std::log,std::logf,std::logl (3) - Linux Manuals

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


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


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).


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


// Run this code

  #include <iostream>
  #include <cmath>
  #include <cerrno>
  #include <cstring>
  #include <cfenv>
  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::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

log10l computes common (base 10) logarithm (log10(x))


log2l base 2 logarithm of the given number (log2(x))

log1pl natural logarithm (to base e) of 1 plus the given number (ln(1+x))

expl returns e raised to the given power (ex)

                   complex natural logarithm with the branch cuts along the negative real axis
log(std::complex) (function template)
                   applies the function std::log to each element of valarray
log(std::valarray) (function template)