Req->Doc #53208 [Opn->Asn]: Modulo does not calc right (C99 not mathematical correct)

Edit report at http://bugs.php.net/bug.php?id=53208&edit=1 ID: 53208 Updated by: aharvey@php.net Reported by: fibrefox at dynamicfiles dot de Summary: Modulo does not calc right (C99 not mathematical correct) -Status: Open +Status: Assigned -Type: Feature/Change Request +Type: Documentation Problem -Package: Math related +Package: Documentation problem Operating System: any PHP Version: Irrelevant -Assigned To: +Assigned To: aharvey Block user comment: N New Comment: Agreed; this is a documentation issue, since there's no way this is going to be changed at this point in PHP's life. There's already a minimal note about this on the arithmetic operators page; I'll expand it out to note that this is the same as the C99 implementation of %. Previous Comments: ------------------------------------------------------------------------ [2010-10-31 00:05:24] rasmus@php.net Note also that the example you gave of a BC break was a very minor one. It was a BC break for people who had written code specifically for PHP 5.3.x using an old- style constructor. The number of people who would write 5.3-specific code with a PHP4-style constructor is tiny. It may in fact be the case that nobody had written code like that. That's why we deemed it ok to break this to fix a mistake. Changing the modulus operator is a completely different thing as it would touch every piece of PHP code ever written that uses modulus. ------------------------------------------------------------------------ [2010-10-30 23:57:49] cataphract@php.net PHP doesn't have a formal specification; so the "correct" behavior is an fluid concept. What the documentation says has some weight, but it's not normative. Definitions can be more or less consistent, but they are arbitrary. Considering only the case when the second operand n is positive, it is indeed more common in mathematics to define the modulus so it's a number between 0 and n - 1 (and that's indeed the behavior of e.g. gmp_mod). In this case, it would certainly break a lot applications to make the change you propose and the benefits just aren't compelling enough, especially given that several other programming languages define the modulus operator in the same fashion. ------------------------------------------------------------------------ [2010-10-30 22:40:11] fibrefox at dynamicfiles dot de I see the problem in here, but as i see on the website there are changes anyway in meanings: "Backwards incompatible change: Methods with the same name as the last element of a namespaced class name will no longer be treated as constructor." The term "modulus" which the symbol "%" stands for is just wrong here. To gain the compatibility the documentation should be remade ... or the "real-modulus" has to be added in the core by offering a new function, maybe "euclid_mod" or something like that. It is really important to NOT make programming in some kind of struggeling through the documentation or the code to find the problem, as you mentioned, of such a basic operator. I really think there a lot of coders are wondering about this flaw in PHP (but it is a sign of C99) and are trying to make workarounds to "fix" this. Built-in functions should be a little bit faster in here, so there also will be some kind of performance-boost here. Anyway, i think the operator should do what he says he would do ... producing the modulus, and not the remainder. ------------------------------------------------------------------------ [2010-10-30 21:27:33] cataphract@php.net You are of course right, and this merits a mention in the documentation, but changing the meaning of such a basic operator is a huge BC break. ------------------------------------------------------------------------ [2010-10-30 19:45:59] fibrefox at dynamicfiles dot de Description: ------------ I found a bug-report that already noted it, but since this is VERY old, i think it should be re-opened or re-mentioned: http://bugs.php.net/bug.php?id=20365 Modulo operator DOES NOT calc right. (-4) mod 10 should be 6, thats the mathematical definition. modulo is NOT the distance to zero. modulo is NOT the remainder while division. the problem is, that the underlying code is written in c/c++, but c99 does NOT have a real modulus, its a remainder-function. here is a site which describes it: http://www.alecjacobson.com/weblog/?p=1140 Test script: --------------- <?php // euclidean result: 6; php result: -4 .... due to the bug in c++ echo (-4)%10; ?> Expected result: ---------------- 6 Actual result: -------------- -1 ------------------------------------------------------------------------ -- Edit this bug report at http://bugs.php.net/bug.php?id=53208&edit=1