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# std::ldexp

From cppreference.com
< cpp‎ | numeric‎ | math

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).

### [edit] Parameters

x | - | floating point value
exp | - | integer value

### [edit] 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.

### [edit] 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

### [edit] 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.

### [edit] 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

### [edit] See also

frexp | decomposes a number into significand and a power of 2 (function) [edit]
scalbn scalbln (C++11)(C++11) | multiplies a number by FLT_RADIX raised to a power (function) [edit]
C documentation for ldexp

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