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From: greg@gryphon.CTS.COM (Greg Laskin)
Newsgroups: misc.misc,comp.lang.c
Subject: Re: Printing binary (was: Re: Why college?)
Message-ID: <2030@gryphon.CTS.COM>
Date: Sat, 24-Oct-87 21:19:35 EST
Article-I.D.: gryphon.2030
Posted: Sat Oct 24 21:19:35 1987
Date-Received: Tue, 27-Oct-87 02:27:38 EST
References: <35@ateng.UUCP> <3194@sol.ARPA> <2783@xanth.UUCP> <235@snark.UUCP> <2072@watcgl.waterloo.edu>
Reply-To: greg@gryphon.CTS.COM (Greg Laskin)
Organization: Trailing Edge Technology, Redondo Beach, CA
Lines: 55
Xref: mnetor misc.misc:1970 comp.lang.c:5139

In article <2072@watcgl.waterloo.edu> smvorkoetter@watmum.waterloo.edu (Stefan M. Vorkoetter) writes:
>In article <235@snark.UUCP> eric@snark.UUCP (Eric S. Raymond) writes:
>>I thought about the standard ways to do this, (essentially, crack the sucker
>>into powers-of-two internally via repeated integer-divide and modulo
>>operations) went *ugh*, and then had a flash that gave me an amusing
>>alternate solution. Here it is: (warning! this is untested and off-the-cuff)
>
>There is a very quick way to do this using only the <<, >>, and & operators:
>
>#define	BITS		32			/* or 16 */
>
>void print_binary(n)
>int n;
>{
>	int i;
>
>	for(i = 0; i < BITS; i++) {
>		printf("%1d",(n >> (BITS - 1)) & 1);
                              ^^^^^^^^^^^^^^^
			      not a constant expression
>		n <<= 1;
>	}
>}



#define BitsPerUnit 8

void print_binary(n)
	int n;
{
	int i;

	for (i=0; i < (sizeof (int) * BitsPerUnit) ; ++i,n<<=1)
		putchar(( n & (1<<(sizeof (int) * BitsPerUnit) -1)) ? '1' : '0');
			      /*   a constant expression         */
}

Using the constant expression is "quick"er.  On my 16 bit machine, the compiler
can resolve the expression to:
	putchar((n & 0x8000 ) ? '1' : '0');
at compile time instead of shifting n BITS bits BITS times at run time.

Using putchar() instead of printf() is picking a nit.

Comments on portability to other environments solicited (by email please).



-- 
Greg Laskin   
"When everybody's talking and nobody's listening, how can we decide?"
INTERNET:     Greg.Laskin@gryphon.CTS.COM
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