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pow

(PHP 4, PHP 5)

powExponential expression

Описание

number pow ( number $base , number $exp )

Returns base raised to the power of exp .

Внимание

In PHP 4.0.6 and earlier pow() always returned a float, and did not issue warnings.

Список параметров

base

The base to use

exp

The exponent

Возвращаемые значения

base raised to the power of exp . If the result can be represented as integer it will be returned as type integer, else it will be returned as type float. If the power cannot be computed FALSE will be returned instead.

Список изменений

Версия Описание
Since 4.0.6 The function will now return integer results if possible, before this it always returned a float result. For older versions, you may receive a bogus result for complex numbers.
Since 4.2.0 PHP stops to emit a warning if the value can't be computed, it will now silently return FALSE only.

Примеры

Пример #1 Some examples of pow()

<?php

var_dump
(pow(28)); // int(256)
echo pow(-120); // 1
echo pow(00); // 1

echo pow(-15.5); // PHP >4.0.6  NAN
echo pow(-15.5); // PHP <=4.0.6 1.#IND
?>

Смотрите также

  • exp() - Calculates the exponent of e
  • sqrt() - Square root
  • bcpow() - Raise an arbitrary precision number to another
  • gmp_pow() - Raise number into power


User Contributed Notes
pow
Anonymous
25-Apr-2010 09:24
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: https://doc0.ru/phpe/language.types.float.php
Matt Dudley
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
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
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
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
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
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
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
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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