round.c

数学运算库源码

/*							round.c
 *
 *	Round double to nearest or even integer valued double
 *
 *
 *
 * SYNOPSIS:
 *
 * double x, y, round();
 *
 * y = round(x);
 *
 *
 *
 * DESCRIPTION:
 *
 * Returns the nearest integer to x as a double precision
 * floating point result.  If x ends in 0.5 exactly, the
 * nearest even integer is chosen.
 * 
 *
 *
 * ACCURACY:
 *
 * If x is greater than 1/(2*MACHEP), its closest machine
 * representation is already an integer, so rounding does
 * not change it.
 */

/*
Cephes Math Library Release 2.1:  January, 1989
Copyright 1984, 1987, 1989 by Stephen L. Moshier
Direct inquiries to 30 Frost Street, Cambridge, MA 02140
*/
#include "mconf.h"
#ifdef ANSIPROT
double floor ( double );
#else
double floor();
#endif

double round(x)
double x;
{
double y, r;

/* Largest integer <= x */
y = floor(x);

/* Fractional part */
r = x - y;

/* Round up to nearest. */
if( r > 0.5 )
	goto rndup;

/* Round to even */
if( r == 0.5 )
	{
	r = y - 2.0 * floor( 0.5 * y );
	if( r == 1.0 )
		{
rndup:
		y += 1.0;
		}
	}

/* Else round down. */
return(y);
}