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std::round, std::lround, std::llround

From cppreference.com
< cpp‎ | numeric‎ | math
 
 
 
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Defined in header <cmath>
float round( float arg );
(1) (since C++11)
double round( double arg );
(2) (since C++11)
long double round( long double arg );
(3) (since C++11)
double round( Integral arg );
(4) (since C++11)
long lround( float arg );
(5) (since C++11)
long lround( double arg );
(6) (since C++11)
long lround( long double arg );
(7) (since C++11)
long lround( Integral arg );
(8) (since C++11)
long long llround( float arg );
(9) (since C++11)
long long llround( double arg );
(10) (since C++11)
long long llround( long double arg );
(11) (since C++11)
long long llround( Integral arg );
(12) (since C++11)
1-3) Computes the nearest integer value to arg (in floating-point format), rounding halfway cases away from zero, regardless of the current rounding mode.
5-7, 9-11) Computes the nearest integer value to arg (in integer format), rounding halfway cases away from zero, regardless of the current rounding mode.
4,8,12) A set of overloads or a function template accepting an argument of any integral type. Equivalent to 2), 6), or 10), respectively (the argument is cast to double).

Contents

[edit] Parameters

arg - floating point value

[edit] Return value

If no errors occur, the nearest integer value to arg, rounding halfway cases away from zero, is returned.

Return value
math-round away zero.svg
Argument

If a domain error occurs, an implementation-defined value is returned

[edit] Error handling

Errors are reported as specified in math_errhandling

If the result of std::lround or std::llround 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::round function:
  • The current rounding mode has no effect.
  • If arg is ±∞, it is returned, unmodified
  • If arg is ±0, it is returned, unmodified
  • If arg is NaN, NaN is returned
For std::lround and std::llround functions:
  • FE_INEXACT is never raised
  • The current rounding mode has no effect.
  • If arg is ±∞, 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 arg is NaN, FE_INVALID is raised and an implementation-defined value is returned

[edit] Notes

FE_INEXACT may be (but isn't required to be) raised by std::round when rounding a non-integer finite value.

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

POSIX specifies that all cases where std::lround or std::llround raise FE_INEXACT are domain errors.

The double version of std::round behaves as if implemented as follows:

#include <cmath>
#include <cfenv>
#pragma STDC FENV_ACCESS ON
double round(double x)
{
    std::fenv_t save_env;
    std::feholdexcept(&save_env);
    double result = std::rint(x);
    if (std::fetestexcept(FE_INEXACT)) {
        auto const save_round = std::fegetround();
        std::fesetround(FE_TOWARDZERO);
        result = std::rint(std::copysign(0.5 + std::fabs(x), x));
        std::fesetround(save_round);
    }
    std::feupdateenv(&save_env);
    return result;
}

[edit] Example

#include <iostream>
#include <cmath>
#include <cfenv>
#include <climits>
 
#pragma STDC FENV_ACCESS ON
 
int main()
{
    // round
    std::cout << "round(+2.3) = " << std::round(2.3)
              << "  round(+2.5) = " << std::round(2.5)
              << "  round(+2.7) = " << std::round(2.7) << '\n'
              << "round(-2.3) = " << std::round(-2.3)
              << "  round(-2.5) = " << std::round(-2.5)
              << "  round(-2.7) = " << std::round(-2.7) << '\n';
 
    std::cout << "round(-0.0) = " << std::round(-0.0)  << '\n'
              << "round(-Inf) = " << std::round(-INFINITY) << '\n';
 
    // lround
    std::cout << "lround(+2.3) = " << std::lround(2.3)
              << "  lround(+2.5) = " << std::lround(2.5)
              << "  lround(+2.7) = " << std::lround(2.7) << '\n'
              << "lround(-2.3) = " << std::lround(-2.3)
              << "  lround(-2.5) = " << std::lround(-2.5)
              << "  lround(-2.7) = " << std::lround(-2.7) << '\n';
 
    std::cout << "lround(-0.0) = " << std::lround(-0.0)  << '\n'
              << "lround(-Inf) = " << std::lround(-INFINITY) << '\n';
 
    // error handling
    std::feclearexcept(FE_ALL_EXCEPT);
    std::cout << "std::lround(LONG_MAX+1.5) = "
              << std::lround(LONG_MAX+1.5) << '\n';
    if(std::fetestexcept(FE_INVALID))
              std::cout << "    FE_INVALID was raised\n";
}

Possible output:

round(+2.3) = 2  round(+2.5) = 3  round(+2.7) = 3
round(-2.3) = -2  round(-2.5) = -3  round(-2.7) = -3
round(-0.0) = -0
round(-Inf) = -inf
lround(+2.3) = 2  lround(+2.5) = 3  lround(+2.7) = 3
lround(-2.3) = -2  lround(-2.5) = -3  lround(-2.7) = -3
lround(-0.0) = 0
lround(-Inf) = -9223372036854775808
std::lround(LONG_MAX+1.5) = -9223372036854775808
    FE_INVALID was raised

[edit] See also

nearest integer not greater than the given value
(function) [edit]
nearest integer not less than the given value
(function) [edit]
(C++11)
nearest integer not greater in magnitude than the given value
(function) [edit]
C documentation for round