算术运算符

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bxm AT cs d0t nott d0t ac d0t uk
06-Jun-2006 07:06


An implementation of the classical modulus operator (as in mathematics or Java). An alternative to the one given below, which was more elegant. This implementation maybe more efficient?



// Computes...

// Mathematics: $a (mod $b)

// Java: $a % $b

function modulus( $a, $b )

{

    if( $a < 0 )

    {

        return $b - ( abs( $a ) % $b );

    }

    else

    {

        return $a % $b;

    }

}

andr3a at 3site dot it
05-Jun-2006 08:14


[ops ... wrong example :D]

just a note about module, it works only with integers.



<?php

$a = pow(2, 31);

$b = ($a / 2) - 1;

echo implode('<br />', array(

    $a + $b, // OK => 3221225472

    $a - $b, // OK => 1073741825

    $a * $b, // OK => 2.3058430070662E+018

    $a / $b, // OK => 2.0000000018626

    $a % $b  // WTF ? => -2

));

?>



maybe bc_mod with a string cast should be a solution



bc_mod((string)pow(2,31), (pow(2,31)/2)-1)

shmee35 at hotmail dot com
15-Dec-2005 07:00


Note that constructs which evaluate to 0 (such as empty strings, false, or an explicit NULL) will be treated as 0 in a arithmetic operation. Therefore:

<?php

echo 5*"", '<br>';

echo 5+false, '<br>';

echo 5/NULL, '<br>';

?>

Will produce:

0

5

Warning: Division by zero

pww8 at cornell dot edu
18-Aug-2005 05:25


It appears floating-point infinity (INF) is not returned from divide by zero (in PHP 5.0.0).  Instead a warning is given and Boolean FALSE is returned.



I searched the various manuals and did not find relevant explanation, so am adding this.

Gemini6Ice
13-Aug-2005 06:59


For integer division, just use floor($a / $b).

rz at daimi dot au dot dk
05-Apr-2005 08:04


This is a really simple observation.

There is no integer division operator. But if you need to divide by a power of 2, you can use the right-shift operator instead. 257/32 gives 8.03125, but 257>>5 gives 8.

areshankar at gmail dot com
17-Mar-2005 06:40


Most of the mail services/bulletin boards display the time when the message was posted or the mail was received in the  form  - 3 days ago, 15 mins ago etc.



Here is  a nifty little PHP function which does the same by basically  combining the usage of "/" and "%" operators.



You need to pass the unix timestamp of the time when the message was posted to the function.



function intervalCalc($postedtime)

{

    $now = time();

    $interval_secs = $now  - $postedtime;

    

    if ($interval_secs > 86400)

    { 

        $interval = (($interval_secs - ($interval_secs%86400))/86400);

        

        $interval.= " days ago";

    }

    elseif ($interval_secs > 3600)

    {

        $interval = ($interval_secs - ($interval_secs%3600)/3600);

        

        $interval.= " hours ago";

    }

    elseif ($interval_secs > 60)

    {

        $interval = ($interval_secs - ($interval_secs%60)/60);

        

        $interval.= " minutes ago";

    }

    

    return  $interval;

    

}

sputnik13 at gmail dot com
14-Feb-2005 10:49


I just had to make a comment about gilthans' use of the bitwise xor operator (^)...  I'm guessing from the line if(($x < 0 ^ $y < 0) && $x != 0) that he wants to verify that only x or only y is less than 0 AND x != 0...  Bitwise operators should not be used for truth values like this, that's what xor is for.

justin at koivi dot com
26-Jan-2005 05:08


A note about the documentation on this page that I found while teaching a PHP class:



The above statment: "The division operator ("/") returns a float value anytime, even if the two operands are integers (or strings that get converted to integers)." is NOT TRUE.



Take the following:

<?php

$a=5;

$b=10;

$c=$b/$a;

$d=10/5;

var_dump($c);

var_dump($d);

var_dump($b/$a);

var_dump(10/5);

?>

All 4 instances will print "int(2)"



Therefore, the division operator "/" returns an integer value if the numbers are evenly divisible AND they are both integers (or strings that have been converted to integer values).

no at email dot com
14-Jan-2005 05:08


If you need "div" function like in pascal ( 123 div 10 == 12) you can try this:

$number "div" 10 = ($number-($number % 10)) / 10;

jphansen at uga dot edu
07-Jan-2005 02:12


If the second operand of modulus is of a precision type, modulus will return a boolean instead of a numeric type.

gilthans at gmail dot com
21-Dec-2004 05:13


Me and a few friends were talking about how math works in coding languages, and after a while of discussion, we started wondering how does the sqrt() function works, and how does the division work, using only addition, substraction and multiplication (because that could be easily done using only addition).

I have best knowledge in PHP, so I had my shot at division, and a while of testing proved that my code works. I'm posting this here for the curious, even though the normal PHP division works 7 times faster, so I think there is some way of making my code more efficient.

divide($x, $y, $a, 0) is similar to number_format($x/$y, $a);, and divide($x, $y, $a, 1) is similar to Array(floor($x/$y), $x%$y).

<?php

function divide($x, $y, $a=6, $z=false){

    // returns $x/$y

    $t = '';

    if($y == 0)

    return 0;

    if(($x < 0 ^ $y < 0) && $x != 0)

    $t = '-';

    $x = abs($x);

    $y = abs($y);

    $in = 0;

    while(($in+1)*$y <= $x)

        $in++;

    $r = $x-($in*$y);

    if($z)

    return Array($in, $r);

    $d = '';

    $c = 0;

    while($r != 0 && $c < $a){

    list($u, $r) = divide($r*10, $y, $a, 1);

    $c++;

    if($c >= $a){

        $b = divide($r*10, $y, $a, 1);

        $u += ($b[0] > 4) ? 1 : 0;

    }

    $d .= $u;

    }

    return intval($t.$in.(($d != '') ? ".".$d : ""));

}

?>

adam.pippin at accesscomm dot ca
03-Dec-2004 05:15


To factor a number:



<?php



function factornumber($number)

{

    for ($i=1; $i<=($number/2); $i++)

    {

        if ($number % $i == 0)

        {

            echo "$i * ".($number/$i)." = $number\n";

        }

    }

}



factornumber(50);



?>



That script outputs:



1 * 50 = 50

2 * 25 = 50

5 * 10 = 50

10 * 5 = 50

25 * 2 = 50



As you may notice, it makes no attempts at removing duplicates (eg: 5*10 and 10*5), although it would be trivial to make it do so.

glenn at benge dot co dot nz
06-Oct-2004 02:28


a real simple method to reset an integer to a the next lowest multiple of a divisor



$startSeq = $startSeq - ($startSeq % $entriesPerPage);



if $startSeq was already a multiple, then " $startSeq % $entriesPerPage " will return 0 and $startSeq will not change.

arjini at gmail dot com
09-Sep-2004 12:48


When dealing purely with HTML, especially tables, or other things in "grids"  the modulous operator is really useful for splitting up the data with a seperator.



This snippet reads any gif files from the directory the script is in, prints them out and puts in a break every 5th image.



<?php

    $d = dir('./');

    $i = 0;

    while(false !== ($e = $d->read())){

        if(strpos($e,'.gif')){

            ++$i;

            echo '<img src="'.$e.'"/>'.chr(10);

            if(!($i%5))

                echo '<br/>';

        }

    }

?>



For tables just put </tr><tr> in place of the break.

csaba at alum dot mit dot edu
16-May-2004 08:32


The function below is not a replacement for http://php.net/fmod  Rather, it is the classic (mathematician's) modulo where the resultant value is always non-negative, in the range [0,$base).  For example, mod(-9,7) => 5.  When both arguments are integers, you get the classic notion of the % operator.



function mod($num, $base) {

    return ($num - $base*floor($num/$base)); }



Csaba Gabor

info at sima-pc dot com
02-May-2004 05:48


Note that operator % (modulus) works just with integers (between -214748348 and 2147483647) while fmod() works with short and large numbers.



Modulus with non integer numbers will give unpredictable results.

php AT internethoofdkantoor.nl
13-Jan-2004 08:15


For the mathematicians among us:

The mod operator % works a bit differently than in mathematics (e.g. in group theory), in that negative remainders aren't represented by their positive equivalence classes, as is common in mathematics.



An example:

-1%7 returns -1, instead of 6.



it depends a bit on how you interpret 'remainder of a division', i.e. if you want the remainder to be always positive.

soren at byu dot edu
20-Nov-2003 11:49


Exponentiation doesn't use ^ or ** as you might be used to with other languages.  To calculate "z equals y to the x" use:



$z = pow(y,x)

darnell at uga dot edu
29-Oct-2003 11:57


Also, when automatically converting floats to integers, PHP truncates them rather than rounding.  So in the previous note, 2.4 gets truncated to 2 before the modulus operator is applied.

todd at magnifisites dot com
17-Oct-2003 07:22


There isn't much detail in the PHP Manual regarding the PHP modulus operator (%).  It seems as though the PHP modulus operator (%) works on integer operands. If PHP operates like Perl, then the modulus operator (%) converts its operands to integers before finding the remainder according to integer division.  Testing seems to prove the theory:



<?php

$dividend = 7;

$divisor = 2.4;

print $dividend%$divisor; // prints 1

print fmod($dividend, $divisor); // prints 2.2

?>

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