log, logf, logl
From cppreference.com
Defined in header <math.h>
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float logf( float arg ); |
(1) | (since C99) |
double log( double arg ); |
(2) | |
long double logl( long double arg ); |
(3) | (since C99) |
Defined in header <tgmath.h>
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#define log( arg ) |
(4) | (since C99) |
1-3) Computes the natural (base e) logarithm of
arg
.4) Type-generic macro: If
arg
has type long double, logl
is called. Otherwise, if arg
has integer type or the type double, log
is called. Otherwise, logf
is called. If arg
is complex or imaginary, then the macro invokes the corresponding complex function (clogf, clog, clogl).Parameters
arg | - | floating point value |
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 <stdio.h> #include <math.h> #include <float.h> #include <errno.h> #include <fenv.h> #pragma STDC FENV_ACCESS ON int main(void) { printf("log(1) = %f\n", log(1)); printf("base-5 logarithm of 125 = %f\n", log(125)/log(5)); // special values printf("log(1) = %f\n", log(1)); printf("log(+Inf) = %f\n", log(INFINITY)); //error handling errno = 0; feclearexcept(FE_ALL_EXCEPT); printf("log(0) = %f\n", log(0)); if(errno == ERANGE) perror(" errno == ERANGE"); if(fetestexcept(FE_DIVBYZERO)) puts(" FE_DIVBYZERO raised"); }
Output:
log(1) = 0.000000 base-5 logarithm of 125 = 3.000000 log(1) = 0.000000 log(+Inf) = inf log(0) = -inf errno == ERANGE: Numerical result out of range FE_DIVBYZERO raised
References
- C11 standard (ISO/IEC 9899:2011):
- 7.12.6.7 The log functions (p: 244-245)
- 7.25 Type-generic math <tgmath.h> (p: 373-375)
- F.10.3.7 The log functions (p: 522)
- C99 standard (ISO/IEC 9899:1999):
- 7.12.6.7 The log functions (p: 225)
- 7.22 Type-generic math <tgmath.h> (p: 335-337)
- F.9.3.7 The log functions (p: 459)
- C89/C90 standard (ISO/IEC 9899:1990):
- 4.5.4.4 The log function
See also
(C99)(C99) |
computes common (base-10) logarithm (log10(x)) (function) |
(C99)(C99)(C99) |
computes base-2 logarithm (log2(x)) (function) |
(C99)(C99)(C99) |
computes natural (base-e) logarithm of 1 plus the given number (ln(1+x)) (function) |
(C99)(C99) |
computes e raised to the given power (ex) (function) |
(C99)(C99)(C99) |
computes the complex natural logarithm (function) |