The lockers in use

Problem

Eufemia looks after the lockers at the swimming club. The board by the door is one of the old ones: instead of lighting a little lamp per locker, it shows a single number. That number, written in binary, says which lockers are in use: every digit 1 is a locker with its key in, and every digit 0 is a free one.

Write a function that takes that number, from 0 up, and returns how many lockers are in use, that is, how many ones the number has when you write it in binary.

With 16 you return 1: in binary 16 is 10000, and it carries a single one. With 89 you return 4, because 89 in binary is 1011001. With 0 you return 0: the board all zeros, no locker in use.

Binary digits come out by dividing by 2 and looking at what is left over: if 1 is left over, that digit is a one and that locker is in use; if 0 is left over, it is free. Then you keep the result of the division and go on, until the number runs out. You do not have to build the binary text: here you only have to count.

Eufemia's board holds up to 31 lockers.

Examples

• The one from the example

  16 → 1

• Eighty-nine

  89 → 4

• Eight lockers in a row

  255 → 8

• Two in use and one free

  6 → 2

Besides these, the challenge has hidden tests that are revealed when you submit your solution.

You start with this

def taken(n):
    pass

function taken(n) {
}