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std::ilogb,std::ilogbf,std::ilogbl(3) | C++ Standard Libary | std::ilogb,std::ilogbf,std::ilogbl(3) |

# 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)*

2022.07.31 | http://cppreference.com |