Note that the benefit of this function for small argument values is lost if PHP is compiled against a C library that that not have builtin support for the log1p() function.
In this case, log1p() will be compiled by using log() instead, and the precision of the result will be identical to log(1), i.e. it will always be 0 for small numbers.
Sample log1p(1.0e-20):
- returns 0.0 if log1p() is approximated by using log()
- returns something very near from 1.0e-20, if log1p() is supported by the underlying C library.
One way to support log1p() correctly on any platform, so that the magnitude of the expected result is respected:
function log1p($x) {
return ($x>-1.0e-8 && $x<1.0e-8) ? ($x - $x*$x/2) : log(1+$x);
}
If you want better precision, you may use a better limited development, for small positive or negative values of x:
log(1+x) = x - x^2/2 + x^3/3 - ... + (-1)^(n-1)*x^n/n + ...
(This serial sum converges only for values of x in [0 ... 1] inclusive, and the ^ operator in the above formula means the exponentiation operator, not the PHP xor operation)
Note that log1p() is undefined for arguments lower than or equal to -1, and that the implied base of the log function is the Neperian "e" constant.
log1p
(PHP 4 >= 4.1.0, PHP 5, PHP 7, PHP 8)
log1p — Devuelve log(1 + numero), calculado de tal forma que no pierde precisión incluso cuando el valor del numero se aproxima a cero.
Descripción
log1p() devuelve el equivalente a log(1 +
number) calculado de tal forma que no pierde precisión
incluso cuando el valor de number se aproxima a cero.
log() puede devolver sólo log(1) en este caso debido a la falta de precisión.
Parámetros
-
number -
El argumento a procesar
Valores devueltos
log(1 + number)
Historial de cambios
| Versión | Descripción |
|---|---|
| 5.3.0 | Esta función está disponible en todas las plataformas. |
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Anonymous ¶
22 years ago
