656: Mainline

To compile trades.c, the C compiler also needs to know about the trade structure declared in trades.h. Accordingly, you provide access to the trades.h file via another #include declaration:

/* Trades definition file (c extension) */ 
#include "trades.h" 
double mean_price (struct trade **array, int length) { 
  int counter; double sum = 0.0;  
  for (counter = 0; counter < length; ++counter) 
    sum = sum + array[counter] -> price; 
  return sum / counter; 
} 
double mean_size (struct trade **array, int length) { 
  int counter; double sum = 0.0;  
  for (counter = 0; counter < length; ++counter) 
    sum = sum + array[counter] -> number; 
  return sum / counter; 
}