table of contents
std::round,std::roundf,std::roundl,std::lround,std::lroundf,std::lroundl,(3) | C++ Standard Libary | std::round,std::roundf,std::roundl,std::lround,std::lroundf,std::lroundl,(3) |
NAME¶
std::round,std::roundf,std::roundl,std::lround,std::lroundf,std::lroundl, - std::round,std::roundf,std::roundl,std::lround,std::lroundf,std::lroundl,
Synopsis¶
Defined in header <cmath>
float round ( float arg ); (1) (since C++11)
(constexpr since C++23)
float roundf( float arg ); (2) (since C++11)
(constexpr since C++23)
double round ( double arg ); (3) (since C++11)
(constexpr since C++23)
long double round ( long double arg ); (4) (since C++11)
(constexpr since C++23)
long double roundl( long double arg ); (5) (since C++11)
(constexpr since C++23)
double round ( IntegralType arg ); (6) (since C++11)
(constexpr since C++23)
long lround ( float arg ); (7) (since C++11)
(constexpr since C++23)
long lroundf( float arg ); (8) (since C++11)
(constexpr since C++23)
long lround ( double arg ); (9) (since C++11)
(constexpr since C++23)
long lround ( long double arg ); (10) (since C++11)
(constexpr since C++23)
long lroundl( long double arg ); (11) (since C++11)
(constexpr since C++23)
long lround ( IntegralType arg ); (12) (since C++11)
(constexpr since C++23)
long long llround ( float arg ); (13) (since C++11)
(constexpr since C++23)
long long llroundf( float arg ); (14) (since C++11)
(constexpr since C++23)
long long llround ( double arg ); (15) (since C++11)
(constexpr since C++23)
long long llround ( long double arg ); (16) (since C++11)
(constexpr since C++23)
long long llroundl( long double arg ); (17) (since C++11)
(constexpr since C++23)
long long llround ( IntegralType arg ); (18) (since C++11)
(constexpr since C++23)
1-5) Computes the nearest integer value to arg (in floating-point format),
rounding
halfway cases away from zero, regardless of the current rounding mode.
7-11,13-17) Computes the nearest integer value to arg (in integer format),
rounding
halfway cases away from zero, regardless of the current rounding mode.
6,12,18) A set of overloads or a function template accepting an argument of
any
integral type. Equivalent to (3), (9), or (15),
respectively (the argument is cast
to double).
Parameters¶
arg - floating point value.
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.
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.
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;
}
Example¶
// Run this code
#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
See also¶
floor
floorf nearest integer not greater than the given value
floorl (function)
(C++11)
(C++11)
ceil
ceilf nearest integer not less than the given value
ceill (function)
(C++11)
(C++11)
trunc
truncf
truncl nearest integer not greater in magnitude than the given value
(C++11) (function)
(C++11)
(C++11)
2022.07.31 | http://cppreference.com |