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