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

Subtract 1 from the number, then and it with the original number. If the result is zero, it was a power of two.

if (((n-1) & n) == 0) {
    // power of two!
}

(sorry, my PHP is rusty...)