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Groups > comp.lang.c > #148003

Re: OT: improve this simple algorithm?

From James Kuyper <jameskuyper@alumni.caltech.edu>
Newsgroups comp.lang.c
Subject Re: OT: improve this simple algorithm?
Date 2019-11-14 15:25 -0500
Organization A noiseless patient Spider
Message-ID <qqkd7k$tse$1@dont-email.me> (permalink)
References (2 earlier) <_85zF.5075$II7.2317@fx17.iad> <qqj564$2m4$1@dont-email.me> <fxgzF.21520$1S4.4613@fx01.iad> <qqk7lv$op4$1@dont-email.me> <P5izF.26147$aI.67@fx44.iad>

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On 11/14/19 2:42 PM, DFS wrote:
...
> The compiler flags certain data type comparisons, but not invalid 
> variable assignments that violate the range of the data type.
> 
> for(unsigned short i = -1; i < (sizeof(seq) / sizeof(seq[0])) - 1; i++)

That's because the range of the data type has not been violated. The
behavior of that code is well-defined: -1 gets implicitly converted to
unsigned short; the result of that conversion is USHRT_MAX. Since
USHRT_MAX is greater than sizeof seq/sizeof seq[0], the for condition
fails the very first time it is tested, and the loop never executes.

Unsigned types are a special case:
"A computation involving unsigned operands can never overflow, because a
result that cannot be represented by the resulting unsigned integer type
is reduced modulo the number that is one greater than the largest value
that can be represented by the resulting type" (6.2.5p9).

In general, out-of-range values cause problems, but they are never
required to result in a compile-time diagnostic:

Conversion to a signed integer type of an integer value that cannot be
represented in that type: "either the result is implementation-defined
or an implementation-defined signal is raised." (6.3.1.3p3).

Conversion of a real floating value to an integer type other than _Bool
that cannot represent the integer part of that value: "the behavior is
undefined." (6.3.1.4p1).

Conversion to a floating point type of an integer or floating real value
that is outside the range that can be represented by that type: "the
behavior is undefined" (6.3.1.4p2, 6.3.1.5p1).

If the implementation-defined result of converting a pointer to an
integer type cannot be represented in that type, "the behavior is
undefined" (6.3.2.3p6).

"If an integer constant cannot be represented by any type in
its list" (see 6.4.4.1p5) "and has no extended integer type, then the
integer constant has no type." (6.4.4.1p6).

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OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-12 15:55 -0500
  Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-12 21:20 +0000
    Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-12 23:28 -0500
      Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-13 10:52 +0100
        Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-13 10:16 -0500
          Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-13 16:53 +0100
      Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-13 11:18 +0000
  Re: OT: improve this simple algorithm? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-12 23:30 +0000
    Re: OT: improve this simple algorithm? Ian Collins <ian-news@hotmail.com> - 2019-11-15 05:12 +1300
      Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 16:34 +0000
        Re: OT: improve this simple algorithm? Ian Collins <ian-news@hotmail.com> - 2019-11-15 10:07 +1300
          Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 22:25 +0000
            Re: OT: improve this simple algorithm? Ian Collins <ian-news@hotmail.com> - 2019-11-15 12:02 +1300
              Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 23:27 +0000
                Re: OT: improve this simple algorithm? Ian Collins <ian-news@hotmail.com> - 2019-11-15 19:06 +1300
                Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-15 10:45 +0000
                Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-15 13:28 +0100
                Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-15 12:58 +0000
                Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-15 15:39 +0100
                Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-15 15:04 +0000
                Re: OT: improve this simple algorithm? Richard Damon <Richard@Damon-Family.org> - 2019-11-15 12:19 -0500
                Re: OT: improve this simple algorithm? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-11-15 09:46 -0800
                Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-15 18:59 +0000
                Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-16 11:59 +0000
                Re: OT: improve this simple algorithm? Ian Collins <ian-news@hotmail.com> - 2019-11-16 09:02 +1300
              Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-15 11:17 +0100
                Re: OT: improve this simple algorithm? Jorgen Grahn <grahn+nntp@snipabacken.se> - 2019-11-15 21:38 +0000
    Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-14 11:44 -0500
      Re: OT: improve this simple algorithm? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-14 18:11 +0000
        Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-14 13:33 -0500
          Re: OT: improve this simple algorithm? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-15 01:49 +0000
      Re: OT: improve this simple algorithm? Barry Schwarz <schwarzb@delq.com> - 2019-11-14 10:38 -0800
    Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-14 12:56 -0500
      Re: OT: improve this simple algorithm? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-14 13:50 -0500
        Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-14 14:42 -0500
          Re: OT: improve this simple algorithm? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-14 15:25 -0500
      Re: OT: improve this simple algorithm? Barry Schwarz <schwarzb@delq.com> - 2019-11-14 11:03 -0800
        Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-14 16:08 -0500
          Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-15 11:30 +0100
            Re: OT: improve this simple algorithm? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-15 11:17 +0000
              Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-15 17:35 -0500
                Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-15 17:39 -0500
                Re: OT: improve this simple algorithm? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-15 23:19 +0000
                Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-15 18:53 -0500
                Re: OT: improve this simple algorithm? Richard Damon <Richard@Damon-Family.org> - 2019-11-15 19:08 -0500
                Re: OT: improve this simple algorithm? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-16 01:30 +0000
                Re: OT: improve this simple algorithm? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-16 07:39 -0500
                Re: OT: improve this simple algorithm? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-16 07:37 -0500
          Re: OT: improve this simple algorithm? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-15 09:46 -0500
            Re: OT: improve this simple algorithm? Richard Damon <Richard@Damon-Family.org> - 2019-11-15 12:38 -0500
              Re: OT: improve this simple algorithm? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-11-15 23:07 +0000
              Re: OT: improve this simple algorithm? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-28 16:54 -0800
                Re: OT: improve this simple algorithm? Richard Damon <Richard@Damon-Family.org> - 2019-11-28 20:17 -0500
                Re: OT: improve this simple algorithm? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-29 06:08 -0800
      Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 20:45 +0100
    Re: OT: improve this simple algorithm? Keith Thompson <kst-u@mib.org> - 2019-11-14 10:10 -0800
      Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 20:19 +0100
    Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-12 23:57 -0500
      Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-13 10:34 +0100
    Re: OT: improve this simple algorithm? DFS <nospam@dfs.com> - 2019-11-13 23:57 -0500
      Re: OT: improve this simple algorithm? Barry Schwarz <schwarzb@delq.com> - 2019-11-13 23:00 -0800
      Re: OT: improve this simple algorithm? Ike Naar <ike@sdf.lonestar.org> - 2019-11-14 07:53 +0000
        Re: OT: improve this simple algorithm? guinness.tony@gmail.com - 2019-11-14 02:34 -0800
      Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 10:01 +0100
        Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 11:27 +0000
          Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 12:46 +0100
            Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 12:01 +0000
              Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 14:05 +0100
  Re: OT: improve this simple algorithm? Barry Schwarz <schwarzb@delq.com> - 2019-11-12 18:34 -0800
    Re: OT: improve this simple algorithm? Keith Thompson <kst-u@mib.org> - 2019-11-12 19:01 -0800
      Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 17:08 +0100
        Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 17:06 +0000
        Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 19:11 +0000
      Re: OT: improve this simple algorithm? Ian Collins <ian-news@hotmail.com> - 2019-11-15 05:15 +1300
      Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 18:13 +0000
        Re: OT: improve this simple algorithm? Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 20:16 +0100
          Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 20:03 +0000
            Re: OT: improve this simple algorithm? Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-15 04:51 +0100
      Re: OT: improve this simple algorithm? Keith Thompson <kst-u@mib.org> - 2019-11-14 10:15 -0800
        Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 20:52 +0100
      Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-13 10:57 +0100
        Re: OT: improve this simple algorithm? Ian Collins <ian-news@hotmail.com> - 2019-11-13 23:23 +1300
        Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-13 11:53 +0000
          Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-13 14:02 +0100
            Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-13 13:40 +0000
              Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-13 17:01 +0100
                Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-13 16:28 +0000
                Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-13 22:49 +0100
                Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-13 22:38 +0000
                Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-13 23:06 +0000
                Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 10:10 +0100
                Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 10:42 +0000
                Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 12:53 +0100
                Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 12:28 +0000
                Re: OT: improve this simple algorithm? David Brown <david.brown@hesbynett.no> - 2019-11-14 14:21 +0100
                Re: OT: improve this simple algorithm? Bart <bc@freeuk.com> - 2019-11-14 14:28 +0000
                Re: OT: improve this simple algorithm? Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 15:38 +0100
                Re: OT: improve this simple algorithm? Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 13:37 +0100
                Re: OT: improve this simple algorithm? Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 13:38 +0100
          Re: OT: improve this simple algorithm? Thiago Adams <thiago.adams@gmail.com> - 2019-11-13 05:21 -0800

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