Figure 5.9  Program to Monitor Gasoline Storage Tank
/*
 * Monitor gasoline supply in storage tank. Issue warning when supply
 * falls below MIN_PCT % of tank capacity.
 */

#include <stdio.h>

/* constant macros */
#define CAPACITY     80000.0 /* number of barrels tank can hold        */
#define MIN_PCT      10      /* warn when supply falls below this 
		percent of capacity               */
#define GALS_PER_BRL 42.0    /* number of U.S. gallons in one barrel   */

/* Function prototype */
double monitor_gas(double min_supply, double start_supply);

int
main(void)
{
        double start_supply, /* input - initial supply in barrels	*/
               min_supply,   /* minimum number of barrels left without 
warning	*/
               current;      /* output - current supply in barrels	*/
                                
        /* Compute minimum supply without warning  */
        min_supply = MIN_PCT / 100.0 * CAPACITY;

        /* Get initial supply */
        printf("Number of barrels currently in tank> ");
        scanf("%lf", &start_supply);

        /* Subtract amounts removed and display amount remaining
           as long as minimum supply remains.	*/
        current = monitor_gas(min_supply, start_supply);

        /* Issue warning 	*/
        printf("only %.2f barrels are left.\n\n", current);
        printf("*** WARNING ***\n");
     	  printf("Available supply is less than %d percent of tank's\n",
	       MIN_PCT);
        printf("%.2f-barrel capacity.\n", CAPACITY);

        return (0);
}

/*
 * Computes and displays amount of gas remaining after each delivery 
 * Pre : min_supply and start_supply are defined.
 * Post: Returns the supply available (in barrels) after all permitted 
 *       removals. The value returned is the first supply amount that is
 *       less than min_supply.
 */
double
monitor_gas(double min_supply, double start_supply)
{
        double remov_gals,   /* input - amount of current delivery	*/
               remov_brls,   /*         in barrels and gallons	*/
               current;      /* output - current supply in barrels	*/

        for  (current = start_supply;
              current >= min_supply;
              current -= remov_brls) {
            printf("%.2f barrels are available.\n\n", current);
            printf("Enter number of gallons removed> ");
            scanf("%lf", &remov_gals);
            remov_brls = remov_gals / GALS_PER_BRL;

            printf("After removal of %.2f gallons (%.2f barrels),\n",
		   remov_gals, remov_brls);
        }

	return (current);
}

Number of barrels currently in tank> 8500.5
8500.50 barrels are available.

Enter number of gallons removed> 5859.0
After removal of 5859.00 gallons (139.50 barrels),
8361.00 barrels are available.

Enter number of gallons removed> 7568.4
After removal of 7568.40 gallons (180.20 barrels),
8180.80 barrels are available.

Enter number of gallons removed> 8400.0
After removal of 8400.00 gallons (200.00 barrels),
only 7980.80 barrels are left.

*** WARNING ***
Available supply is less than 10 percent of tank's
80000.00-barrel capacity.