NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part1/2284

Internet Message Format | 1996-08-05 | 2.0 KB

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From: Lawrence Kirby <[e-mail]>
Newsgroups: comp.lang.c
Subject: Re: quick decision: is n a power of 2?
Date: Sat, 20 Jan 96 01:05:12 GMT
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In article <4dp8cr$[e-mail]> [e-mail] "Robert Fry" writes:

>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)

Bill said he was using unsigned long ints which are better suited to this
(it doesn't work when num is negative).

>{
>    return((( num - 1) & num) == 0);
>}

It is also important to consider the special case of 0. This classes it
as a power of 2 which may or may not be what is wanted (or it may not
matter).

>(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))

Best to fully parenthesize the macro arguments i.e.

#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.

Right. Naming conventions have been regularly discussed on comp.lang.c.
Maybe it would be a good idea to adopt a naming convention which
clearly distinhuishes macros that are unsafe for side-effects.

--
-----------------------------------------
Lawrence Kirby | [e-mail]
Wilts, England | [e-mail]
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