Problem - Number of 1 Bits

Here's your first problem: count how many bits are set to 1 in the binary representation of a positive integer $n$.

For example, if $n = 13 = 1101$, the answer is 3 (three 1s). If $n = 7 = 111$, the answer is 3.

Before reading the solution, think: how would you check each bit? How many bits do you need to check?