log1p, log1pf, log1pl

Article
06/05/2017

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The latest version of this topic can be found at log1p, log1pf, log1pl2.

Computes the natural logarithm of 1 plus the specified value.

Syntax

double log1p(  
   double x  
);  
  
float log1p(  
   float x  
); //C++ only  
  
long double log1p(  
   long double x  
); //C++ only  
  
float log1pf(  
   float x  
);  
  
long double log1pl(  
   long double x  
);

Parameters

x
The floating-point argument.

Return Value

If successful, returns the natural (base-e) log of (x+1).

Otherwise, may return one of the following values:

Input	Result	SEH exception	errno
+inf	+inf
Denormals	Same as input	UNDERFLOW
±0	Same as input
-1	-inf	DIVBYZERO	ERANGE
< -1	nan	INVALID	EDOM
-inf	nan	INVALID	EDOM
±SNaN	Same as input	INVALID
±QNaN, indefinite	Same as input

The errno value is set to ERANGE if x = -1. The errno value is set to EDOM if x < −1.

Remarks

The log1p functions may be more accurate than using log(x+1) when x is near 0.

Because C++ allows overloading, you can call overloads of log1p that take and return float and long double types. In a C program, log1p always takes and returns a double.

If x is a natural number, this function returns the logarithm of the factorial of (x-1).

Requirements

Function	C header	C++ header
`log1p`, `log1pf`, `log1pl`	<math.h>	<cmath>

For additional compatibility information, see Compatibility.

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