Log
Defined in header <cmath>
.
Description
Computes the natural (base e
) logarithm of num
.
The library provides overloads of std::log
for all cv-unqualified floating-point types as the type of the parameter num
.
Declarations
- C++23
- C++11
// 1)
/* floating-point-type */ log( /* floating-point-type */ num );
// 2)
float logf( float num );
// 3)
long double logl( long double num );
// 4)
template< class Integer >
double log ( Integer num );
// 1)
float log ( float num );
// 2)
double log ( double num );
// 3)
long double log ( long double num );
// 4)
float logf( float num );
// 5)
long double logl( long double num );
// 6)
template< class Integer >
double log ( Integer num );
Parameters
num
- floating-point or integer value
Return value
If no errors occur, the natural (base e
) logarithm of num
(ln(num) or loge(num)) is returned.
If a domain error occurs, an implementation-defined value is returned (NaN where supported)
If a pole error occurs, -HUGE_VAL
, -HUGE_VALF
, or -HUGE_VALL
is returned.
Error handling
Errors are reported as specified in math_errhandling.
Domain error occurs if num
is less than zero.
Pole error may occur if num
is zero.
If the implementation supports IEEE floating-point arithmetic (IEC 60559),
If the argument is ±0
, -∞
is returned and FE_DIVBYZERO
is raised.
If the argument is 1
, +0
is returned
If the argument is negative, NaN is returned and FE_INVALID
is raised.
If the argument is +∞
, +∞
is returned
If the argument is NaN, NaN is returned
Notes
The additional overloads are not required to be provided exactly as Additional Overloads.
They only need to be sufficient to ensure that for their argument num
of integer type,
std::log(num)
has the same effect as std::log(static_cast<double>(num))
.
Examples
#include <cerrno>
#include <cfenv>
#include <cstring>
#include <cmath>
#include <iostream>
// #pragma STDC FENV_ACCESS ON
int main()
{
std::cout << "log(1) = " << std::log(1) << '\n'
<< "base-5 logarithm of 125 = " << std::log(125)/std::log(5) << '\n';
// special values
std::cout << "log(1) = " << std::log(1) << '\n'
<< "log(+Inf) = " << std::log(INFINITY) << '\n';
// error handling
errno = 0;
std::feclearexcept(FE_ALL_EXCEPT);
std::cout << "log(0) = " << std::log(0) << '\n';
if (errno == ERANGE)
std::cout << " errno == ERANGE: " << std::strerror(errno) << '\n';
if (std::fetestexcept(FE_DIVBYZERO))
std::cout << " FE_DIVBYZERO raised\n";
}
log(1) = 0
base-5 logarithm of 125 = 3
log(1) = 0
log(+Inf) = inf
log(0) = -inf
errno == ERANGE: Numerical result out of range
FE_DIVBYZERO raised