Also known as average streaming.
The following function adds a number to an average. average is the current average, size is the current number of values in the average, and value is the number to add to the average:
double addToAverage(double average, int size, double value)
return (size * average + value) / (size + 1);
Likewise, the following function removes a number from the average:
double subtractFromAverage(double average, int size, double value)
// if (size == 1) return 0; // wrong but then adding a value "works"
// if (size == 1) return NAN; // mathematically proper
// assert(size > 1); // debug-mode check
// if(size < 2) throw(...) // always check
return (size * average - value) / (size - 1);