to test whether a number is a power of 2

[ravi]

Give a one-line C expression to test whether a number is a power of 2.
[No loops allowed]

[Default User]

ravi wrote:

Give a one-line C expression to test whether a number is a power of 2.
[No loops allowed]

Do your own homework.

Brian

[CBFalconer]

ravi wrote:

>
Give a one-line C expression to test whether a number is a power
of 2. [No loops allowed]

For unsigned integers:

if (!((n - 1) & n)) puts("n is power of 2");

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[Erik de Castro Lopo]

ravi wrote:

Give a one-line C expression to test whether a number is a power of 2.
[No loops allowed]

static inline int
is_power_of_2 (int x)
{
return ((x & (x - 1)) == 0) ;
}
--
-----------------------------------------------------------------
Erik de Castro Lopo
-----------------------------------------------------------------
"Hamas: Islam will conquer US and Britain."
-- http://www.pmw.org.il/LatestBulletins.htm#b220606

[Richard Heathfield]

Erik de Castro Lopo said:

ravi wrote:

>Give a one-line C expression to test whether a number is a power of
2.
[No loops allowed]

static inline int
is_power_of_2 (int x)
{
return ((x & (x - 1)) == 0) ;
}

Having proved willing to do homework for free, you and Chuck might want
to brace yourselves for the rush.

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Army1987]

On Sun, 08 Jul 2007 17:23:03 +1000, Erik de Castro Lopo wrote:

ravi wrote:

>Give a one-line C expression to test whether a number is a power of 2.
[No loops allowed]

static inline int
is_power_of_2 (int x)
{
return ((x & (x - 1)) == 0) ;

What if x <= 0 (in which case x cannot be a power of two)?

}

--
Army1987 (Replace "NOSPAM" with "email")
"Never attribute to malice that which can be adequately explained
by stupidity." -- R. J. Hanlon (?)

[CBFalconer]

Richard Heathfield wrote:

Erik de Castro Lopo said:

>ravi wrote:

>>Give a one-line C expression to test whether a number is a power
of 2.

static inline int
is_power_of_ 2 (int x)
{
return ((x & (x - 1)) == 0) ;
}

Having proved willing to do homework for free, you and Chuck might want
to brace yourselves for the rush.

I like my version better :-) In all seriousness, this is not such
a simple thing, so I doubt it was homework. If he hadn't used such
rude terminology I would be virtually sure it is not.

--
<http://www.cs.auckland .ac.nz/~pgut001/pubs/vista_cost.txt>
<http://www.securityfoc us.com/columnists/423>
<http://www.aaxnet.com/editor/edit043.html>
cbfalconer at maineline dot net

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[Christopher Benson-Manica]

CBFalconer <cb********@yah oo.comwrote:

[OP wrote:]

>Give a one-line C expression to test whether a number is a power
of 2.

I like my version better :-) In all seriousness, this is not such
a simple thing, so I doubt it was homework. If he hadn't used such
rude terminology I would be virtually sure it is not.

Given the number of ridiculous questions that are routinely asked here
that clearly *are* homework, I'm surprised you find it unlikely that
the above question is not homework. Why else would there be
constraints such as "one line" or "no loops allowed" (snipped from
above context)? I personally find it highly unlikely that anyone
asking the question seriously would phrase it so tersely.

--
C. Benson Manica | I *should* know what I'm talking about - if I
cbmanica(at)gma il.com | don't, I need to know. Flames welcome.

[websnarf]

On Jul 8, 5:54 am, Army1987 <army1...@NOSPA M.itwrote:

On Sun, 08 Jul 2007 17:23:03 +1000, Erik de Castro Lopo wrote:

ravi wrote:

Give a one-line C expression to test whether a number is a power of 2.
[No loops allowed]

static inline int
is_power_of_2 (int x)
{
return ((x & (x - 1)) == 0) ;

What if x <= 0 (in which case x cannot be a power of two)?

Oh for crying out loud! The other answers had the OP perfectly set up
with the wrong answer (brilliant coordination, BTW, guys -- all of you
getting it simultaneously wrong in exactly the same way). Now the OP
can figure it out completely correctly without any research himself.
Good job.

--
Paul Hsieh
http://www.pobox.com/~qed/
http://bstring.sf.net/

[Nick Keighley]

On 8 Jul, 23:37, Harald van D k <true...@gmail. comwrote:

Flash Gordon wrote:

Eric Sosman wrote, On 08/07/07 13:27:

ravi wrote:
On Jul 8, 10:22 am, Martin Ambuhl <mamb...@earthl ink.netwrote:
ravi wrote:

>>>Give a one-line C expression to test whether a number is a power of 2.
[No loops allowed]

<snip>

[...] Had you enough sense you would have instantly known the answer
was:

int powerof2( double num ) { return 1; }

After all, the OP did not specify integer power of 2, or even real power
of 2, and I think if you allow complex powers, all numbers will qualify.

What about zeroes, infinities, and NaNs? I would think it should return 0
for those

ah, but the spec said test a *number* and I don't think infinity is a
number and as for NaN...

So it should indicate some sort of domain error or assert on a debug
build.
--
Nick Keighley