http://en.wikipedia.org/wiki/Time_complexity[^],

http://en.wikipedia.org/wiki/Big_O_notation[^].

For example, for

`(int index = 0; index < length; ++index) { doSomething(index); }`

the answer is O(n). How do we know that? Because, apparently, the number of calls to `doSomething`

and, hence, time, is a linear function of `length`

. When we can figure out and classify such function, we can find out a *complexity class*.

It is also apparent that it's not always possible. After all, it is proven that it's possible to create an algorithm with unpredictable behavior, so finding time complexity may be a theoretically unresolvable problem, or the notion of time complexity may be not applicable at all.

—SA