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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.

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.

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);
}

Log base 2 of 65536.000000 is 16.000000

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