/*
 * scope.c - illustrates the use of global and local variables and 
 * global function prototypes.
 *
 * Revised code taken from pg. 330 (Program 7.1) (Bronson) "First 
 * Book on ANSI C"
 * 
 * CS50
 */

#include <stdio.h>

/* firstnum is a global variable not defined on the main() stack. It has full scope of all functions in the file scope.c. Any code in the file can read and write to it. Once main() teriminates the variable is dellocated and no longer exists */
int firstnum; // create a global variable named firstnum

void valfun(); // global function prototype

int main()
{
  int secnum; //create a local variable named secnum
  firstnum = 10; //store a value into the global variable
  secnum = 20; // store a value into the local variable

  printf("\nFrom main(): firstnum = %d",firstnum);
  printf("\nFrom main(): secnum = %d\n",secnum);

  valfun(); // call the function valfun

  printf("\nFrom main() again: firstnum = %d",firstnum);
  printf("\nFrom main() again: secnum = %d\n",secnum);

  return 0;
}

void valfun() // no values are passed to this function
{
  /* secum is a local variable created on the stack when valfun() executes. When valfun() exits the stack is deallocated and the variable no longer exists. It is local and its scope is valfun()*/
  int secnum; // create a second local variable named secnum 
  secnum = 30; // this only affects this local variable's value

  printf("\nFrom valfun(): firstnum = %d",firstnum);
  printf("\nFrom valfun(): secnum = %d\n",secnum);
  firstnum = 40; // this changes firstnum for both functions
}