std::frexp,std::frexpf,std::frexpl (3) - Linux Manuals

std::frexp,std::frexpf,std::frexpl: std::frexp,std::frexpf,std::frexpl


std::frexp,std::frexpf,std::frexpl - std::frexp,std::frexpf,std::frexpl


Defined in header <cmath>
float frexp ( float arg, int* exp );
float frexpf( float arg, int* exp ); (since C++11)
double frexp ( double arg, int* exp ); (1) (2)
long double frexp ( long double arg, int* exp );
long double frexpl( long double arg, int* exp ); (3) (since C++11)
double frexp ( IntegralType arg, int* exp ); (4) (since C++11)

1-3) Decomposes given floating point value arg into a normalized fraction and an integral power of two.
4) A set of overloads or a function template accepting an argument of any integral_type. Equivalent to (2) (the argument is cast to double).


arg - floating point value
exp - pointer to integer value to store the exponent to

Return value

If arg is zero, returns zero and stores zero in *exp.
Otherwise (if arg is not zero), if no errors occur, returns the value x in the range (-1;-0.5], [0.5; 1) and stores an integer value in *exp such that x×2(*exp)
If the value to be stored in *exp is outside the range of int, the behavior is unspecified.
If arg is not a floating-point number, the behavior is unspecified.

Error handling

This function is not subject to any errors specified in math_errhandling.
If the implementation supports IEEE floating-point arithmetic (IEC 60559),

* If arg is ±0, it is returned, unmodified, and 0 is stored in *exp.
* If arg is ±∞, it is returned, and an unspecified value is stored in *exp.
* If arg is NaN, NaN is returned, and an unspecified value is stored in *exp.
* No floating-point exceptions are raised.
* If FLT_RADIX is 2 (or a power of 2), the returned value is exact, the_current_rounding_mode is ignored


On a binary system (where FLT_RADIX is 2), frexp may be implemented as

      *exp = (value == 0) ? 0 : (int)(1 + std::logb(value));
      return std::scalbn(value, -(*exp));

The function std::frexp, together with its dual, std::ldexp, can be used to manipulate the representation of a floating-point number without direct bit manipulations.


Compares different floating-point decomposition functions
// Run this code

  #include <iostream>
  #include <cmath>
  #include <limits>

  int main()
      double f = 123.45;
      std::cout << "Given the number " << f << " or " << std::hexfloat
                << f << std::defaultfloat << " in hex,\n";

      double f3;
      double f2 = std::modf(f, &f3);
      std::cout << "modf() makes " << f3 << " + " << f2 << '\n';

      int i;
      f2 = std::frexp(f, &i);
      std::cout << "frexp() makes " << f2 << " * 2^" << i << '\n';

      i = std::ilogb(f);
      std::cout << "logb()/ilogb() make " << f/std::scalbn(1.0, i) << " * "
                << std::numeric_limits<double>::radix
                << "^" << std::ilogb(f) << '\n';

Possible output:

  Given the number 123.45 or 0x1.edccccccccccdp+6 in hex,
  modf() makes 123 + 0.45
  frexp() makes 0.964453 * 2^7
  logb()/ilogb() make 1.92891 * 2^6

See also

ldexpl multiplies a number by 2 raised to a power


logbl extracts exponent of the number

ilogbl extracts exponent of the number

modfl decomposes a number into integer and fractional parts