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

From cppreference.com
< cpp‎ | numeric‎ | math
 
 
 
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Defined in header <cmath>
(1)
float       log ( float num );

double      log ( double num );

long double log ( long double num );
(until C++23)
/* floating-point-type */
            log ( /* floating-point-type */ num );
(since C++23)
(constexpr since C++26)
float       logf( float num );
(2) (since C++11)
(constexpr since C++26)
long double logl( long double num );
(3) (since C++11)
(constexpr since C++26)
Additional overloads (since C++11)
Defined in header <cmath>
template< class Integer >
double      log ( Integer num );
(A) (constexpr since C++26)
1-3) 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.(since C++23)
A) Additional overloads are provided for all integer types, which are treated as double.
(since C++11)

Contents

[edit] Parameters

num - floating-point or integer value

[edit] Return value

If no errors occur, the natural (base-e) logarithm of num (ln(num) or log
e
(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.

[edit] 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.

[edit] Notes

The additional overloads are not required to be provided exactly as (A). 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)).

[edit] Example

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

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

[edit] See also

(C++11)(C++11)
computes common (base 10) logarithm (log10(x))
(function) [edit]
(C++11)(C++11)(C++11)
base 2 logarithm of the given number (log2(x))
(function) [edit]
(C++11)(C++11)(C++11)
natural logarithm (to base e) of 1 plus the given number (ln(1+x))
(function) [edit]
(C++11)(C++11)
returns e raised to the given power (ex)
(function) [edit]
complex natural logarithm with the branch cuts along the negative real axis
(function template) [edit]
applies the function std::log to each element of valarray
(function template) [edit]