ID: 25328
User updated by: hakon at haugnes dot name
Reported By: hakon at haugnes dot name
Status: Open
Bug Type: *Math Functions
Operating System: Redhat 8.0
PHP Version: 4.3.2
New Comment:
I just realized that the [mal]functioning of % for large numbers is
probably why the function BCMOD has been created...
Previous Comments:
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[2003-08-30 18:39:00] hakon at haugnes dot name
Description:
------------
The modulus operator % returns wrong results if the number gets large.
In a program used to calculate primes, it will find that large numbers,
ending in 5, are prime. The following are calculations using % on such
a large number, note that all numbers used in the MOD calculation are
whole integers (and should not suffer rounding by MOD):
MOD: 53253252355 % 3=1
DIV: 53253252355 / 3=17751084118.333332061767578125
MOD: 53253252355 % 4=3
DIV: 53253252355 / 4=13313313088.75
MOD: 53253252355 % 5=2
DIV: 53253252355 / 5=10650650471
Obviously, any number ending in 5 cannot be prime, since it will always
be divisible by 5. The MOD result on 5 is therefore wrong, as the
division proves.
Reproduce code:
---------------
$i=53253252355;
ini_set("precision",35);
for ($m=2; $m<=$i; $m++){
echo "MOD: $i % $m=". $i % $m ;
echo "DIV: $i / $m=". $i/$m ."<Br>";
}
Expected result:
----------------
I would expect to see
MOD: 53253252355 % 3=1
DIV: 53253252355 / 3=17751084118.333332061767578125
MOD: 53253252355 % 4=3
DIV: 53253252355 / 4=13313313088.75
MOD: 53253252355 % 5=0
DIV: 53253252355 / 5=10650650471
Actual result:
--------------
However, this is produced: ([THE ERROR] added for your convenience)
MOD: 53253252355 % 3=1
DIV: 53253252355 / 3=17751084118.333332061767578125
MOD: 53253252355 % 4=3
DIV: 53253252355 / 4=13313313088.75
MOD: 53253252355 % 5=2 [THE ERROR]
DIV: 53253252355 / 5=10650650471
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Edit this bug report at http://bugs.php.net/?id=25328&edit=1
