http://www.perlmonks.org?node_id=435792

Baratski has asked for the wisdom of the Perl Monks concerning the following question:

Hello.

Is there a better way, other than my following example, to find if a number is a multiple of a number?

My klunky way would be to keep subtracting x from y and look for a zero value. This seems like a long haul if I have to keep subtracting say, 4 from 123456789, or something like that. 

$x = 4;
$y = 123456789;

while($y > 0)
{

$y -= $x;

if($y == 0)
{
#.....
}

}

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Is there a better way?

Thank you.

Replies are listed 'Best First'.
Re: Find if a number is a multiple of a number.
by gellyfish (Monsignor) on Mar 02, 2005 at 12:36 UTC

    You can use the modulus operator : 

    if ( ! ($y % $x) )
{
   # it is an exact multiple
}

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    /J\

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Re: Find if a number is a multiple of a number.
by borisz (Canon) on Mar 02, 2005 at 12:41 UTC
    use % for positive integer values. 
    $x = 4;
$y = 12345679;
if ( $y % $x == 0 ) {
 ...
}

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    Boris
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      Reply to all. Sweeeet! Works great! And much, much faster than my code. Thanks!
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      use % for positive integer values.
      What does make you think that the use % of is limited to positive integers? 
      $ perl -le 'print for map $_%-3, -18..-15'
0
-2
-1
0

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        The docs: from perlop 
        Binary "%" computes the modulus of two numbers.Given integer operands
$a and $b: If $b is positive, then "$a % $b" is $a minus the largest
multiple of $b that is not greater than $a.  If $b is negative then
"$a % $b" is $a minus the smallest multiple of $b that is not less tha
+n
$a (i.e. the result will be less than or equal to zero).  Note that
when "use integer" is in scope "%" gives you direct access to the mod-
ulus operator as implemented by your C compiler.  This operator is not
as well defined for negative operands, but it will execute faster.

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        Boris
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Re: Find if a number is a multiple of a number.
by Anonymous Monk on Mar 02, 2005 at 12:36 UTC
    Use the modulus operator: %. For details, see the perlop manual page.
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Re: Find if a number is a multiple of a number.
by g0n (Priest) on Mar 02, 2005 at 12:36 UTC
    Could you be looking for the modulus operator '%'?

    perl -e 'print 123456789%4'

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    VGhpcyBtZXNzYWdlIGludGVudGlvbmFsbHkgcG9pbnRsZXNz
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Re: Find if a number is a multiple of a number.
by ambrus (Abbot) on Mar 02, 2005 at 13:06 UTC
    Use 
    sub divides1 { 
    my($d, $n) = @_; 
    $d < $n ? 
        $d & 1 ? 
            $n & 1 ? divides($d, $n - $d) : divides($d, $n >> 1) : 
            $n & 1 ? 0 : divides($d >> 1, $n >> 1) :
        $d == $n; 
} 
sub divides { 
    0 == $_[1] || divides1(abs $_[0], abs $_[1]) 
}

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    or simply 
    sub divides { 
    0 == $_[0] ? 0 == $_[1] : 0 == $_[1] % $_[0] 
}

    [download]

    Update: fixed newline in divides1 subroutine.

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