NAME¶

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

Synopsis¶

Defined in header <cmath>
float frexp ( float arg, int* exp ); (1) (constexpr since C++23)
float frexpf( float arg, int* exp ); (2) (since C++11)
(constexpr since C++23)
double frexp ( double arg, int* exp ); (3) (constexpr since C++23)
long double frexp ( long double arg, int* exp ); (4) (constexpr since C++23)
long double frexpl( long double arg, int* exp ); (5) (since C++11)
(constexpr since C++23)
double frexp ( IntegralType arg, int* exp ); (6) (since C++11)
(constexpr since C++23)

1-5) Decomposes given floating point value arg into a normalized fraction and an
integral power of two.
6) A set of overloads or a function template accepting an argument of any integral
type. Equivalent to (3) (the argument is cast to double).

Parameters¶

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)
== arg

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

Notes¶

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.

Example¶

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¶

ldexp
ldexpf multiplies a number by 2 raised to a power
ldexpl (function)
(C++11)
(C++11)
logb
logbf
logbl extracts exponent of the number
(C++11) (function)
(C++11)
(C++11)
ilogb
ilogbf
ilogbl extracts exponent of the number
(C++11) (function)
(C++11)
(C++11)
modf
modff decomposes a number into integer and fractional parts
modfl (function)
(C++11)
(C++11)