pow

(PHP 4, PHP 5)

pow — Exponential expression

Descrierea

number pow ( number $base , number $exp )

Returns base raised to the power of exp .

Avertizare

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

Parametri

base: The base to use
exp: The exponent

Valorile întroarse

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.

Istoria schimbărilor

Versiunea	Descriere
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.

Exemple

Example #1 Some examples of pow()


<?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
?>

Vedeţi de asemenea

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

rad2deg

Last updated: Fri, 06 Nov 2009

add a note User Contributed Notes
pow

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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rad2deg

Last updated: Fri, 06 Nov 2009