std::ilogb
| Defined in header <cmath>
|
||
| int ilogb( float arg ); |
(1) | (since C++11) |
| int ilogb( double arg ); |
(2) | (since C++11) |
| int ilogb( long double arg ); |
(3) | (since C++11) |
| int ilogb( IntegralType arg ); |
(4) | (since C++11) |
| #define FP_ILOGB0 /*implementation-defined*/ |
(5) | (since C++11) |
| #define FP_ILOGBNAN /*implementation-defined*/ |
(6) | (since C++11) |
arg, and returns it as a signed integer value. Formally, the unbiased exponent is the integral part of log
r|arg| as a signed integral value, for non-zero arg, where r is std::numeric_limits<T>::radix and T is the floating-point type of arg.
Parameters
| arg | - | floating point value |
Return value
If no errors occur, the unbiased exponent of arg is returned as a signed int value.
If arg is zero, FP_ILOGB0 is returned.
If arg is infinite, INT_MAX is returned.
If arg is a NaN, FP_ILOGBNAN is returned.
If the correct result is greater than INT_MAX or smaller than INT_MIN, the return value is unspecified.
Error handling
Errors are reported as specified in math_errhandling
A domain error or range error may occur if arg is zero, infinite, or NaN.
If the correct result is greater than INT_MAX or smaller than INT_MIN, a domain error or a range error may occur
If the implementation supports IEEE floating-point arithmetic (IEC 60559),
- If the correct result is greater than INT_MAX or smaller than INT_MIN, FE_INVALID is raised.
- If
argis 0, , or NaN, FE_INVALID is raised. - In all other cases, the result is exact (FE_INEXACT is never raised) and the current rounding mode is ignored
Notes
If arg is not zero, infinite, or NaN, the value returned is exactly equivalent to static_cast<int>(std::logb(arg))
POSIX requires that a domain error occurs if arg is zero, infinite, NaN, or if the correct result is outside of the range of int.
POSIX also requires that, on XSI-conformant systems, the value returned when the correct result is greater than INT_MAX is INT_MAX and the value returned when the correct result is less than INT_MIN is INT_MIN.
The correct result can be represented as int on all known implementations. For overflow to occur, INT_MAX must be less than LDBL_MAX_EXP*log2(FLT_RADIX) or INT_MIN must be greater than LDBL_MIN_EXP-LDBL_MANT_DIG)*log2(FLT_RADIX).
The value of the exponent returned by std::ilogb is always 1 less than the exponent retuned by std::frexp because of the different normalization requirements: for the exponent e returned by std::ilogb, |arg*r-e
| is between 1 and r (typically between 1 and 2), but for the exponent e returned by std::frexp, |arg*2-e
| is between 0.5 and 1.
Example
Compares different floating-point decomposition functions
#include <iostream> #include <cmath> #include <limits> #include <cfenv> #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 << "ilogb(0) = " << std::ilogb(0) << '\n'; if(std::fetestexcept(FE_INVALID)) std::cout << " FE_INVALID 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
ilogb(0) = -2147483648
FE_INVALID raisedSee also
| decomposes a number into significand and a power of 2 (function) | |
| (C++11) |
extracts exponent of the number (function) |
| (C++11)(C++11) |
multiplies a number by FLT_RADIX raised to a power (function) |