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log, logf, log10, log10f

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Calculates logarithms.

Syntax

  
      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.

Return Value

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.

Remarks

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.

Requirements

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.

Example

// 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);  
}

Output

Log base 2 of 65536.000000 is 16.000000

.NET Framework Equivalent

See Also

Floating-Point Support
exp, expf
_matherr
pow, powf, powl
_CIlog
_CIlog10