Calculating averages can be an expensive operation. Say you have 10 million customer database records and you want to keep track of the average age of your customers. <!–more–> You can either:
- Recalculate the average everytime it is required, accessing all 10 million database records and summing them up in the process, or;
- Cache an average and update it every time a new customer is added to the database.
This equation is what you need:
$${{a}_{new}}{=}{\frac {nr+x} {n+1}}$$
Where n is the number of records, a is the current average and r is the new value to be added.