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std::logb — cppreference.com

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Déclaré dans l'en-tête `<cmath>`
`float logb( float arg );`		(depuis C++11)
`double logb( double arg );`		(depuis C++11)
`long double logb( long double arg );`		(depuis C++11)
`double logb( Integral arg );`		(depuis C++11)

Extrait la valeur de l'exposant de la arg argument en virgule flottante, et retourne une valeur à virgule flottante. Formellement, le résultat est la partie intégrante de log
r|arg| signé en tant que valeur à virgule flottante, pour les non-nul arg, où r est std::numeic_limits<T>::radix et T est le type à virgule flottante de arg. Si arg est inférieure à la normale, elle est traitée comme si elle était normalisée .

Original:

Extracts the value of the exponent from the floating-point argument arg, and returns it as a floating-point value. Formally, the result is the integral part of log
r|arg| as a signed floating-point value, for non-zero arg, where r is std::numeic_limits<T>::radix and T is the floating-point type of arg. If arg is subnormal, it is treated as though it was normalized.

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Paramètres

arg

-

valeur du point flottant

Original:

floating point value

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Retourne la valeur

L'exposant virgule flottante .

Original:

The floating-point exponent.

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Erreur de domaine ou une plage peut se produire si arg est nul .

Original:

Domain or range error may occur if arg is zero.

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Notes

La valeur de l'exposant retourné par std::logb est toujours inférieur de 1 l'exposant retuned par std::frexp en raison des exigences de normalisation différents: pour le e exposant retourné par std::logb, |arg*r-e
| est comprise entre 1 et r (typiquement entre 1 et 2), mais pour le exposant e retourné par std::frexp, |arg*2-e
| se situe entre 0.5 et 1 .

Original:

The value of the exponent returned by std::logb is always 1 less than the exponent retuned by std::frexp because of the different normalization requirements: for the exponent e returned by std::logb, |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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Exemple

Compare les différentes fonctions de décomposition en virgule flottante

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';
}

Résultat :

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

Voir aussi


frexp	se décompose en un certain nombre mantisse et d'une puissance de `2` Original: decomposes a number into significand and a power of `2` The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (fonction) [edit]

ilogb (C++11)	extrait exposant du nombre Original: extracts exponent of the number The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (fonction) [edit]

scalbn scalbln (C++11) (C++11)	multiplie par un nombre FLT_RADIX élevé à une puissance Original: multiplies a number by FLT_RADIX raised to a power The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (fonction) [edit]

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