How does one determine if a given year is a leap year? It's a common belief that if the year is evenly divisible by 4, that it is a leap year. That is true most of the time, but not always.
Occasionally the needs arises to determine if a number is odd or even.
Some structures, like an object with member arrays, will have you in a straight-jacket long before you determine where a particular value falls within the structure and how you should construct a reference to it.