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
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Edit this bug report at http://bugs.php.net/?id=22527&edit=1
