pow

说明

number pow ( number base, number exp )

返回 base 的 exp 次方的幂。如果可能，本函数会返回 integer。

如果不能计算幂，将发出一条警告，pow() 将返回 FALSE。PHP 4.2.0 版开始 pow() 不要产生任何的警告。

注: PHP 不能处理负数的 base。

例子 1. pow() 例子


<?php

var_dump(pow(2, 8)); // int(256)
echo pow(-1, 20); // 1
echo pow(0, 0); // 1

echo pow(-1, 5.5); // error

?>

警告

在 PHP 4.0.6 及之前版本中，pow() 总是返回 float，并且不产生警告。

参见 exp()，sqrt()，bcpow() gmp_pow() 和。

add a note User Contributed Notes

moikboy (nospam) moikboy (nospam) hu
10-May-2006 04:27


Here is a function for calculating the $k-th root of $a :



<?php

function root($a,$k){return(($a<0&&$k%2>0)?-1:1)*pow(abs($a),1/$k);};

?>

admin at mattwilko dot com
08-Apr-2005 12:32


Here's a function that works with negative powers:



<?php

function newpow($base, $power)

{

if ($power < 0) {

$npower = $power - $power - $power;

return 1 / pow($base, $npower);

}

else

{

return pow($base, $power);

}

}

?>

louis [at] mulliemedia.com
01-Jan-2005 12:02


Here's a pow() function that allows negative bases :

<?php

function npow($base, $exp)

{

    $result = pow(abs($base), $exp);

    if ($exp % 2 !== 0) {

        $result = - ($result);

    } 

    return $result;

} 

?>

janklopper .AT. gmail dot.com
10-Nov-2004 10:26


since pow doesn't support decimal powers, you can use a different sollution,



thanks to dOt for doing the math!



a^b = e^(b log a)

which is no the 10log but the e-log (aka "ln")



so instead of: pow( $a , 0.6 ) use something like: exp( 0.6 * log($a) )

matthew underscore kay at ml1 dot net
18-Mar-2004 03:03


As of PHP5beta4, pow() with negative bases appears to work correctly and without errors (from a few cursory tests):



pow(-3, 3) = -27

pow(-3, 2) = 9

pow(-5, -1) = -0.2

bishop
18-Jul-2003 11:01


A couple of points on pow():

1. One of the official examples of pow(2,8) is not pragmatic; use 1 << 8 as it's substantially faster

2. When passing variables to pow(), cast them otherwise you might get warnings on some versions of PHP

3. All the rules of algebra apply: b**(-e) is 1/(b**e), b**(p/q) is the qth root of b**p



So, e.g., sqrt($x) === pow($x, .5); but sqrt() is faster.

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