std::ldexp

 

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Defined in header <cmath>
float       ldexp( float x, int exp );	(1)
double      ldexp( double x, int exp );	(2)
long double ldexp( long double x, int exp );	(3)
double      ldexp( Integral x, int exp );	(4)	(since C++11)

1-3) Multiplies a floating point value x by the number 2 raised to the exp power.

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).

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	decomposes a number into significand and a power of 2 (function)
scalbnscalbln (C++11)(C++11)	multiplies a number by FLT_RADIX raised to a power (function)
C documentation for ldexp