pi
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
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。可能な場合、この関数は、integer 型の値を 返します。累乗が計算できない場合は FALSE を返します。
変更履歴
| バージョン | 説明 |
|---|---|
| 4.0.6 以降 | 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。 |
| 4.2.0 以降 | 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、 単に FALSE を返すだけとなりました。 |
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
?>
add a note
User Contributed Notespow
Anonymous
25-Apr-2010 09:24
25-Apr-2010 09:24
Matt Dudley
16-Jul-2008 10:14
16-Jul-2008 10:14
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 05:33
04-May-2007 05:33
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 03:50
15-Dec-2006 03:50
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 08:27
10-May-2006 08:27
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/$k);};
?>
louis [at] mulliemedia.com
31-Dec-2004 04:02
31-Dec-2004 04:02
Here's a pow() function that allows negative bases :
<?php
function npow($base, $exp)
{
$result = pow(abs($base), $exp);
if ($exp % 2 !== 0) {
$result = - ($result);
}
return $result;
}
?>
janklopper .AT. gmail dot.com
10-Nov-2004 02:26
10-Nov-2004 02:26
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 07:03
18-Mar-2004 07:03
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 03:01
18-Jul-2003 03:01
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.