Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #15992

Re: C arithmetic question.

Show all headers | View raw

On 2012-01-20, Michael Press <rubrum@pacbell.net> wrote:
> In article <slrn3vfsjhflpb.8kc.ike@sverige.freeshell.org>,
>  Ike Naar <ike@sverige.freeshell.org> wrote:
>
>> > x = y (mod m) if and only if x - y  is divisible by m.
>> 
>> The mod operator does not violate that law.
>> Your problem is not with the mod operator, it's
>> that the numbers that are given to the mod operator
>> are not what you think they are.
>
> You consider that more important than
> the mod operator being consistent. I do not.

what do you want % to do?

3 % 4 == 3      /* everyone agrees, hopefully */
3 % -4 == ?
-3 % 4 == ?
-3 % -4 == ?

Common Lisp has two functions: mod and rem. (mod 3 4) and (rem 3 4) is 3.
But:

[1]> (mod -3 -4)
-3
[2]> (mod -3 4)
1
[3]> (mod 3 -4)
-1
[4]> (rem -3 -4)
-3
[5]> (rem -3 4)
-3
[6]> (rem 3 -4)

In some languages, there is a mod which always returns the least positive
residue. (But does that even mean when the modulus is negative?)
Note that the Lisp mod does give you a positive least residue value if the
modulus is positive.

Would you want the C % operator to be like Lisp's mod?

But note that in Lisp, there are two divisions too: mod pairs with floor (which
truncates toward minus infinity), and rem pairs with truncate (which truncates
toward zero).

Just like in C we have "(a/b)*b + a%b shall equal a", in Lisp we have
a similar relationship between floor and mod, and truncate and rem:

  (defun mod-test (a b)
    (values (= a (+ (* b (floor a b)) (mod a b)))
            (= a (+ (* b (truncate a b)) (rem a b)))))

 (mod-test 3 -4) -> T ; T
 (mod-test 3 4) -> T ; T
 (mod-test -3 -4) -> T ; T
 (mod-test 3 4) -> T ; T

As of C99, it appears that C's operators are like truncate and rem.

I think, IMO, that if you're only going to have one kind of truncating division
and one kind of mod, it's best to have a division which truncates toward zero,
and a modulus which is like the Lisp mod: the sign is always positive if the
modulus is positive, giving you a least positive residue.

Does the equality  (a/b)*b + a%b  == a  have any practical value?

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

C arithmetic question. Michael Press <rubrum@pacbell.net> - 2012-01-18 10:32 -0800
  Re: C arithmetic question. James Kuyper <jameskuyper@verizon.net> - 2012-01-18 14:03 -0500
    Re: C arithmetic question. TheGunslinger <mikiesrunsbaal.sec@sbcglobal.net> - 2012-01-18 19:30 -0800
      Re: C arithmetic question. James Kuyper <jameskuyper@verizon.net> - 2012-01-19 08:10 -0500
  Re: C arithmetic question. Markus Wichmann <nullplan@gmx.net> - 2012-01-18 21:14 +0100
    Re: C arithmetic question. "BartC" <bc@freeuk.com> - 2012-01-19 10:44 +0000
      Re: C arithmetic question. Seebs <usenet-nospam@seebs.net> - 2012-01-21 21:32 +0000
  Re: C arithmetic question. Peter Nilsson <airia@acay.com.au> - 2012-01-18 13:51 -0800
    Re: C arithmetic question. Michael Press <rubrum@pacbell.net> - 2012-01-18 15:51 -0800
      Re: C arithmetic question. James Kuyper <jameskuyper@verizon.net> - 2012-01-18 20:50 -0500
        Re: C arithmetic question. Michael Press <rubrum@pacbell.net> - 2012-01-20 12:41 -0800
          Re: C arithmetic question. Keith Thompson <kst-u@mib.org> - 2012-01-20 14:11 -0800
            Re: C arithmetic question. gazelle@shell.xmission.com (Kenny McCormack) - 2012-01-20 22:43 +0000
          Re: C arithmetic question. James Kuyper <jameskuyper@verizon.net> - 2012-01-20 17:19 -0500
            Re: C arithmetic question. James Kuyper <jameskuyper@verizon.net> - 2012-01-20 17:32 -0500
              And jacob navia <jacob@spamsink.net> - 2012-01-20 23:52 +0100
                Re: And Markus Wichmann <nullplan@gmx.net> - 2012-01-21 19:23 +0100
                Re: And Keith Thompson <kst-u@mib.org> - 2012-01-21 13:20 -0800
                Re: And jacob navia <jacob@spamsink.net> - 2012-01-21 23:02 +0100
                Re: And Markus Wichmann <nullplan@gmx.net> - 2012-01-22 15:24 +0100
                Re: And Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-22 15:40 +0000
                Re: And blp@cs.stanford.edu (Ben Pfaff) - 2012-01-21 11:02 -0800
          Re: C arithmetic question. Rui Maciel <rui.maciel@gmail.com> - 2012-01-24 18:53 +0000
      Re: C arithmetic question. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-19 02:09 +0000
      Re: C arithmetic question. Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2012-01-18 23:00 -0700
        Re: C arithmetic question. Michael Press <rubrum@pacbell.net> - 2012-01-18 23:14 -0800
          Re: C arithmetic question. Ike Naar <ike@sverige.freeshell.org> - 2012-01-19 08:41 +0000
            Re: C arithmetic question. Michael Press <rubrum@pacbell.net> - 2012-01-20 12:43 -0800
              Re: C arithmetic question. Kaz Kylheku <kaz@kylheku.com> - 2012-01-20 22:17 +0000
                Re: C arithmetic question. Michael Press <rubrum@pacbell.net> - 2012-01-21 00:12 -0800
              Re: C arithmetic question. Seebs <usenet-nospam@seebs.net> - 2012-01-21 21:32 +0000
                Re: C arithmetic question. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-21 23:21 +0000
                Re: C arithmetic question. Robert Wessel <robertwessel2@yahoo.com> - 2012-01-22 00:48 -0600
                Re: C arithmetic question. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-22 11:45 +0000
                Re: C arithmetic question. Robert Wessel <robertwessel2@yahoo.com> - 2012-01-22 21:46 -0600
                Re: C arithmetic question. Kaz Kylheku <kaz@kylheku.com> - 2012-01-23 04:17 +0000
                Re: C arithmetic question. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-23 11:19 +0000
                Re: C arithmetic question. Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-26 10:41 -0800
                Re: C arithmetic question. Seebs <usenet-nospam@seebs.net> - 2012-01-23 07:58 +0000
                Re: C arithmetic question. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-23 11:06 +0000
                Re: C arithmetic question. James Kuyper <jameskuyper@verizon.net> - 2012-01-23 07:03 -0500
                Re: C arithmetic question. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-23 17:27 +0000
          Re: C arithmetic question. Keith Thompson <kst-u@mib.org> - 2012-01-19 02:38 -0800
            Re: C arithmetic question. Dr Nick <3-nospam@temporary-address.org.uk> - 2012-01-20 07:30 +0000
          Re: C arithmetic question. Nomen Nescio <nobody@dizum.com> - 2012-01-19 12:28 +0100
            Re: C arithmetic question. Michael Press <rubrum@pacbell.net> - 2012-01-20 12:44 -0800
              Re: C arithmetic question. 88888 Dihedral <dihedral88888@googlemail.com> - 2012-01-20 13:14 -0800
          Re: C arithmetic question. James Kuyper <jameskuyper@verizon.net> - 2012-01-19 08:45 -0500
          Re: C arithmetic question. Chad <cdalten@gmail.com> - 2012-01-24 08:14 -0800
            Re: C arithmetic question. James Kuyper <jameskuyper@verizon.net> - 2012-01-24 12:15 -0500
              Re: C arithmetic question. Chad <cdalten@gmail.com> - 2012-01-24 09:26 -0800
                Re: C arithmetic question. James Kuyper <jameskuyper@verizon.net> - 2012-01-24 12:39 -0500
                Re: C arithmetic question. Chad <cdalten@gmail.com> - 2012-01-24 10:03 -0800
                Re: C arithmetic question. James Kuyper <jameskuyper@verizon.net> - 2012-01-24 13:23 -0500
                Re: C arithmetic question. Chad <cdalten@gmail.com> - 2012-01-24 10:40 -0800
                Re: C arithmetic question. James Kuyper <jameskuyper@verizon.net> - 2012-01-24 14:35 -0500
                Re: C arithmetic question. Chad <cdalten@gmail.com> - 2012-01-24 12:04 -0800
                Re: C arithmetic question. Chad <cdalten@gmail.com> - 2012-01-24 12:02 -0800
                Re: C arithmetic question. Kleuske <kleuske@nowhere.net> - 2012-01-24 20:26 +0000
                Re: C arithmetic question. Chad <cdalten@gmail.com> - 2012-01-24 13:45 -0800
                Re: C arithmetic question. James Kuyper <jameskuyper@verizon.net> - 2012-01-24 15:40 -0500
                Re: C arithmetic question. Chad <cdalten@gmail.com> - 2012-01-24 12:09 -0800
      Re: C arithmetic question. Rui Maciel <rui.maciel@gmail.com> - 2012-01-24 18:48 +0000
    Re: C arithmetic question. "osmium" <r124c4u102@comcast.net> - 2012-01-19 08:12 -0600
  Re: C arithmetic question. Geoff <geoff@invalid.invalid> - 2012-01-20 15:33 -0800
    Re: C arithmetic question. Kleuske <kleuske@nowhere.net> - 2012-01-27 08:17 +0000
      Re: C arithmetic question. Geoff <geoff@invalid.invalid> - 2012-01-27 13:37 -0800

csiph-web