Re: #22527 [Opn->Bgs]: Modulus returned negative value

This is actually an interesting question.  Should we be truncating towards
zero?  I'd say yes, but then I tested Perl, Python and Tcl, and they all
say that -27 % 7 is 1 which means they truncate towards negative infinity.

Too late to change at this point in the game, but perhaps it calls for a
modulus() function which leans the other way?

-Rasmus

On Mon, 4 Mar 2003 [email protected] wrote:

>  ID:               22527
>  Updated by:       [email protected]
>  Reported By:      jaypedhskr at aol dot com
> -Status:           Open
> +Status:           Bogus
>  Bug Type:         Math related
>  Operating System: Unix
>  PHP Version:      4.2.3
>  New Comment:
>
> Modulus has never been well-defined for negative values in computer
> languages.  It all comes down to whether the language truncates towards
> zero or towards negative infinity.  Computers have traditionally
> truncated towards zero, whereas mathematicians tend to truncate towards
> negative infinity.  Some languages like Fortran and Ada actually have
> two different modulus operators for this reason.  PHP just has one, and
> it truncates towards zero as has been the traditional and expected
> thing for programming languages to do.
>
> So, given that, let's look at your numbers.  Modulus has to satisfy the
> relation: (a/b)*b + a%b = a
> where a/b is an integer division where our truncation direction comes
> in.
>
>   (-27/7) * 7 + -27%7 = -27
>   ( -3  ) * 7 + -27%7 = -27
>         -21 + -27%7   = -27
>               -27%7   = -27+21
>               -27%7   = -6
>
> In fact, the ISO standard for the C programming language, in which PHP
> is written, defines integer division and modulus operators to perform
> truncation towards 0 and not towards negative infinity.  We don't
> really have a strict language definition for PHP, but if we did, we
> would most likely follow the lead of languages like Fortran, C and C++
> and specify truncation towards zero to be as consistent as possible
> with other languages.
>
>
>
> Previous Comments:
> ------------------------------------------------------------------------
>
> [2003-03-03 22:29:46] jaypedhskr at aol dot com
>
> Hi,
>
> I had the following result from a PHP script, as seen from printing
> output variables:
>
>     -27 % 7 == -6
>
> My understanding of the modulus function is that it returns the
> remainder from division, and that the remainder can never be negative.
>
> I believe that is the standard mathematical definition.
>
> So I think what should have been returned is 1 instead of -6.
>
> That is
>
>   -27 = 7*(-4) + 1
>
> instead of
>
>   -27 = 7*(-3) -6
>
> I got around this by adding the the modulus value if the result was
> less than zero.  But I think a result of less than zero shouldn't
> occur.
>
> Code snippet that ran into this:
>
>       $T5 = $D + $T1 + $Y + $T2 + $T3 - $T4;
>       $weekday = $T5 % 7;
>
> Added code to handle this:
>       if ($weekday < 0) {
>           $weekday += 7;  // -27 % 7 = -6 case
>       }
>
> Specific problem case:
>     $T5 = 3 + 2 + 3 + 5 + 0 - 40; // -27
>     $weekday = %T5 % 7; // -27 % 7 = -6
>
> Thanks!
>
> -Jay Pedersen
>
>
> ------------------------------------------------------------------------
>
>
> --
> Edit this bug report at http://bugs.php.net/?id=22527&edit=1
>