std::round, std::lround, std::llround - cppreference.com

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

From cppreference.com
< cpp‎ | numeric‎ | math

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).

### [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
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)) {
            std::fesetround(FE_TOWARDZERO);
            result = std::rint(std::copysign(0.5 + std::fabs(x), x));
        }
        std::feupdateenv(&save_env);
        return result;
    }

### [edit] 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

### [edit] See also

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

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