NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part1/2229

Internet Message Format | 1996-08-05 | 1011 b

Path: crl.crl.com!not-for-mail
From: [e-mail] (Robert Fry)
Newsgroups: comp.lang.c
Subject: Re: quick decision: is n a power of 2?
Date: 19 Jan 1996 15:10:19 -0800
Organization: CRL Dialup Internet Access
Message-ID: <4dp8cr$[e-mail]>
References: <[e-mail]>     <4dorr8$[e-mail]> <[e-mail]>
NNTP-Posting-Host: crl.com

Someone was asking for a quick way to determine if a number is a power of
2. The solutions I've seen involved ounting every bit and seeing if the
number of 'on' bits is 1. But why not take advantage of the binary
representation of numbers and use:

int is_power_of_2( long num)
{
    return((( num - 1) & num) == 0);
}

(You could also make a macro of it if you need better speed in exchange
for reduced maintainability):

#define IS_POWER_OF_2(num) (!(( num - 1) & num))

Be careful with this, of course. IS_POWER_OF_2(x++) has undefined results,
for example.

  Bob