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Message-ID: <260f63050525135825d3128c@mail.gmail.com>
Date: Wed, 25 May 2005 15:58:48 -0500
From: Larry Ziegenbein <larryzie AT gmail DOT com>
To: djgpp AT delorie DOT com
Subject: Re: to check given no. is power of 2 ( formatted text...)
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Hi Rui,

I think when you treat a double as an int, for example comparing
fracpart to 0 in this line --

if (fracpart == 0) return 1; 

you will most often be disappointed.  Doubles will generally have a
bit of imprecision in the last few placeholders and throw you off
unless you plan for it in advance.  Give it a try and step through
this code so you understand.  ;-)  You could round the result to the
appropriate number of digits and recast it as an int before the
compare maybe to make it work, but the other solutions posted are
pretty nice looking to me!!

Anyway, this code generally won't work, and it is dangerous to do
things like this, at least for newbies like me, I avoid this like the
plague.  But I screw up in other places to make up for it.

happy hacking,
Larry


On 5/25/05, cosmos <cosmos AT tvtel DOT pt> wrote:
> 
> ----- Original Message -----
> From: "cosmos" <cosmos AT tvtel DOT pt>
> To: <djgpp AT delorie DOT com>
> Sent: Wednesday, May 25, 2005 8:27 PM
> Subject: Re: to check given no. is power of 2
> 
> 
> > Hi,
> >
> > Mathematically, a given number "N" is a power of 2 if "log(N) / log(2)" is
> > an integer, right? So:
> >
> > #include <stdio.h>
> > #include <math.h>
> >   double param, fractpart, intpart;
> >
> >   param = log(N) / log(2);
> >   fractpart = modf (param , &intpart);
> >   if (fracpart == 0) return 1; else return 0;
> 
> 
> Regards,
> 
> Rui Fernandes
> >
> >
> > ----- Original Message -----
> > From: "Martin Ambuhl" <mambuhl AT earthlink DOT net>
> > Newsgroups: comp.os.msdos.djgpp
> > To: <djgpp AT delorie DOT com>
> > Sent: Wednesday, May 25, 2005 7:39 PM
> > Subject: Re: to check given no. is power of 2
> >
> >
> > > Gerd Termathe wrote:
> > > > "aveo" <rahul DOT h AT gmail DOT com> schrieb im Newsbeitrag
> > > > news:1116997063 DOT 905915 DOT 71670 AT z14g2000cwz DOT googlegroups DOT com...
> > > >
> > > >>hi all
> > > >>i need a C code that checks given no. is power of 2 or not without
> > > >>checking any condition.
> > > >>
> > > >
> > > >
> > > > int is_power_of_2 (int N)
> > > > {
> > > >  if ( N!=0 && (N&(N-1))==0 ) return 1; else return 0;
> > > > }
> > >
> > > Why an if ... else?
> > > Why use uppercase for non-macros?
> > > Why use signed integers?
> > >
> > > inline unsigned is_power_of_2 (unsigned n)
> > > {
> > >    return (n && !(n&(n-1)));
> > > }
> > >
> >
> >
> 
> 
>