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Re: setting pointers to NULL and testing question.

From David Brown <david.brown@hesbynett.no>
Newsgroups comp.lang.c
Subject Re: setting pointers to NULL and testing question.
Date 2016-06-15 22:31 +0200
Organization A noiseless patient Spider
Message-ID <njsdvg$i86$1@dont-email.me> (permalink)
References (4 earlier) <dsb7ghFocnaU1@mid.individual.net> <7861f1fe-7e13-2e0b-5a0d-6de024da95c2@verizon.net> <22eb816c-faf2-403f-8438-f829caa9e252@googlegroups.com> <njqvp0$5nh$1@dont-email.me> <5761ae6f.6916500@news.xs4all.nl>

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On 15/06/16 21:43, Richard Bos wrote:
> David Brown <david.brown@hesbynett.no> wrote:
>
>> The neatest way would be to simply allow
>>
>>       double dx = monster.x-player.x;
>>       double dy = monster.y-player.y
>>       double monster_distance = sqrt(dx*dx+dy*dy);
>>       ...
>>       double dx = shot.x-player.x;
>>       double dy = shot.y-player.y
>>       double shot_distance = sqrt(dx*dx+dy*dy);
>>
>> and say that a re-definition of "double dx" creates a new variable in a
>> new scope, just as if the second part were inside its own block.
>
> Ah, but then you'd get people wanting to make the second set of dx/dy a
> different type from the first. So you'd have to allow that. And _then_
> you'd get people complaining about how that is type-unsafe and
> confusing. And in any case, this would only create a new scope where the
> new variables are needed, _not_ end the old scope where the old
> variables aren't needed any more, which is what was wanted, really.
>

You can, I think, pretend that whenever you see a definition such as 
"double dx = ...", that there is a new block scope started just before 
it, and which exits at the end of the current block scope.  Then your 
new "double dx" can be of any type you want.  The previous "dx" is still 
alive, and still in scope - but it is hidden (shadowed) by the new one. 
  It is up to the compiler to realise that it can optimise on that basis.

> You'd have to create a new statement, instead: an undeclare statement.
> It should work much like #undef does for #define. Then you could do:
>
>    double dx = monster.x-player.x;
>    double dy = monster.y-player.y
>    double monster_distance = sqrt(dx*dx+dy*dy);
>    undeclare dx, dy;
>    ...
>    /* Note that you cannot use dx and dy here; it's as if
>       they'd never been declared in the first place. */
>    ...
>    long int dx = shot.x-player.x;
>    long int dy = shot.y-player.y
>    long int shot_distance = sqrt(dx*dx+dy*dy);
>    undeclare dx, dy;
>
> and everybody would be happy *flashes most naive smile ever*.
>

An "undeclare" statement would be a fine idea, and give exact control 
over scopes.  But I suspect that it would be very rarely useful in practice.


> Richard
>

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Thread

setting pointers to NULL and testing question. G G <gdotone@gmail.com> - 2016-06-12 16:21 -0700
  Re: setting pointers to NULL and testing question. Richard Heathfield <rjh@cpax.org.uk> - 2016-06-13 00:50 +0100
    Re: setting pointers to NULL and testing question. G G <gdotone@gmail.com> - 2016-06-12 17:03 -0700
    Re: setting pointers to NULL and testing question. Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2016-06-12 20:25 -0600
      Re: setting pointers to NULL and testing question. James Kuyper <jameskuyper@verizon.net> - 2016-06-12 22:57 -0400
        Re: setting pointers to NULL and testing question. Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2016-06-13 10:03 -0600
          Re: setting pointers to NULL and testing question. Tim Rentsch <txr@alumni.caltech.edu> - 2016-06-15 14:43 -0700
      Re: setting pointers to NULL and testing question. supercat@casperkitty.com - 2016-06-13 08:06 -0700
        Re: setting pointers to NULL and testing question. Richard Damon <Richard@Damon-Family.org> - 2016-06-16 08:27 -0400
          Re: setting pointers to NULL and testing question. supercat@casperkitty.com - 2016-06-16 06:27 -0700
            Re: setting pointers to NULL and testing question. supercat@casperkitty.com - 2016-06-16 07:31 -0700
            Re: setting pointers to NULL and testing question. Richard Damon <Richard@Damon-Family.org> - 2016-06-18 13:39 -0400
              Re: setting pointers to NULL and testing question. supercat@casperkitty.com - 2016-06-18 11:37 -0700
                Re: setting pointers to NULL and testing question. Richard Damon <Richard@Damon-Family.org> - 2016-06-18 17:17 -0400
                Re: setting pointers to NULL and testing question. supercat@casperkitty.com - 2016-06-18 17:22 -0700
      Re: setting pointers to NULL and testing question. James Kuyper <jameskuyper@verizon.net> - 2016-06-13 14:10 -0400
        Re: setting pointers to NULL and testing question. Keith Thompson <kst-u@mib.org> - 2016-06-13 11:28 -0700
        Re: setting pointers to NULL and testing question. Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2016-06-13 12:31 -0600
    Re: setting pointers to NULL and testing question. Keith Thompson <kst-u@mib.org> - 2016-06-12 19:38 -0700
      Re: setting pointers to NULL and testing question. Richard Heathfield <rjh@cpax.org.uk> - 2016-06-13 07:35 +0100
    Re: setting pointers to NULL and testing question. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-06-13 11:40 +0100
      Re: setting pointers to NULL and testing question. BartC <bc@freeuk.com> - 2016-06-13 11:56 +0100
        Re: setting pointers to NULL and testing question. Martin Shobe <mashobe9@gmail.com> - 2016-06-13 06:22 -0500
          Re: setting pointers to NULL and testing question. BartC <bc@freeuk.com> - 2016-06-13 12:39 +0100
            Re: setting pointers to NULL and testing question. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-06-13 15:22 +0100
              Re: setting pointers to NULL and testing question. BartC <bc@freeuk.com> - 2016-06-13 17:34 +0100
                Re: setting pointers to NULL and testing question. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-06-13 19:31 +0100
                Re: setting pointers to NULL and testing question. BartC <bc@freeuk.com> - 2016-06-13 20:17 +0100
                Re: setting pointers to NULL and testing question. Keith Thompson <kst-u@mib.org> - 2016-06-13 12:47 -0700
                Re: setting pointers to NULL and testing question. Tim Rentsch <txr@alumni.caltech.edu> - 2016-06-15 16:44 -0700
                Re: setting pointers to NULL and testing question. Keith Thompson <kst-u@mib.org> - 2016-06-15 17:20 -0700
      Re: setting pointers to NULL and testing question. Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-06-13 04:01 -0700
        Re: setting pointers to NULL and testing question. Tim Rentsch <txr@alumni.caltech.edu> - 2016-06-16 12:57 -0700
      Re: setting pointers to NULL and testing question. Richard Heathfield <rjh@cpax.org.uk> - 2016-06-13 12:18 +0100
        Re: setting pointers to NULL and testing question. David Brown <david.brown@hesbynett.no> - 2016-06-13 14:01 +0200
          Re: setting pointers to NULL and testing question. Richard Heathfield <rjh@cpax.org.uk> - 2016-06-13 14:15 +0100
            Re: setting pointers to NULL and testing question. David Brown <david.brown@hesbynett.no> - 2016-06-13 15:37 +0200
              Re: setting pointers to NULL and testing question. Richard Heathfield <rjh@cpax.org.uk> - 2016-06-13 14:48 +0100
        Re: setting pointers to NULL and testing question. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-06-13 15:43 +0100
  Re: setting pointers to NULL and testing question. Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-06-12 17:54 -0700
  Re: setting pointers to NULL and testing question. John Bode <jfbode1029@gmail.com> - 2016-06-14 08:36 -0700
    Re: setting pointers to NULL and testing question. Richard Heathfield <rjh@cpax.org.uk> - 2016-06-14 16:41 +0100
    Re: setting pointers to NULL and testing question. Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-06-14 09:01 -0700
    Re: setting pointers to NULL and testing question. "James R. Kuyper" <jameskuyper@verizon.net> - 2016-06-14 13:25 -0400
      Re: setting pointers to NULL and testing question. supercat@casperkitty.com - 2016-06-14 10:48 -0700
        Re: setting pointers to NULL and testing question. Ian Collins <ian-news@hotmail.com> - 2016-06-15 08:24 +1200
          Re: setting pointers to NULL and testing question. "James R. Kuyper" <jameskuyper@verizon.net> - 2016-06-14 16:57 -0400
            Re: setting pointers to NULL and testing question. supercat@casperkitty.com - 2016-06-14 14:29 -0700
              Re: setting pointers to NULL and testing question. BartC <bc@freeuk.com> - 2016-06-14 22:41 +0100
                Re: setting pointers to NULL and testing question. supercat@casperkitty.com - 2016-06-14 15:16 -0700
                Re: setting pointers to NULL and testing question. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-06-15 00:25 +0100
                Re: setting pointers to NULL and testing question. Keith Thompson <kst-u@mib.org> - 2016-06-14 16:58 -0700
                Re: setting pointers to NULL and testing question. David Brown <david.brown@hesbynett.no> - 2016-06-15 09:28 +0200
                Re: setting pointers to NULL and testing question. Keith Thompson <kst-u@mib.org> - 2016-06-15 09:57 -0700
              Re: setting pointers to NULL and testing question. David Brown <david.brown@hesbynett.no> - 2016-06-15 09:23 +0200
                Re: setting pointers to NULL and testing question. Richard Heathfield <rjh@cpax.org.uk> - 2016-06-15 08:35 +0100
                Re: setting pointers to NULL and testing question. Richard Heathfield <rjh@cpax.org.uk> - 2016-06-15 08:37 +0100
                Re: setting pointers to NULL and testing question. David Brown <david.brown@hesbynett.no> - 2016-06-15 10:42 +0200
                Re: setting pointers to NULL and testing question. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-06-15 13:28 +0100
                Re: setting pointers to NULL and testing question. supercat@casperkitty.com - 2016-06-15 11:41 -0700
                Re: setting pointers to NULL and testing question. raltbos@xs4all.nl (Richard Bos) - 2016-06-15 19:43 +0000
                Re: setting pointers to NULL and testing question. supercat@casperkitty.com - 2016-06-15 13:09 -0700
                Re: setting pointers to NULL and testing question. David Brown <david.brown@hesbynett.no> - 2016-06-15 22:31 +0200
                Re: setting pointers to NULL and testing question. Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-06-15 17:11 -0400
                Re: setting pointers to NULL and testing question. raltbos@xs4all.nl (Richard Bos) - 2016-06-18 20:44 +0000
                Re: setting pointers to NULL and testing question. Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-06-18 17:20 -0400
        Re: setting pointers to NULL and testing question. Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-06-14 23:22 -0700
          Re: setting pointers to NULL and testing question. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-06-15 13:23 +0100
      Re: setting pointers to NULL and testing question. John Bode <jfbode1029@gmail.com> - 2016-06-20 13:36 -0700
    Re: setting pointers to NULL and testing question. Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-06-14 14:59 -0400
      Re: setting pointers to NULL and testing question. "James R. Kuyper" <jameskuyper@verizon.net> - 2016-06-14 15:17 -0400
      Re: setting pointers to NULL and testing question. Richard Heathfield <rjh@cpax.org.uk> - 2016-06-14 20:25 +0100
      Re: setting pointers to NULL and testing question. Tim Rentsch <txr@alumni.caltech.edu> - 2016-06-15 19:48 -0700
        Re: setting pointers to NULL and testing question. Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-06-15 23:25 -0400
          Re: setting pointers to NULL and testing question. James Kuyper <jameskuyper@verizon.net> - 2016-06-16 12:11 -0400
            Re: setting pointers to NULL and testing question. supercat@casperkitty.com - 2016-06-16 09:25 -0700
            Re: setting pointers to NULL and testing question. Tim Rentsch <txr@alumni.caltech.edu> - 2016-06-16 21:46 -0700
              Re: setting pointers to NULL and testing question. Gareth Owen <gwowen@gmail.com> - 2016-06-17 06:13 +0100
                Re: setting pointers to NULL and testing question. Robert Wessel <robertwessel2@yahoo.com> - 2016-06-17 02:03 -0500
                Re: setting pointers to NULL and testing question. Martin Shobe <mashobe9@gmail.com> - 2016-06-17 07:07 -0500
                Re: setting pointers to NULL and testing question. supercat@casperkitty.com - 2016-06-17 03:18 -0700
                Re: setting pointers to NULL and testing question. James Kuyper <jameskuyper@verizon.net> - 2016-06-17 09:16 -0400
                Re: setting pointers to NULL and testing question. Tim Rentsch <txr@alumni.caltech.edu> - 2016-06-21 11:03 -0700
                Re: setting pointers to NULL and testing question. Tim Rentsch <txr@alumni.caltech.edu> - 2016-06-21 10:58 -0700
              Re: setting pointers to NULL and testing question. Keith Thompson <kst-u@mib.org> - 2016-06-21 11:28 -0700
          Re: setting pointers to NULL and testing question. Tim Rentsch <txr@alumni.caltech.edu> - 2016-06-16 21:39 -0700

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