NAME¶

std::log,std::logf,std::logl - std::log,std::logf,std::logl

Synopsis¶

Defined in header <cmath>
float log ( float arg );
float logf( float arg ); (since C++11)
double log ( double arg ); (1) (2)
long double log ( long double arg );
long double logl( long double arg ); (3) (since C++11)
double log ( IntegralType arg ); (4) (since C++11)

1-3) Computes the natural (base e) logarithm of arg.
4) A set of overloads or a function template accepting an argument of any integral
type. Equivalent to (2) (the argument is cast to double).

Parameters¶

arg - value of floating-point or Integral type

Return value¶

If no errors occur, the natural (base-e) logarithm of arg (ln(arg) or log
e(arg)) 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 arg is less than zero.

Pole error may occur if arg 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

Example¶

// Run this code

#include <iostream>
#include <cmath>
#include <cerrno>
#include <cstring>
#include <cfenv>
// #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";
}

Possible output:¶

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

See also¶

log10
log10f computes common (base 10) logarithm (\({\small \log_{10}{x}
log10l }\)log[10](x))
(C++11) (function)
(C++11)
log2
log2f base 2 logarithm of the given number (\({\small \log_{2}{x}
log2l }\)log[2](x))
(C++11) (function)
(C++11)
(C++11)
log1p
log1pf natural logarithm (to base e) of 1 plus the given number
log1pl (\({\small \ln{(1+x)} }\)ln(1+x))
(C++11) (function)
(C++11)
(C++11)
exp
expf returns e raised to the given power (\({\small e^x}\)e^x)
expl (function)
(C++11)
(C++11)
complex natural logarithm with the branch cuts along the negative
log(std::complex) real axis
(function template)
log(std::valarray) applies the function std::log to each element of valarray
(function template)