PHP 5.4.36 Released

expm1

(PHP 4 >= 4.1.0, PHP 5)

expm1 Returns exp(number) - 1, computed in a way that is accurate even when the value of number is close to zero

Description

float expm1 ( float $arg )

expm1() returns the equivalent to 'exp(arg) - 1' computed in a way that is accurate even if the value of arg is near zero, a case where 'exp (arg) - 1' would be inaccurate due to subtraction of two numbers that are nearly equal.

Parameters

arg

The argument to process

Return Values

'e' to the power of arg minus one

Changelog

Version Description
5.3.0 This function is now available on all platforms

See Also

  • log1p() - Returns log(1 + number), computed in a way that is accurate even when the value of number is close to zero
  • exp() - Calculates the exponent of e

add a note add a note

User Contributed Notes 2 notes

up
1
hagen at von-eitzen dot de
11 years ago
Compare this to log1p (which is its inverse).

Also, You may have to use a similar workaraound in case the underlying C library
does not support expm1:

<?php
function expm1($x) {
     return (
$x>-1.0e-6 && $x<1.0e-6) ? ($x + $x*$x/2) : (exp($x)-1);
}
?>
up
0
brettz9 AAT yah
5 years ago
Note that exp(x)-1 can be approximated by x + x^2/2! + ... + x^n/n!  and for any value
To Top