PHP Unconference Europe 2015

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

설명

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.

인수

arg

The argument to process

반환값

'e' to the power of arg minus one

변경점

버전 설명
5.3.0 This function is now available on all platforms

참고

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

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User Contributed Notes 2 notes

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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);
}
?>
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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
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