Finding if a number is a power of 2

Question

Just out of curiosity, how can you tell if a number x is a power of two (x = 2^n) without using recursion.

Thanks

Answer 1

For completeness, if the number is a float, you can test if it's a power of two by chacking if the mantissa is all zeros:

<?php
$number = 1.2379400392853803e27;
$d = unpack("h*", pack("d", $number)); $d = reset($d);
$isPowerOfTwo = substr($d, 0, 13) == "0000000000000";
var_dump($isPowerOfTwo); // bool(true)

Exercise for the reader: corner cases and big-endian machines.