log, logf, log10, log10f

Статья
10/16/2008

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
);

Параметры

x
Value whose logarithm is to be found.

Возвращаемое значение

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.

Заметки

C++ allows overloading, so you can call overloads of log and log10. In a C program, log and log10 always take and return a double.

Требования

Routine	Required header
log, logf, log10, log10f	<math.h>

For additional compatibility information, see Compatibility in the Introduction.

Libraries

All versions of the C run-time libraries.

Пример

// 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 );
}

Output

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);
}