6.11.118 log2 Function

Calculates the base-2 logarithm of a double-precision floating-point value.

Include

<math.h>

Prototype

double log2(double x);

Argument

x

any double-precision floating-point positive number

Return Value

Returns the base-2 logarithm of x. If x is +/-0, -infinity is returned. If x is a negative number, NaN is returned .

Remarks

If x is less than 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 = log2(x);
  if(errno)
    perror("Error");
  printf("The base-2 logarithm of %f is %f\n", x, y);

  errno = 0;
  x = 0.0;
  y = log2(x);
  if(errno)
    perror("Error");
  printf("The base-2 logarithm of %f is %f\n", x, y);

  errno = 0;
  x = -2.0;
  y = log2(x);
  if(errno)
    perror("Error");
  printf("The base-2 logarithm of %f is %f\n", x, y);
}

Example Output

The base-2 logarithm of 2.000000 is 1.000000
The base-2 logarithm of 0.000000 is -inf
Error: domain error
The base-2 logarithm of -2.000000 is nan