log, logf, log10, log10f
Calculates logarithms.
double log( double x ); float log( float x ); // C++ only long double log( long double x ); // C++ only float logf( float x ); double log10( double x ); float log10( float x ); // C++ only long double log10( long double x ); // C++ only float log10f ( float x );
Parameters
- x
-
Value whose logarithm is to be found.
The log functions return the natural logarithm (base e) of x if successful. The log10 functions return the base-10 logarithm. If x is negative, these functions return an indefinite, by default. If x is 0, they return INF (infinite).
| Input | SEH Exception | Matherr Exception |
|---|---|---|
| ± QNAN,IND | none | _DOMAIN |
| ± 0 | ZERODIVIDE | _SING |
| x < 0 | INVALID | _DOMAIN |
log and log10 has an implementation that uses Streaming SIMD Extensions 2 (SSE2). See _set_SSE2_enable for information and restrictions on using the SSE2 implementation.
| Routine | Required header | Compatibility |
|---|---|---|
| log, logf, log10, log10f | <math.h> | ANSI, Windows 95, Windows 98, Windows 98 Second Edition, Windows Millennium Edition, Windows Millennium Edition, Windows NT 4.0, Windows 2000, Windows XP Home Edition, Windows XP Professional, Windows Server 2003 |
For additional compatibility information, see Compatibility in the Introduction.
Libraries
All versions of the C run-time libraries.
// crt_log.c
/* This program uses log and log10
* to calculate the natural logarithm and
* the base-10 logarithm of 9,000.
*/
#include <math.h>
#include <stdio.h>
int main( void )
{
double x = 9000.0;
double y;
y = log( x );
printf( "log( %.2f ) = %f\n", x, y );
y = log10( x );
printf( "log10( %.2f ) = %f\n", x, y );
}
log( 9000.00 ) = 9.104980 log10( 9000.00 ) = 3.954243
To generate logarithms for other bases, use the mathematical relation: log base b of a == natural log (a) / natural log (b).
// logbase.cpp
#include <math.h>
#include <stdio.h>
double logbase(double a, double base)
{
return log(a) / log(base);
}
int main()
{
double x = 65536;
double result;
result = logbase(x, 2);
printf("Log base 2 of %lf is %lf\n", x, result);
} 