NAME¶

std::ilogb,std::ilogbf,std::ilogbl - std::ilogb,std::ilogbf,std::ilogbl

Synopsis¶

Defined in header <cmath>
int ilogb ( float arg ); (1) (since C++11)
int ilogbf( float arg ); (constexpr since C++23)
int ilogb ( double arg ); (2) (since C++11)
(constexpr since C++23)
int ilogb ( long double arg ); (3) (since C++11)
int ilogbl( long double arg ); (constexpr since C++23)
int ilogb ( IntegralType arg ); (4) (since C++11)
(constexpr since C++23)
#define FP_ILOGB0 /*implementation-defined*/ (5) (since C++11)
#define FP_ILOGBNAN /*implementation-defined*/ (6) (since C++11)

1-3) Extracts the value of the unbiased exponent from the floating-point argument
arg, and returns it as a signed integer value.
4) A set of overloads or a function template accepting an argument of any integral
type. Equivalent to (2) (the argument is cast to double).
5) Expands to integer constant expression whose value is either INT_MIN or -INT_MAX.
6) Expands to integer constant expression whose value is either INT_MIN or +INT_MAX.

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 arg is ±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

// Run this code

#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

See also¶

frexp
frexpf decomposes a number into significand and a power of 2
frexpl (function)
(C++11)
(C++11)
logb
logbf
logbl extracts exponent of the number
(C++11) (function)
(C++11)
(C++11)
scalbn
scalbnf
scalbnl
scalbln
scalblnf
scalblnl multiplies a number by FLT_RADIX raised to a power
(C++11) (function)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)