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Top Forums Programming Problem with rounding using lrint Post 302985088 by migurus on Friday 4th of November 2016 03:52:17 PM
Old 11-04-2016
Problem with rounding using lrint

I run into a situation when integer result of 81 * 0.5 is 40 (expected 41) when using lrint() function.

When I use a simple +0.5 approach results are right.

The code is:
Code:
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
 int main(int argc, char *argv[])
{
        const int       X       = 81;
        double          prc     = (argc == 2) ? atof(argv[1]) : 0.5;
        double          dval    = X * prc;
        long            ival    = lrint(dval);
        printf("%i * %.16f = %.16f  IVAL = %li  ", X, prc, dval, ival);
        ival = (X * prc) + 0.5;
        printf("USING +0.5 IVAL = %li\n", ival);
        return(0);
}

results are:
Code:
~$ c 0.5
81 * 0.5000000000000000 = 40.5000000000000000  IVAL = 40  USING +0.5 IVAL = 41

Using gcc ver 5.4.0 on Linux, running on 2 years old Xeon processor, if that matters. I checked floating point rounding defaults in compiler and it is a regular "round to the nearest integer" rule.

My question is - should I abandon rint-family functions and use +0.5 approach?
 

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RINT(3) 						   BSD Library Functions Manual 						   RINT(3)

NAME
rint, lrint, llrint -- round to integral value SYNOPSIS
#include <math.h> double rint(double x); long double rintl(long double x); float rintf(float x); long int lrint(double x); long int lrintl(long double x); long int lrintf(float x); long long int llrint(double x); long long int llrintl(long double x); long long int llrintf(float x); DESCRIPTION
The rint() functions return the integral value nearest to x (according to the prevailing rounding mode) in floating-point format. The lrint() and llrint() functions return the integral value nearest to x (according to the prevailing rounding mode) in the return formats specified. If the rounded value is outside the range of the return type, the numeric result is unspecified and the "invalid" floating-point exception is raised. A range error may occur if the magnitude of x is too large. SPECIAL VALUES
rint(+-0) returns +-0 for all rounding modes. rint(+-infinity) returns +-infinity for all rounding modes. All these functions raise the "inexact" floating-point exception if the result differs in value from the argument (except when they raise some other floating-point exception, such as "invalid"). NOTE
The rinttol() function is deprecated. Please use the C99 function lrint() instead. SEE ALSO
abs(3), fabs(3), ceil(3), floor(3), math(3) STANDARDS
The rint() , lrint() , and llrint() functions conform to ISO/IEC 9899:2011. BSD
July 02, 2008 BSD
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