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

Formally, the unbiased exponent is the integral part of log r |num| as a signed integral value, for non-zero num , where r is std:: numeric_limits < T > :: radix and T is the floating-point type of num .

Parameters

Return value

If no errors occur, the unbiased exponent of num is returned as a signed int value.

If num is zero, FP_ILOGB0 is returned.

If num is infinite, INT_MAX is returned.

If num 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 num 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 num 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 num is not zero, infinite, or NaN, the value returned is exactly equivalent to static_cast < int > ( std:: logb ( num ) ) .

POSIX requires that a domain error occurs if num 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 * std:: log2 ( FLT_RADIX ) or INT_MIN must be greater than LDBL_MIN_EXP - LDBL_MANT_DIG ) * std:: 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 , |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 :: ilogb ( num ) has the same effect as std :: ilogb ( static_cast < double > ( num ) ) .

Example

#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 << "ilogb(0) = " << std::ilogb(0) << '\n';
    if (std::fetestexcept(FE_INVALID))
        std::cout << "    FE_INVALID raised\n";
}

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