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std::ilogb

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Defined in header <cmath>
int         ilogb( float arg );
(seit C++11)
int         ilogb( double arg );
(seit C++11)
int         ilogb( long double arg );
(seit C++11)
double      ilogb( Integral arg );
(seit C++11)
#define FP_ILOGB0 /*implementation-defined*/
(seit C++11)
#define FP_ILOGBNAN /*implementation-defined*/
(seit C++11)

Extracts the value of the exponent from the floating-point argument arg, and returns it as a signed integer value. Formally, the result 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.

Inhaltsverzeichnis

[Bearbeiten] Parameter

arg -
Floating-Point-Wert
Original:
floating point value
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[Bearbeiten] Rückgabewert

The floating-point exponent, cast to integer, as if by static_cast<int>(std::logb(arg)).

Domain oder den Bereich Fehler kann auftreten, wenn arg Null ist, wird FP_ILOGB0 in diesem Fall zurückgegeben .
Original:
Domain or range error may occur if arg is zero, FP_ILOGB0 is returned in that case.
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Domain oder den Bereich Fehler kann auftreten, wenn arg unendlich ist, wird MAX_INT in diesem Fall zurückgegeben .
Original:
Domain or range error may occur if arg is infinite, MAX_INT is returned in that case.
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Domain oder den Bereich Fehler kann auftreten, wenn arg NaN ist, wird FP_ILOGBNAN in diesem Fall zurückgegeben .
Original:
Domain or range error may occur if arg is NaN, FP_ILOGBNAN is returned in that case.
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Wenn das Ergebnis nicht als int dargestellt werden, ist das Ergebnis nicht definiert .
Original:
If the result cannot be represented as int, the result is undefined.
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[Bearbeiten] Notes

Der Wert des Exponenten zurückschickt std::ilogb immer 1 kleiner als der Exponent durch std::frexp wegen der unterschiedlichen Anforderungen Normalisierung retuned: für den Exponenten e durch std::ilogb zurückgekehrt ist |arg*r-e
|
zwischen 1 und r (typischerweise zwischen 1 und 2), aber für die Exponenten e durch std::frexp zurückgegeben wird, ist |arg*2-e
|
zwischen 0.5 und 1 .
Original:
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.
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[Bearbeiten] Beispiel

Vergleicht verschiedene Floating-Point-Zersetzung Funktionen
Original:
Compares different floating-point decomposition functions
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#include <iostream>
#include <cmath>
#include <limits>
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';
}

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

[Bearbeiten] Siehe auch

zersetzt eine Zahl in Mantisse und einer Leistung von 2
Original:
decomposes a number into significand and a power of 2
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(Funktion) [edit]
(C++11)
extrahiert Exponent der Zahl
Original:
extracts exponent of the number
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(Funktion) [edit]
(C++11)
(C++11)
multipliziert eine Zahl durch FLT_RADIX potenziert
Original:
multiplies a number by FLT_RADIX raised to a power
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(Funktion) [edit]