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Top Forums Programming Problem with rounding using lrint Post 302985096 by Don Cragun on Friday 4th of November 2016 05:29:41 PM
Old 11-04-2016
You need to determine what rounding rules you want to use for tie-breaking cases (see Wikipedia's discussion on rounding)
and you need to realize that even in double precision floating point, the result of a floating point calculation is not always exact (even when computing a value using decimal calculations would be exact). For example, the awk program (which uses double precision floating point for its calculations):
Code:
printf '.1 .5\n.3 .5\n' | awk '{printf("%.40f\n", $1 * $2)}'

produces the output:
Code:
0.0500000000000000027755575615628913510591
0.1499999999999999944488848768742172978818

not the output you might expect:
Code:
0.0500000000000000000000000000000000000000
0.1500000000000000000000000000000000000000

Which shows two examples where decimal arithmetic produces exact results, but binary arithmetic produced one result that was a little bit high and one result that was a little bit low.
 

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LRINT(3)						   BSD Library Functions Manual 						  LRINT(3)

NAME
llrint, llrintf, llrintl, lrint, lrintf, lrintl -- convert to integer LIBRARY
Math Library (libm, -lm) SYNOPSIS
#include <math.h> long long llrint(double x); long long llrintf(float x); long long llrintl(long double x); long lrint(double x); long lrintf(float x); long lrintl(long double x); DESCRIPTION
The lrint() function returns the integer nearest to its argument x according to the current rounding mode. If the rounded result is too large to be represented as a long value, an invalid exception is raised and the return value is undefined. Otherwise, if x is not an inte- ger, lrint() raises an inexact exception. When the rounded result is representable as a long, the expression lrint(x) is equivalent to (long)rint(x) (although the former may be more efficient). The llrint(), llrintf(), llrintl(), lrintf(), and lrintl() functions differ from lrint() only in their input and output types. SEE ALSO
lround(3), math(3), rint(3), round(3) STANDARDS
These functions conform to ISO/IEC 9899:1999 (``ISO C99''). HISTORY
The llrint(), llrintf(), lrint(), and lrintf() routines first appeared in FreeBSD 5.4. The long double variants were introduced in FreeBSD 8.0. BSD
January 13, 2008 BSD
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