#include <stdio.h>     /* Directive to compiler: include standard input/output header file*/

#define CARBON 12   /* Macro definition for a constant CARBON; it can be used in the program */

#define  PI 3.14   /* we define yet another constant PI */

/* The following are examples of function declarations */

int mySum(int n, int k);

double myProduct(double x, double y);

int factorial(int f);

/* This is a sample comment. It can take any number of lines. 
    But it starts with forward-slash-followed-by-star
    and it must end with a star-followed-by-forward-slash.
*/


int main( void )
{
/* We start our program with declaring and initializing a set of local variables */

int n=4, k=7, sum;

/* We group declarations of variables according to their purposes 
	for mnemonic reasons */
int argument, fct;

double x=2.0, y=5.0, product;

/* The following declares that variable  outp  points to a file. Note that 
   the word  FILE must be uppercase. Pointers are to be discussed later.
*/
FILE  * outp;

/* Call the function  mySum  to compute a sum of the given arguments n and k */
sum = mySum(n,k);

/* Call function  myProduct  to multiply the given arguments x and y */

product = PI * myProduct(x,y);

/* Ask a program user to enter an argument for the factorial function */

printf("Type argument for factorial (a positive integer less than 13) and press Enter = ");

/* Ampersend preceeding a variable  argument  is required here to convert
   a variable into a pointer. This operation will be discussed later. 
*/
scanf("%d", &argument);

fct = factorial(argument);

/* Print the computed values on the screen - the standard output for programs */

printf("sum= %d\n",sum);
printf("factorial= %d\n",fct);
printf("product= %lf\n",product);

/* Print the computed values of all variables to a file named results.txt */

outp = fopen("results.txt", "w");   /* open a file for reading/writing */

fprintf(outp,"sum= %d\n",sum);
fprintf(outp,"factorial= %d\n",fct);
fprintf(outp,"product= %lf\n",product);

/* Now, we have to close file and thereby release memory */
fclose(outp);

/* This just a statement to end our program when a program returns normally. */
return(0);
}


int mySum(int n, int k)
{
int s;
s = n+k;
return(s);
}

double myProduct(double x, double y)
{
double p;
p = x*y;
return(p);
}

int factorial(int f)
{
int previous;

if (f < 1) {  /* We use the curved braces to enclose a consecutive pieces of code
		to be executed together. */
	printf("Error: wrong value of argument f=%d. Argument must be an integer between 1 and 12.\n",f); 
	return(0);
	}

if (f > 13) {
	printf("Error: wrong value of argument f=%d. Argument must be an integer between 1 and 12.\n",f); 
	return(0);
	}

if (f == 1) return(1);

if (f==2) return(2);
else {
		previous = f -1;   /* decrement value of f */
		/* We compute factorial recursively and return the resulting value.
			Recall that factorial is defined as  n! = n*(n-1)!  */
		return( f * factorial(previous) );  
	}
}