std::round, std::roundf, std::roundl, std::lround, std::lroundf, std::lroundl, std::llround, std::llroundf
Defined in header <cmath>
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float round ( float arg ); float roundf( float arg ); |
(1) | (since C++11) |
double round ( double arg ); |
(2) | (since C++11) |
long double round ( long double arg ); long double roundl( long double arg ); |
(3) | (since C++11) |
double round ( IntegralType arg ); |
(4) | (since C++11) |
long lround ( float arg ); long lroundf( float arg ); |
(5) | (since C++11) |
long lround ( double arg ); |
(6) | (since C++11) |
long lround ( long double arg ); long lroundl( long double arg ); |
(7) | (since C++11) |
long lround ( IntegralType arg ); |
(8) | (since C++11) |
long long llround ( float arg ); long long llroundf( float arg ); |
(9) | (since C++11) |
long long llround ( double arg ); |
(10) | (since C++11) |
long long llround ( long double arg ); long long llroundl( long double arg ); |
(11) | (since C++11) |
long long llround ( IntegralType arg ); |
(12) | (since C++11) |
arg
(in floating-point format), rounding halfway cases away from zero, regardless of the current rounding mode.arg
(in integer format), rounding halfway cases away from zero, regardless of the current rounding mode.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.
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
andstd::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
#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
(C++11)(C++11) |
nearest integer not greater than the given value (function) |
(C++11)(C++11) |
nearest integer not less than the given value (function) |
(C++11)(C++11)(C++11) |
nearest integer not greater in magnitude than the given value (function) |