NAME¶

std::ldexp,std::ldexpf,std::ldexpl - std::ldexp,std::ldexpf,std::ldexpl

Synopsis¶

Defined in header <cmath>
float ldexp ( float x, int exp ); (1) (constexpr since C++23)
float ldexpf( float x, int exp ); (2) (since C++11)
(constexpr since C++23)
double ldexp ( double x, int exp ); (3) (constexpr since C++23)
long double ldexp ( long double x, int exp ); (4) (constexpr since C++23)
long double ldexpl( long double x, int exp ); (5) (since C++11)
(constexpr since C++23)
double ldexp ( IntegralType x, int exp ); (6) (since C++11)
(constexpr since C++23)

1-5) Multiplies a floating point value x by the number 2 raised to the exp power.
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¶

x - floating point value
exp - integer value

Return value¶

If no errors occur, x multiplied by 2 to the power of exp (x×2exp
) is returned.

If a range error due to overflow occurs, ±HUGE_VAL, ±HUGE_VALF, or ±HUGE_VALL is
returned.

If a range error due to underflow occurs, the correct result (after rounding) is
returned.

Error handling¶

Errors are reported as specified in math_errhandling.

If the implementation supports IEEE floating-point arithmetic (IEC 60559),

* Unless a range error occurs, FE_INEXACT is never raised (the result is exact)
* Unless a range error occurs, the current rounding mode is ignored
* If x is ±0, it is returned, unmodified
* If x is ±∞, it is returned, unmodified
* If exp is 0, then x is returned, unmodified
* If x is NaN, NaN is returned

Notes¶

On binary systems (where FLT_RADIX is 2), std::ldexp is equivalent to std::scalbn.

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

On many implementations, std::ldexp is less efficient than multiplication or
division by a power of two using arithmetic operators.

Example¶

// Run this code

#include <iostream>
#include <cmath>
#include <cerrno>
#include <cstring>
#include <cfenv>

// #pragma STDC FENV_ACCESS ON
int main()
{
std::cout << "ldexp(7, -4) = " << std::ldexp(7, -4) << '\n'
<< "ldexp(1, -1074) = " << std::ldexp(1, -1074)
<< " (minimum positive subnormal double)\n"
<< "ldexp(nextafter(1,0), 1024) = "
<< std::ldexp(std::nextafter(1,0), 1024)
<< " (largest finite double)\n";
// special values
std::cout << "ldexp(-0, 10) = " << std::ldexp(-0.0, 10) << '\n'
<< "ldexp(-Inf, -1) = " << std::ldexp(-INFINITY, -1) << '\n';
// error handling
errno = 0;
std::feclearexcept(FE_ALL_EXCEPT);
std::cout << "ldexp(1, 1024) = " << std::ldexp(1, 1024) << '\n';
if (errno == ERANGE)
std::cout << " errno == ERANGE: " << std::strerror(errno) << '\n';
if (std::fetestexcept(FE_OVERFLOW))
std::cout << " FE_OVERFLOW raised\n";
}

Output:¶

ldexp(7, -4) = 0.4375
ldexp(1, -1074) = 4.94066e-324 (minimum positive subnormal double)
ldexp(nextafter(1,0), 1024) = 1.79769e+308 (largest finite double)
ldexp(-0, 10) = -0
ldexp(-Inf, -1) = -inf
ldexp(1, 1024) = inf
errno == ERANGE: Numerical result out of range
FE_OVERFLOW raised

See also¶

frexp
frexpf decomposes a number into significand and a power of 2
frexpl (function)
(C++11)
(C++11)
scalbn
scalbnf
scalbnl
scalbln
scalblnf
scalblnl multiplies a number by FLT_RADIX raised to a power
(C++11) (function)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)