673: Mainline

Suppose, for example, that you want your analyze_trades program to have two ways of operating, one fast and approximate and another slower, but more accurate. The mean_price function, as defined, is completely accurate:

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; 
} 

Here is another, faster version of mean_price that bases its result on only every second element in the array:

double mean_price (struct trade **array, int length) { 
  int counter, selector; double sum = 0.0;  
  for (counter = 0, selector = 0; 
       selector < length; 
       ++counter, selector = selector + 2) 
    sum = sum + array[selector] -> price; 
  return sum / counter; 
} 

You can combine the two versions of mean_price, controlling speed versus accuracy via a global variable named mode. If the value of mode is 0, you have high speed; if the value of mode is 1, you have high accuracy:

int mode = default value, 0 or 1 
... 
double mean_price (struct trade **array, int length) { 
  int counter, selector, increment; double sum = 0.0;  
  if (mode) 
    increment = 2; 
  else 
    increment = 1; 
  for (counter = 0, selector = 0; 
       selector < length; 
       ++counter, selector = selector + increment) 
    sum = sum + array[selector] -> price; 
  return sum / counter; 
}