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pow (PHP 3, PHP 4, PHP 5) pow -- Exponential expression Descriptionnumber pow ( number base, number exp )
Returns base raised to the power of
exp. If possible, this function will
return an integer.
If the power cannot be computed, a warning will be issued,
and pow() will return FALSE. Since PHP 4.2.0
pow() doesn't issue any warning.
Замечание:
PHP cannot handle negative bases.
Пример 1. Some examples of pow() |
<?php
var_dump(pow(2, 8)); echo pow(-1, 20); echo pow(0, 0); echo pow(-1, 5.5); ?>
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| Внимание |
In PHP 4.0.6 and earlier pow() always returned
a float, and did not issue warnings.
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See also:
exp(),
sqrt(),
bcpow(), and
gmp_pow(),
Lant
30-Jun-2007 09:25
Its not just x86 system that return pow(2, 1024) as INF.
On my x64 system running php complied for x64 I still find that the maximum I can do is pow(2, 1023).
Perhaps this is because of the x86_64 instructions and the limit can only be pushed further on "true" x64 systems like IA64.
Docey
04-May-2007 10:33
no integer breaking here, pow just silently switches to using floats instead of integers.
pow(2, 31) = integer value
pow(2, 32) = float value.
the manual says the limit for floats is machine dependent so i did a little loop to see how far it will go before becomming infinit. the result is 1023.
pow(2, 1023) = float
pow(2, 1024) = ifinit.
tested on php 4.4.1 under windows2000 on an AMD AthlonXP 2800+. i gues this is thus the same for all 32bit i386 systems.
adverneo at gmx dot de
19-Apr-2007 10:11
Be aware of breaking the integer-limit. $var = pow(2,32); will produce a buffer overflow in PHP 5.0.3 (already reported)
gilthansREMOVEME at gmail dot com
15-Dec-2006 07:50
Note that pow(0, 0) equals to 1 on PHP 4 (only tested it there), although mathematically this is undefined.
moikboy (nospam) moikboy (nospam) hu
10-May-2006 01:27
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/$k);};
?>
admin at mattwilko dot com
07-Apr-2005 09:32
Here's a function that works with negative powers:
<?php
function newpow($base, $power)
{
if ($power < 0) {
$npower = $power - $power - $power;
return 1 / pow($base, $npower);
}
else
{
return pow($base, $power);
}
}
?>
louis [at] mulliemedia.com
31-Dec-2004 08:02
Here's a pow() function that allows negative bases :
<?php
function npow($base, $exp)
{
$result = pow(abs($base), $exp);
if ($exp % 2 !== 0) {
$result = - ($result);
}
return $result;
}
?>
janklopper .AT. gmail dot.com
10-Nov-2004 06:26
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
17-Mar-2004 11:03
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
17-Jul-2003 08:01
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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