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Showing content from https://en.cppreference.com/w/cpp/language/../numeric/random/../math/rint.html below:

std::rint, std::rintf, std::rintl, std::lrint, std::lrintf, std::lrintl, std::llrint, std::llrintf

Rounding to floating-point types

(1) float rint ( float num ); double rint ( double num ); long double rint ( long double num ); (since C++11)
(until C++23)

/* floating-point-type */ rint( /* floating-point-type */ num );

(since C++23)

float rintf( float num );

(2) (since C++11)

long double rintl( long double num );

(3) (since C++11)

Rounding to long

(4) long lrint ( float num ); long lrint ( double num ); long lrint ( long double num ); (since C++11)
(until C++23)

long lrint ( /* floating-point-type */ num );

(since C++23)

long lrintf( float num );

(5) (since C++11)

long lrintl( long double num );

(6) (since C++11)

Rounding to long long

(7) long long llrint ( float num ); long long llrint ( double num ); long long llrint ( long double num ); (since C++11)
(until C++23)

long long llrint ( /* floating-point-type */ num );

(since C++23)

long long llrintf( float num );

(8) (since C++11)

long long llrintl( long double num );

(9) (since C++11)

template< class Integer > double rint( Integer num );

(A) (since C++11)

template< class Integer > long lrint( Integer num );

(B) (since C++11)

template< class Integer > long long llrint( Integer num );

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::rint for all cv-unqualified floating-point types as the type of the parameter num.(since C++23) 4-9)

Rounds the floating-point argument

num

to an integer value, using the

current rounding mode

The library provides overloads of std::lrint and std::llrint for all cv-unqualified floating-point types as the type of the parameter num.(since C++23)

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

[edit] Parameters num - floating-point or integer value [edit] Return value

If no errors occur, the nearest integer value to num, according to the current rounding mode, is returned.

[edit] Error handling

Errors are reported as specified in math_errhandling.

If the result of std::lrint or std::llrint is outside the range representable by the return type, a domain error or a range error may occur.

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

For the std::rint function:

If num is Â±â, it is returned, unmodified.
If num is Â±0, it is returned, unmodified.
If num is NaN, NaN is returned.

For std::lrint and std::llrint functions:

If num is Â±â, FE_INVALID is raised and an implementation-defined value is returned.
If the result of the rounding is outside the range of the return type, FE_INVALID is raised and an implementation-defined value is returned.
If num is NaN, FE_INVALID is raised and an implementation-defined value is returned.

[edit] Notes

POSIX specifies that all cases where std::lrint or std::llrint raise FE_INEXACT are domain errors.

As specified in math_errhandling, FE_INEXACT may be (but isn't required to be on non-IEEE floating-point platforms) raised by std::rint when rounding a non-integer finite value.

The only difference between std::rint and std::nearbyint 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::rint 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_DOWNWARD, then std::rint is equivalent to std::floor.
FE_UPWARD, then std::rint is equivalent to std::ceil.
FE_TOWARDZERO, then std::rint is equivalent to std::trunc.
FE_TONEAREST, then std::rint differs from std::round in that halfway cases are rounded to even rather than away from zero.

The additional overloads are not required to be provided exactly as (A-C). They only need to be sufficient to ensure that for their argument num of integer type:

std::rint(num) has the same effect as std::rint(static_cast<double>(num)).
std::lrint(num) has the same effect as std::lrint(static_cast<double>(num)).
std::llrint(num) has the same effect as std::llrint(static_cast<double>(num)).

[edit] Example

#include <cfenv>
#include <climits>
#include <cmath>
#include <iostream>
// #pragma STDC FENV_ACCESS ON
 
int main()
{
    std::fesetround(FE_TONEAREST);
    std::cout << "Rounding to nearest (halfway cases to even):\n"
              << "  rint(+2.3) = " << std::rint(2.3) << '\n'
              << "  rint(+2.5) = " << std::rint(2.5) << '\n'
              << "  rint(+3.5) = " << std::rint(3.5) << '\n'
              << "  rint(-2.3) = " << std::rint(-2.3) << '\n'
              << "  rint(-2.5) = " << std::rint(-2.5) << '\n'
              << "  rint(-3.5) = " << std::rint(-3.5) << '\n';
 
    std::fesetround(FE_DOWNWARD);
    std::cout << "Rounding down:\n"
              << "  rint(+2.3) = " << std::rint(2.3) << '\n'
              << "  rint(+2.5) = " << std::rint(2.5) << '\n'
              << "  rint(+3.5) = " << std::rint(3.5) << '\n'
              << "  rint(-2.3) = " << std::rint(-2.3) << '\n'
              << "  rint(-2.5) = " << std::rint(-2.5) << '\n'
              << "  rint(-3.5) = " << std::rint(-3.5) << '\n'
              << "Rounding down with lrint:\n"
              << "  lrint(+2.3) = " << std::lrint(2.3) << '\n'
              << "  lrint(+2.5) = " << std::lrint(2.5) << '\n'
              << "  lrint(+3.5) = " << std::lrint(3.5) << '\n'
              << "  lrint(-2.3) = " << std::lrint(-2.3) << '\n'
              << "  lrint(-2.5) = " << std::lrint(-2.5) << '\n'
              << "  lrint(-3.5) = " << std::lrint(-3.5) << '\n'
              << "Special values:\n"
              << "  lrint(-0.0) = " << std::lrint(-0.0) << '\n'
              << std::hex << std::showbase
              << "  lrint(-Inf) = " << std::lrint(-INFINITY) << '\n';
 
    // error handling
    std::feclearexcept(FE_ALL_EXCEPT);
 
    std::cout << "std::rint(0.1) = " << std::rint(.1) << '\n';
    if (std::fetestexcept(FE_INEXACT))
        std::cout << "  FE_INEXACT was raised\n";
 
    std::feclearexcept(FE_ALL_EXCEPT);
 
    std::cout << "std::lrint(LONG_MIN-2048.0) = "
              << std::lrint(LONG_MIN - 2048.0) << '\n';
    if (std::fetestexcept(FE_INVALID))
        std::cout << "  FE_INVALID was raised\n";
}

Possible output:

Rounding to nearest (halfway cases to even):
  rint(+2.3) = 2
  rint(+2.5) = 2
  rint(+3.5) = 4
  rint(-2.3) = -2
  rint(-2.5) = -2
  rint(-3.5) = -4
Rounding down:
  rint(+2.3) = 2
  rint(+2.5) = 2
  rint(+3.5) = 4
  rint(-2.3) = -2
  rint(-2.5) = -2
  rint(-3.5) = -4
Rounding down with lrint:
  lrint(+2.3) = 2
  lrint(+2.5) = 2
  lrint(+3.5) = 3
  lrint(-2.3) = -3
  lrint(-2.5) = -3
  lrint(-3.5) = -4
Special values:
  lrint(-0.0) = 0
  lrint(-Inf) = 0x8000000000000000
std::rint(0.1) = 0
std::lrint(LONG_MIN-2048.0) = 0x8000000000000000
  FE_INVALID was raised

[edit] See also

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