std::log, std::logf, std::logl
Defined in header <cmath>
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(1) | ||
float log ( float num ); double log ( double num ); |
(until C++23) | |
/* floating-point-type */ log ( /* floating-point-type */ num ); |
(since C++23) (constexpr since C++26) |
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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) |
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Defined in header <cmath>
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template< class Integer > double log ( Integer num ); |
(A) | (constexpr since C++26) |
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.
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(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) |
(C++11)(C++11)(C++11) |
base 2 logarithm of the given number (log2(x)) (function) |
(C++11)(C++11)(C++11) |
natural logarithm (to base e) of 1 plus the given number (ln(1+x)) (function) |
(C++11)(C++11) |
returns e raised to the given power (ex) (function) |
complex natural logarithm with the branch cuts along the negative real axis (function template) | |
applies the function std::log to each element of valarray (function template) | |
C documentation for log
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