Defined in header <cmath> | ||
---|---|---|
int ilogb ( float arg ); int ilogbf( float arg ); | (1) | (since C++11) |
int ilogb ( double arg ); | (2) | (since C++11) |
int ilogb ( long double arg ); int ilogbl( 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. double
).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
.
arg | - | floating point 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.
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),
INT_MAX
or smaller than INT_MIN
, FE_INVALID
is raised. arg
is ±0, ±∞, or NaN, FE_INVALID
is raised. FE_INEXACT
is never raised) and the current rounding mode is ignored 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
.
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 raised
(C++11)(C++11) | decomposes a number into significand and a power of 2 (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) |
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