ID: 43624
Updated by: [EMAIL PROTECTED]
Reported By: csaba at alum dot mit dot edu
Status: Open
Bug Type: Math related
Operating System: Win XP
PHP Version: 5.2.5
New Comment:
C code:
---
float a = 6.5, b = 1.3;
double c = 6.5, d = 1.3;
puts("Float:");
printf("%f\n", fmod(a, b));
printf("%f\n", a / b);
printf("%f\n", b - fmod(a, b));
puts("Double:");
printf("%G\n", fmod(c, d));
printf("%G\n", c / d);
printf("%G\n", d - fmod(c, d));
---
Result:
Float:
0.000000
5.000000
1.300000
Double:
1.3
5
2.22045E-16
Previous Comments:
------------------------------------------------------------------------
[2007-12-18 14:36:48] csaba at alum dot mit dot edu
Description:
------------
This continues bug report 43572 at
http://bugs.php.net/bug.php?id=43572&edit=2
which discussion is incomplete, yet has been prematurely closed.
> The value that you expect (0) is obtained using float precision.
> (In C) PHP uses double, and with this type fmod() returns 1.3.
So, to be concrete:
$a = 6.5; print gettype($a); // double
$b = 1.3; print gettype($b); // double
$c = $a / $b; print gettype($c); // double
$d = 5.0; print gettype ($d); // double
print $c - $d; // 0
print (gettype($c - $d)); // double
print ($c===$d) ? "same" : "different"; // same
print gettype ($b - fmod($a, $b)); // double
print $b - fmod($a, $b); // 2.22044604925E-16
In other words, PHP's own (double) math shows that 6.5 / 1.3 === 5.0
Regardless of the internal representation and precision of 6.5 and 1.3,
division of the former ($a) by the latter ($b) results in a double value
identical to 5.0.
fMod should return 0 in this situation.
Csaba Gabor from New York
PS. I would prefer that these comments be moved to bug 43572 and that
it be reopened until the discussion is completed, rather than having
multiple threads.
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Edit this bug report at http://bugs.php.net/?id=43624&edit=1
