Array indices must be positive integers or logical values.

Hello, I am having trouble coding a solution using the Newton Raphson method. I need to input a negative x0 number, but I am getting the error, 'Array indices must be positive integers or logical values.'. Please help!

clc; clear
 
x0=input('Enter intial guess: ');
i=1;
I=100;
err=0.1;
while((err>1e-3) && i<I)
  x=x0;
fx(x)=(3/500)*(x^3 - 18.535*x^2 + 25.697*x + 28.099);
pfx(x)= ((9000*x^2)-(111210*x)+77091)/500000;
x1=x-((fx(x))/(pfx(x)));
err=abs(x-x1);
x=x1;
i=i+1;
end
disp('Root of the function')
x(end)

[Walter Roberson]

Unrecognized function or variable 'fx'.

If your fx were an array then fx(x) would try to index fx at x which could be a problem.

Answer 1

[Walter Roberson]

pfx(x)= ((9000*x^2)-(111210*x)+77091)/500000;

does not define a formula relating a function name pfx and an input value x. Instead, that line of code calculates a numeric value on the right side using the current value of x, and tries to store it in a variable named pfx at index value x . That cannot work if x happens to go fractional or 0 or negative.

To define a formula, you would need

pfx = @(x) ((9000*x^2)-(111210*x)+77091)/500000;

[JM]

I see now, thank you very much!