std::ldexp
| 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) |
x by the number 2 raised to the exp power.Contents |
[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
xis ±0, it is returned, unmodified - If
xis ±∞, it is returned, unmodified - If
expis 0, thenxis returned, unmodified - If
xis 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
#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
| decomposes a number into significand and a power of 2 (function) | |
| (C++11)(C++11) |
multiplies a number by FLT_RADIX raised to a power (function) |
| C documentation for ldexp
| |