std::logb, std::logbf, std::logbl
| Defined in header <cmath>
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| (1) | ||
float logb ( float num ); double logb ( double num ); |
(until C++23) | |
| constexpr /* floating-point-type */ logb ( /* floating-point-type */ num ); |
(since C++23) | |
| float logbf( float num ); |
(2) | (since C++11) (constexpr since C++23) |
| long double logbl( long double num ); |
(3) | (since C++11) (constexpr since C++23) |
| Additional overloads (since C++11) |
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| Defined in header <cmath>
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| template< class Integer > double logb ( Integer num ); |
(A) | (constexpr since C++23) |
std::logb for all cv-unqualified floating-point types as the type of the parameter.(since C++23)|
A) Additional overloads are provided for all integer types, which are treated as double.
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(since C++11) |
Formally, the unbiased exponent is the signed integral part of logr|num| (returned by this function as a floating-point value), for non-zero num, where r is std::numeric_limits<T>::radix and T is the floating-point type of num. If num is subnormal, it is treated as though it was normalized.
Contents |
[edit] Parameters
| num | - | floating-point or integer value |
[edit] Return value
If no errors occur, the unbiased exponent of num is returned as a signed floating-point value.
If a domain error occurs, an implementation-defined value is returned.
If a pole error occurs, -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL is returned.
[edit] Error handling
Errors are reported as specified in math_errhandling.
Domain or range error may occur if num is zero.
If the implementation supports IEEE floating-point arithmetic (IEC 60559),
- If num is ±0, -∞ is returned and FE_DIVBYZERO is raised.
- If num is ±∞, +∞ is returned.
- If num is NaN, NaN is returned.
- In all other cases, the result is exact (FE_INEXACT is never raised) and the current rounding mode is ignored.
[edit] Notes
POSIX requires that a pole error occurs if num is ±0.
The value of the exponent returned by std::logb is always 1 less than the exponent returned by std::frexp because of the different normalization requirements: for the exponent e returned by std::logb, |num*r-e| is between 1 and r (typically between 1 and 2), but for the exponent e returned by std::frexp, |num*2-e| is between 0.5 and 1.
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::logb(num) has the same effect as std::logb(static_cast<double>(num)).
[edit] Example
Compares different floating-point decomposition functions:
#include <cfenv> #include <cmath> #include <iostream> #include <limits> // #pragma STDC FENV_ACCESS ON int main() { double f = 123.45; std::cout << "Given the number " << f << " or " << std::hexfloat << f << std::defaultfloat << " in hex,\n"; double f3; double f2 = std::modf(f, &f3); std::cout << "modf() makes " << f3 << " + " << f2 << '\n'; int i; f2 = std::frexp(f, &i); std::cout << "frexp() makes " << f2 << " * 2^" << i << '\n'; i = std::ilogb(f); std::cout << "logb()/ilogb() make " << f / std::scalbn(1.0, i) << " * " << std::numeric_limits<double>::radix << "^" << std::ilogb(f) << '\n'; // error handling std::feclearexcept(FE_ALL_EXCEPT); std::cout << "logb(0) = " << std::logb(0) << '\n'; if (std::fetestexcept(FE_DIVBYZERO)) std::cout << " FE_DIVBYZERO raised\n"; }
Possible output:
Given the number 123.45 or 0x1.edccccccccccdp+6 in hex,
modf() makes 123 + 0.45
frexp() makes 0.964453 * 2^7
logb()/ilogb() make 1.92891 * 2^6
logb(0) = -Inf
FE_DIVBYZERO raised[edit] See also
| (C++11)(C++11) |
decomposes a number into significand and base-2 exponent (function) |
| (C++11)(C++11)(C++11) |
extracts exponent of the number (function) |
| (C++11)(C++11)(C++11)(C++11)(C++11)(C++11) |
multiplies a number by FLT_RADIX raised to a power (function) |
| C documentation for logb
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