std::round,std::roundf,std::roundl,std::lround,std::lroundf,std::lroundl,std::llround,std::llroundf (3) Linux Manual Page
std::round,std::roundf,std::roundl,std::lround,std::lroundf,std::lroundl,std::llround,std::llroundf – std::round,std::roundf,std::roundl,std::lround,std::lroundf,std::lroundl,std::llround,std::llroundf
Synopsis
Defined in header <cmath>
float round ( float arg ); (1) (since C++11)
float roundf( float arg );
double round ( double arg ); (2) (since C++11)
long double round ( long double arg ); (3) (since C++11)
long double roundl( long double arg );
double round ( IntegralType arg ); (4) (since C++11)
long lround ( float arg ); (5) (since C++11)
long lroundf( float arg );
long lround ( double arg ); (6) (since C++11)
long lround ( long double arg ); (7) (since C++11)
long lroundl( long double arg );
long lround ( IntegralType arg ); (8) (since C++11)
long long llround ( float arg ); (9) (since C++11)
long long llroundf( float arg );
long long llround ( double arg ); (10) (since C++11)
long long llround ( long double arg ); (11) (since C++11)
long long llroundl( long double arg );
long long llround ( IntegralType 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).
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),
* 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
* 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>
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:
See also
floor
floorf
floorl nearest integer not greater than the given value
(C++11)
(C++11)
ceil
ceilf
ceill nearest integer not less than the given value
(C++11)
(C++11)
trunc
truncf
truncl nearest integer not greater in magnitude than the given value
(C++11)
(C++11)
(C++11)