/* ********************************************
   A program to illustrate the use of functions

   Computes the polynomial ax^6+bx^4+cx^2+d=0

   Uses square() and quartic() as simple examples of
   function modules to perform abstracted tasks.
   Square() is located in a second file.

   G.C. Rutledge
   October 2002
   ****************************************** */

#include <stdio.h>
double square(double);              /* This is a function PROTOTYPE */
double quartic(double);

int main(void)
{

  double a=1.0, b=1.0, c=1.0, d=1.0;
  double x, f;

  printf("This program computes x^6+x^4+x^2+1.\n");
  printf("Input a value for x: \n");
  scanf("%lf",&x);

  /* Task 1: initailize f(x) */
  f = d;

  /* Task 2: compute x^2 term */
  f += c*square(x);                 /* this is a function INVOCATION */

  /* Task 3: compute x^4 term */  
  f += b*quartic(x);
  
  /* Task 4: compute x^6 term */
  /* Notice reuse of quartic() and square() functions */
  f += a*quartic(x)*square(x);

  printf("The value of this polynomial at x=%f is %f\n",x,f);

  return (0);
}

/* ************************************************
This is the DEFINITION of the quartic() function.  
It returns the value of z^4.

G.C. Rutledge
October 2002
************************************************ */
double quartic(double z)
{
  double z4;
  /* Note that functions can call other functions */
  z4 = square(z)*square(z); 
  return z4;
}