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 — 指数表現
パラメータ
-
base -
使用する基数。
-
exp -
指数。
返り値
base の exp 乗を返します。
両方の引数が非負の整数で、かつ結果が integer 型の範囲に収まる場合は、結果を integer 型で返します。
それ以外の場合は結果を float 型で返します。
変更履歴
| バージョン | 説明 |
|---|---|
| 4.2.0 | PHP 値が計算できない場合に警告を発生することはなくなりました。 |
| 4.0.6 | 可能な場合は、結果を integer で返すようになりました。 以前は、結果を常に float で返していました。 そのため、値によっては間違った結果となることがありました。 |
例
例1 pow() の例
<?php
var_dump(pow(2, 8)); // int(256)
echo pow(-1, 20); // 1
echo pow(0, 0); // 1
echo pow(-1, 5.5); // エラー
?>
注意
注意:
この関数はすべての入力を (スカラー値以外でも) 数値に変換します。 その結果、予想外の結果を引き起こすことがあります。
add a note
User Contributed Notes
pow
chris at ocportal dot com
21-May-2012 06:05
tagg_maiwald at yahoo dot com
02-Nov-2010 09:34
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 04:24
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 05:14
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 12:33
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 08:50
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 03:27
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/$k);};
?>
louis [at] mulliemedia.com
31-Dec-2004 09:02
Here's a pow() function that allows negative bases :
<?php
function npow($base, $exp)
{
$result = pow(abs($base), $exp);
if ($exp % 2 !== 0) {
$result = - ($result);
}
return $result;
}
?>
janklopper .AT. gmail dot.com
10-Nov-2004 07:26
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 12:03
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 10:01
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.