Using Function Block in Fortran - Newton Raphson

Question

I am trying apply the function block for Newton Raphson and am a bit lost. My script is suppose to ask the user for an initial X as i've hardcoded the equation. My output file seems to just be stating zero's instead of the proper output listing it's convergence. I'm assuming i'm calling my write statement wrong or too early? Any help is appreciated.

On a side note...is it possible to also ask the user for both the equation and init value as in other languages?

program main

  implicit none

  real :: x0, xn, err
  write(*,*) "Please enter an initial guess X0."
  read(*,*) x0
  write(*,*) "x = ", xn, " error = ", err

end program main

real function f(x0)
  real :: x0, xn,

  do
    xn = x0 - ( (x0**3 - (x0) - 1) / ( ( 3*(x0**2) )-1) )
    err = 100*abs( (xn-x0)/x0 )
    x0 = xn
    if (err < 0.000001)exit
    return
  end do

end function

score 1 · Answer 1 · answered May 06 '14 at 01:59

In your code you never call function f, so how do you expect your program to work?

Even if you did call f, the variables xn and err that you declare and set in function f are local to that function, and they will not change the values of x0 and xn in the main program. Use a subroutine to pass multiple values back to the main program.

score 0 · Answer 2 · edited May 23 '17 at 12:11

As @Fortranner stated, you need to call the function. You will find your Fortran programming easier if you place your subroutines and functions in a module and use that module from the calling program. For an example of this organization, see Computing the cross product of two vectors in Fortran 90.

Re "is it possible to also ask the user for both the equation and init value"? That is a very general question. If your equation is always a polynomial, you could easily ask the user for the values of the coefficients. Or you could have a list of equations to select from. But entering a totally arbitrary equation at run-time is something like asking to write a scripting language in Fortran -- definitely non-trivial.

I suggest that while developing a program that you use all available warning and checking options of your compiler. For example, using such options gfortran points out that you don't define the return value of f in function f. Which is a hint that the communication between the main program and function has problem(s).

Using Function Block in Fortran - Newton Raphson

2 Answers2