6.11.109 log Function

Calculates the natural logarithm of a double-precision floating-point value.

Include

<math.h>

Prototype

double log(double x);

Argument

x

any positive value for which to return the log

Return Value

Returns the natural (base-$e$) logarithm of x. If x is 0, infinity is returned. If x is a negative number, NaN is returned.

Remarks

If x < 0, a domain error will occur and errno will be set to EDOM.

Example

See the notes at the beginning of this chapter or section for information on using printf() or scanf() (and other functions reading and writing the stdin or stdout streams) in the example code.

#include <math.h>
#include <stdio.h>
#include <errno.h>

int main(void)
{
  double x, y;

  errno = 0;
  x = 2.0;
  y = log(x);
  if (errno)
    perror("Error");
  printf("The natural logarithm of %f is %f\n",
          x, y);

  errno = 0;
  x = 0.0;
  y = log(x);
  if (errno)
    perror("Error");
  printf("The natural logarithm of %f is %f\n",
          x, y);

  errno = 0;
  x = -2.0;
  y = log(x);
  if (errno)
    perror("Error");
  printf("The natural logarithm of %f is %f\n",
          x, y);
}

Example Output

The natural logarithm of 2.000000 is 0.693147
The natural logarithm of 0.000000 is -inf
Error: domain error
The natural logarithm of -2.000000 is nan