327: Mainline

Now consider the following, revised version of the trade_price function. Conspicuously, no value is returned. Equally conspicuously, a second parameter appears, result, a pointer to a floating-point number:

 *-- No value is returned                      *-- Extra parameter 
 |                                             | 
 v                                             v 
void trade_price (struct trade *tptr, double *result) { 
  *result = tptr -> price * tptr -> number; 
} 

Because result is a pointer parameter, its value must be the address of a chunk of memory allocated before trade_price is called. The assignment statement writes the product of the price and number structure variables into that chunk of memory.

Thus, the computation performed in the trade_price function is passed out via the memory chunk identified by a dereferenced pointer, rather than via a normal return value. Put to use, this version of the trade_price function appears in the following revised version of the analyze_trades program; note the introduction of the floating-point variable, trade_price_result, which provides the chunk of memory that the result pointer parameter identifies:

#include   
/* Define the trade structure */ 
struct trade {double price; int number;}; 
/* Define value-computing function */ 
void trade_price (struct trade *tptr, double *result) { 
  *result = tptr -> price * tptr -> number; 
} 
/* Define trade array */ 
struct trade trades[100]; 
main ( ) { 
  /* Declare various variables */ 
  int limit, counter; 
  double trade_price_result, sum = 0.0; 
  /* Read numbers and stuff them into array */ 
  for (limit = 0; 
       2 == scanf ("%lf%i", 
                   &trades[limit].price, 
                   &trades[limit].number); 
       ++limit) 
    ; 
  /* Display value of shares traded */ 
  for (counter = 0; counter < limit; ++counter) { 
    trade_price (&trades[counter], &trade_price_result); 
    sum = sum + trade_price_result; 
  } 
  printf ("The total value of the %i trades is %f.\n", 
                                  limit,       sum); 
} 
--- Data ---
10.2    600 
12.0    100 
13.2    200 
--- Result --- 
The total value of the 3 trades is 9960.000000.