m . It consists:

function f= f(x)
f=cos(x)-x.*exp(x);
end

When I run it it stucks at first. It doesn't do other iterations. How can I figure it out? I need to have separate function file and call it like this.

Answer 1

[Torsten]

a = 1.0;
e = 1e-6;
N = 30;
syms x
fsym = cos(x)-x*exp(x);
f = matlabFunction(fsym);
g = matlabFunction(diff(fsym,x));

step = 1;
%g = diff(f(x));
while abs(f(a))> e  
    if g(a) == 0
        disp('Division by zero.');
        break;
    end
    b = a - f(a)/g(a);
    fprintf('step=%d\ta=%f\tf(a)=%f\n',step,a,f(a));
    a = b;
    
    if step>N
        disp('Not convergent'); 
        break;
    end
    step = step + 1;
end
fprintf('Root is %f\n', a);

[st0s]

Thanks for answer but can you do it like I did? cos(x)-x*exp(x) this expression musn't be on script. I need call it from my f.m file. Also I need take input from user. I tried to fix it but got error when I did g = matlabFunction (diff(f(x));

[Torsten]

Then you must define f(x) twice:

syms x
fsym = cos(x)-x*exp(x);
g = matlabFunction(diff(fsym,x));

...
function value = f(x)
  value = cos(x)-x*exp(x);
end

Or you define f as a function handle and not as a function:

f = @(x) cos(x)-x*exp(x);
g = eval(['@(x)' char(diff(f(x)))])
%or vectorized:
g = eval(['@(x)' vectorize(char(diff(f(x))))])

Or you calculate g = f'(x) using paper and pencil (my suggestion).