NAME¶

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

Synopsis¶

Defined in header <cmath>
float ldexp ( float num, int exp );

double ldexp ( double num, int exp ); (until C++23)

long double ldexp ( long double num, int exp );
constexpr /* floating-point-type */
ldexp ( /* floating-point-type */ num, (since C++23)
int exp ); (1)
float ldexpf( float num, int exp ); (2) (since C++11)
(constexpr since C++23)
long double ldexpl( long double num, int exp ); (3) (since C++11)
(constexpr since C++23)
Additional overloads (since C++11)
Defined in header <cmath>
template< class Integer > (A) (since C++11)
double ldexp ( Integer num, int exp ); (constexpr since C++23)

1-3) Multiplies a floating point value num by the number 2 raised to the exp power.
The library provides overloads of std::ldexp for all cv-unqualified floating-point
types as the type of the parameter num.
(since C++23)

A) Additional overloads are provided for all integer types, which are (since C++11)
treated as double.

Parameters¶

num - floating-point or integer value
exp - integer value

Return value¶

If no errors occur, num multiplied by 2 to the power of exp (num×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 num is ±0, it is returned, unmodified.
* If num is ±∞, it is returned, unmodified.
* If exp is 0, then num is returned, unmodified.
* If num 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.

The additional overloads are not required to be provided exactly as (A). They only
need to be sufficient to ensure that for their argument num of integer type,
std::ldexp(num, exp) has the same effect as std::ldexp(static_cast<double>(num),
exp).

For exponentiation of 2 by a floating point exponent, std::exp2 can be used.

Example¶

// Run this code

#include <cerrno>
#include <cfenv>
#include <cmath>
#include <cstring>
#include <iostream>
// #pragma STDC FENV_ACCESS ON

int main()
{
std::cout
<< "ldexp(5, 3) = 5 * 8 = " << std::ldexp(5, 3) << '\n'
<< "ldexp(7, -4) = 7 / 16 = " << std::ldexp(7, -4) << '\n'
<< "ldexp(1, -1074) = " << std::ldexp(1, -1074)
<< " (minimum positive subnormal float64_t)\n"
<< "ldexp(nextafter(1,0), 1024) = "
<< std::ldexp(std::nextafter(1,0), 1024)
<< " (largest finite float64_t)\n";

// special values
std::cout << "ldexp(-0, 10) = " << std::ldexp(-0.0, 10) << '\n'
<< "ldexp(-Inf, -1) = " << std::ldexp(-INFINITY, -1) << '\n';

// error handling
std::feclearexcept(FE_ALL_EXCEPT);
errno = 0;
const double inf = std::ldexp(1, 1024);
const bool is_range_error = errno == ERANGE;

std::cout << "ldexp(1, 1024) = " << inf << '\n';
if (is_range_error)
std::cout << " errno == ERANGE: " << std::strerror(ERANGE) << '\n';
if (std::fetestexcept(FE_OVERFLOW))
std::cout << " FE_OVERFLOW raised\n";
}

Possible output:¶

ldexp(5, 3) = 5 * 8 = 40
ldexp(7, -4) = 7 / 16 = 0.4375
ldexp(1, -1074) = 4.94066e-324 (minimum positive subnormal float64_t)
ldexp(nextafter(1,0), 1024) = 1.79769e+308 (largest finite float64_t)
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 base-2 exponent
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)
exp2
exp2f
exp2l returns 2 raised to the given power (\({\small 2^x}\)2^x)
(C++11) (function)
(C++11)
(C++11)
C documentation for
ldexp