pi
Many notations use "^" as a power operator, but in PHP (and other C-based languages) that is actually the XOR operator. You need to use this 'pow' function, there is no power operator.
i.e. 3^2 means "3 XOR 2" not "3 squared".
It is particular confusing as when doing Pythagoras theorem in a 'closet points' algorithm using "^" you get results that look vaguely correct but with an error.
pow
(PHP 4, PHP 5)
pow — 거듭제곱 표현
인수
- base
-
사용할 밑
- exp
-
승수
반환값
base의 exp승. 결과가 정수로 표현될 수 있으면 integer형으로 반환하고, 그렇지 않으면 float형으로 반환합니다. 거듭제곱을 계산할 수 없으면 FALSE를 반환합니다.
변경점
| 버전 | 설명 |
|---|---|
| 4.0.6부터 | 가능하면 결과를 interger로 반환합니다. 이전에는 항상 float로 결과를 반환했습니다. 오래된 버전에서는 복잡한 수에 대해서 이상한 결과를 얻을 수도 있습니다. |
| 4.2.0부터 | PHP가 값을 계산할 수 없을 때, 경고를 발생하지 않습니다. 조용히 FALSE만 반환합니다. |
예제
Example #1 pow()의 몇몇 예제
<?php
var_dump(pow(2, 8)); // int(256)
echo pow(-1, 20); // 1
echo pow(0, 0); // 1
echo pow(-1, 5.5); // PHP >4.0.6 NAN
echo pow(-1, 5.5); // PHP <=4.0.6 1.#IND
?>
add a note
User Contributed Notes
pow
chris at ocportal dot com
21-May-2012 10:05
tagg_maiwald at yahoo dot com
02-Nov-2010 02:34
This function returns the value of a positive base with a signed floating point exponent:
function sf_exp( $fl_x = 1, $fl_y = 0)
{ $fl_exp = 0.0;
if (0 > $fl_x)
{ // Alter this logic container to enable processing of negative bases.
$fl_exp = -1.0;
} else
{ $bool_neg = (0 > $fl_y);
if ($bool_neg)
{ $fl_y = 0 - $fl_y;
}
$fl_xlog = log10( $fl_x);
$fl_xylog = ( $fl_xlog * $fl_y);
$fl_exp = pow( 10, $fl_xylog);
if ($bool_neg)
{ $fl_exp = 1/$fl_exp;
}
}
return $fl_exp;
}
Anonymous
25-Apr-2010 08:24
You can increase the 'precision' php.ini setting a little to work with larger float numbers here, but this comes at at cost of sacrificing decimal accuracy. The default 'precision' is 14. 5 is about the threshhold that php can handle for decimal accuracy before at least some number corruption starts showing or it cannot output the actual number, and 16 for large number accuracy, as demonstrated by throwing this into the table below:
<?php
echo "<td>".pow(10, $i) - 1)."</td>";
?>
See the table below for an example, and adjust your php.ini 'precision' setting according to what your OS and PHP version can handle and what number size you want to work with. Alternatively, you can use the bc math functions for more accuracy all around, and not have to rely on the 'precision' ini setting at all, but this moves out of the realm of strictly floats and into strings.
Also, PHP just prefers to display the 'E' notation of float values where possible after about 5 decimal places rather than the actual decimal number (1.0E-5 vs 0.00005).
<?php
echo "<table>";
for($i = 0; $i < 50; $i++) {
$precision = $i + 1;
ini_set('precision', $precision);
echo "<tr>";
echo "<td>".$precision."</td>";
echo "<td>".pow(10, $i)."</td>";
echo "<td>".pow(10, (-1 * $i))."</td>";
echo "<td>".bcpow('10', (string) $i, $precision)."</td>";
echo "<td>".bcpow('10', (string) (-1 * $i), $precision)."</td>";
echo "</tr>";
}
echo "</table>";
?>
Bottom line though is, if you're working with larger numbers or require very fine decimal precision or prefer displaying the full decimal number, use the bc math functions instead. And, do check out PHP's considerations about the float type here: http://php.net/manual/en/language.types.float.php
Matt Dudley
16-Jul-2008 09:14
Calculate wind chill based on the National Weather Service formula.
$temp = 25;
$wind_speed_mph = 6;
$wind_chill = 35.74+(.6215*$temp_f)-(35.75*(pow($wind_speed_mph, 0.16)))+(.4275*$temp_f*(pow($wind_speed_mph, 0.16)));
Value only valid when the temp is 45 or below.... I used this with a weather script I wrote that reads an xml file. They don't provide wind chill.
Docey
04-May-2007 04:33
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+.
gilthansREMOVEME at gmail dot com
15-Dec-2006 12:50
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
09-May-2006 07:27
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/$k);};
?>
louis [at] mulliemedia.com
31-Dec-2004 01:02
Here's a pow() function that allows negative bases :
<?php
function npow($base, $exp)
{
$result = pow(abs($base), $exp);
if ($exp % 2 !== 0) {
$result = - ($result);
}
return $result;
}
?>
janklopper .AT. gmail dot.com
09-Nov-2004 11:26
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 04:03
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 02:01
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.