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# std::log

From cppreference.com
< cpp‎ | numeric‎ | math

Defined in header `<cmath>` |  |
float log( float arg ); | (1) |
double log( double arg ); | (2) |
long double log( long double arg ); | (3) |
double log( Integral arg ); | (4) | (since C++11)

1-3) Computes the 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).

### [edit] Parameters

arg | - | value of floating-point or Integral type

### [edit] 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.

### [edit] 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

### [edit] 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

### [edit] See also

log10 | computes common (base 10) logarithm (log10(x)) (function) [edit]
log2 (C++11) | base 2 logarithm of the given number (log2(x)) (function) [edit]
log1p (C++11) | natural logarithm (to base e) of 1 plus the given number (ln(1+x)) (function) [edit]
exp | returns e raised to the given power (ex) (function) [edit]
log(std::complex) | complex natural logarithm with the branch cuts along the negative real axis (function template) [edit]
log(std::valarray) | applies the function std::log to each element of valarray (function template) [edit]
C documentation for log

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