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| Nearest integer floating point operations | |||||||||||||||||||||||||||||||||||||||||
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Defined in header <cmath> | ||
| (1) | ||
int ilogb(float num); int ilogb(double num); | (since C++11) (until C++23) | |
constexprint ilogb(/* floating-point-type */ num); | (since C++23) | |
int ilogbf(float num); | (2) | (since C++11) (constexpr since C++23) |
int ilogbl(longdouble num); | (3) | (since C++11) (constexpr since C++23) |
#define FP_ILOGB0 /* implementation-defined */ | (4) | (since C++11) |
#define FP_ILOGBNAN /* implementation-defined */ | (5) | (since C++11) |
Defined in header <cmath> | ||
template<class Integer> int ilogb( Integer num); | (A) | (since C++11) (constexpr since C++23) |
std::ilogb for all cv-unqualified floating-point types as the type of the parameternum.(since C++23)Formally, the unbiased exponent is the integral part oflogr|num| as a signed integral value, for non-zeronum, wherer isstd::numeric_limits<T>::radix andT is the floating-point type ofnum.
Contents |
| num | - | floating-point or integer value |
If no errors occur, the unbiased exponent ofnum is returned as a signed int value.
Ifnum is zero,FP_ILOGB0 is returned.
Ifnum is infinite,INT_MAX is returned.
Ifnum is a NaN,FP_ILOGBNAN is returned.
If the correct result is greater thanINT_MAX or smaller thanINT_MIN, the return value is unspecified.
Errors are reported as specified inmath_errhandling.
A domain error or range error may occur ifnum is zero, infinite, or NaN.
If the correct result is greater thanINT_MAX or smaller thanINT_MIN, a domain error or a range error may occur.
If the implementation supports IEEE floating-point arithmetic (IEC 60559),
Ifnum is not zero, infinite, or NaN, the value returned is exactly equivalent tostatic_cast<int>(std::logb(num)).
POSIX requires that a domain error occurs ifnum is zero, infinite, NaN, or if the correct result is outside of the range ofint.
POSIX also requires that, on XSI-conformant systems, the value returned when the correct result is greater thanINT_MAX isINT_MAX and the value returned when the correct result is less thanINT_MIN isINT_MIN.
The correct result can be represented asint on all known implementations. For overflow to occur,INT_MAX must be less thanLDBL_MAX_EXP*std::log2(FLT_RADIX) orINT_MIN must be greater thanLDBL_MIN_EXP-LDBL_MANT_DIG)*std::log2(FLT_RADIX).
The value of the exponent returned bystd::ilogb is always 1 less than the exponent retuned bystd::frexp because of the different normalization requirements: for the exponente returned bystd::ilogb,|num*r-e
| is between1 andr (typically between1 and2), but for the exponente returned bystd::frexp,|num*2-e
| is between0.5 and1.
The additional overloads are not required to be provided exactly as(A). They only need to be sufficient to ensure that for their argumentnum of integer type,std::ilogb(num) has the same effect asstd::ilogb(static_cast<double>(num)).
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 handlingstd::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 + 0.45frexp() makes 0.964453 * 2^7logb()/ilogb() make 1.92891 * 2^6ilogb(0) = -2147483648 FE_INVALID raised
(C++11)(C++11) | decomposes a number into significand and base-2 exponent (function)[edit] |
(C++11)(C++11)(C++11) | extracts exponent of the number (function)[edit] |
(C++11)(C++11)(C++11)(C++11)(C++11)(C++11) | multiplies a number byFLT_RADIX raised to a power (function)[edit] |
C documentation forilogb | |
