std::nearbyint, std::nearbyintf, std::nearbyintl

 

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Defined in header <cmath>
	(1)
float       nearbyint ( float num ); double      nearbyint ( double num ); long double nearbyint ( long double num );			(until C++23)
/* floating-point-type */             nearbyint ( /* floating-point-type */ num );			(since C++23)
float       nearbyintf( float num );		(2)	(since C++11)
long double nearbyintl( long double num );		(3)	(since C++11)
Additional overloads (since C++11)
Defined in header <cmath>
template< class Integer > double      nearbyint ( Integer num );		(A)

1-3) Rounds the floating-point argument num to an integer value in floating-point format, using the current rounding mode. The library provides overloads of std::nearbyint for all cv-unqualified floating-point types as the type of the parameter.(since C++23)

A) Additional overloads are provided for all integer types, which are treated as double.

(since C++11)

Parameters

num

-

floating-point or integer value

Return value

The nearest integer value to num, according to the current rounding mode, is returned.

Error handling

This function is not subject to any of the errors specified in math_errhandling.

If the implementation supports IEEE floating-point arithmetic (IEC 60559),

• FE_INEXACT is never raised.
• If num is ±∞, it is returned, unmodified.
• If num is ±0, it is returned, unmodified.
• If num is NaN, NaN is returned.

Notes

The only difference between std::nearbyint and std::rint is that std::nearbyint never raises FE_INEXACT.

The largest representable floating-point values are exact integers in all standard floating-point formats, so std::nearbyint never overflows on its own; however the result may overflow any integer type (including std::intmax_t), when stored in an integer variable.

If the current rounding mode is FE_TONEAREST, this function rounds to even in halfway cases (like std::rint, but unlike std::round).

The additional overloads are not required to be provided exactly as (A). They only need to be sufficient to ensure that for their argument num of integer type, std::nearbyint(num) has the same effect as std::nearbyint(static_cast<double>(num)).

Example

Run this code

#include <cfenv>
#include <cmath>
#include <iostream>
#pragma STDC FENV_ACCESS ON
 
int main()
{
    std::fesetround(FE_TONEAREST);
    std::cout << "rounding to nearest: \n"
              << "nearbyint(+2.3) = " << std::nearbyint(2.3)
              << "  nearbyint(+2.5) = " << std::nearbyint(2.5)
              << "  nearbyint(+3.5) = " << std::nearbyint(3.5) << '\n'
              << "nearbyint(-2.3) = " << std::nearbyint(-2.3)
              << "  nearbyint(-2.5) = " << std::nearbyint(-2.5)
              << "  nearbyint(-3.5) = " << std::nearbyint(-3.5) << '\n';
 
    std::fesetround(FE_DOWNWARD);
    std::cout << "rounding down:\n"
              << "nearbyint(+2.3) = " << std::nearbyint(2.3)
              << "  nearbyint(+2.5) = " << std::nearbyint(2.5)
              << "  nearbyint(+3.5) = " << std::nearbyint(3.5) << '\n'
              << "nearbyint(-2.3) = " << std::nearbyint(-2.3)
              << "  nearbyint(-2.5) = " << std::nearbyint(-2.5)
              << "  nearbyint(-3.5) = " << std::nearbyint(-3.5) << '\n';
 
    std::cout << "nearbyint(-0.0) = " << std::nearbyint(-0.0)  << '\n'
              << "nearbyint(-Inf) = " << std::nearbyint(-INFINITY) << '\n';
}

Output:

rounding to nearest: 
nearbyint(+2.3) = 2  nearbyint(+2.5) = 2  nearbyint(+3.5) = 4
nearbyint(-2.3) = -2  nearbyint(-2.5) = -2  nearbyint(-3.5) = -4
rounding down:
nearbyint(+2.3) = 2  nearbyint(+2.5) = 2  nearbyint(+3.5) = 3
nearbyint(-2.3) = -3  nearbyint(-2.5) = -3  nearbyint(-3.5) = -4
nearbyint(-0.0) = -0
nearbyint(-Inf) = -inf

See also

rintrintfrintllrintlrintflrintlllrintllrintfllrintl (C++11)(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)	nearest integer using current rounding mode with exception if the result differs (function)
roundroundfroundllroundlroundflroundlllroundllroundfllroundl (C++11)(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)	nearest integer, rounding away from zero in halfway cases (function)
fegetroundfesetround (C++11)(C++11)	gets or sets rounding direction (function)
C documentation for nearbyint