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.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, thenx
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
#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
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 |
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