C in a Nutshell

Name

frexp

Synopsis

Splits a real number into a mantissa and exponent

#include <math.h>
doublefrexp( double x, int *exp );
float frexpf( float x, int *exp );        (C99)
long double frexpl( long double x, int *exp );        (C99)

The frexp() function expresses a floating-point number x as a normalized fraction f and an integer exponent e to base 2. In other words, if the fraction f is the return value of frexp(x, &e), then x = f × 2^e and 0.5 ≤ f < 1.

The normalized fraction is the return value of the frexp() function. The function places the other part of its “answer,” the exponent, in the location addressed by the pointer argument. If the floating-point argument x is equal to 0, then the function stores the value 0 at the exponent location and returns 0.

Example

double fourthrt( double x )
{
  int exponent, exp_mod_4;
  double mantissa =frexp( x, &exponent );

  exp_mod_4 = exponent % 4;
  exponent -= ( exp_mod_4 ); // Get an exponent that's divisible by four ...
  for ( int i = abs( exp_mod_4 ); i > 0; i− )
  {
    if ( exp_mod_4 > 0 )     // ... and compensate in the mantissa.
      mantissa *= 2.0;
    else
      mantissa /= 2.0;
  }
  return ldexp( sqrt( sqrt( mantissa )), exponent / 4 );
}

See Also

The ldexp() function, which performs the reverse calculation.