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Re: A very simple parser with scanf & C

Message-ID <4F642018.7584@mindspring.com> (permalink)
Date 2012-03-17 00:24 -0500
From pete <pfiland@mindspring.com>
Organization PF
Newsgroups comp.lang.c
Subject Re: A very simple parser with scanf & C
References (17 earlier) <kfnk42vt3d7.fsf@x-alumni2.alumni.caltech.edu> <4F5BB70E.84B@mindspring.com> <kfnobs0m7zy.fsf@x-alumni2.alumni.caltech.edu> <4F613025.6244@mindspring.com> <kfnbonxl6s8.fsf@x-alumni2.alumni.caltech.edu>

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Tim Rentsch wrote:
> 
> pete <pfiland@mindspring.com> writes:
> 
> > Tim Rentsch wrote:
> >>
> >> pete <pfiland@mindspring.com> writes:
> >>
> >> > Tim Rentsch wrote:
> >> >>
> >> >> pete <pfiland@mindspring.com> writes:
> >> >>
> >> >> > Tim Rentsch wrote:
> >> >> >>
> >> >> >> pete <pfiland@mindspring.com> writes:
> >> >> >>
> >> >> >> > Tim Rentsch wrote:
> >> >> >> >
> >> >> >> >> [snip]
> >> >> >
> >> >> > If you're going to say for {char c;}
> >> >> >
> >> >> >>>     isalpha( * (unsigned char*) &c )
> >> >> >>>
> >> >> >>> rather than
> >> >> >>>
> >> >> >>>     isalpha(   (unsigned char )  c )
> >> >> >
> >> >> > then I think we should consider a case where
> >> >> >     ( * (unsigned char*) &c )
> >> >> > is different from
> >> >> >     (   (unsigned char )  c )
> >> >> > and the only case I can think of,
> >> >> > is where (c) is signed, negative and not twos complement.
> >> >>
> >> >> Right (ignoring padding bits in char, which no sane
> >> >> implementation has), although actually there is one other
> >> >> case, namely a two's complement trap representation.  Not
> >> >> very likely perhaps but it is allowed, so for the sake of
> >> >> completeness I think it should be mentioned.  (And now it
> >> >> has been...)
> >> >
> >> > Considering that file operations are at the root of this thread:
> >> >
> >> > If char is signed, and char c is assigned a value of (-1), as in
> >> >     char c;
> >> >     c = -1;
> >> >
> >> > and if (c) is written to a file using fputc,
> >> > then the value written into the file will be ((unsigned char)-1).
> >> >
> >> > If that same byte is read back from the same file using fgetc,
> >> > then the value returned by fgetc will be
> >> > ((int)(unsigned char)-1)
> >> > which is equal to ((unsigned char)c)
> >> > regardless of the representation of negative integers.
> >> >
> >> > The return value of fgetc will be unequal to
> >> > ( * (unsigned char*) &c )
> >> > when negative integers are either sign and magnitude
> >> > or one's complement.
> >>
> >> That's right, of course.  Which form should be used depends
> >> on where the bytes came from in the first place -- if they
> >> had been written with fwrite(), for example, then the type
> >> punning form, rather than the ordinary converting form, is
> >> probably a better choice.  And conversely.
> >>
> >> I suspect this is all a moot point anyway, since the values
> >> that might be affected most likely will all have the same
> >> characteristics vis-a-vis the <ctype.h> functions.  So I
> >> will just say thank you for an interesting discussion.
> >
> > As far as the ctype functions go,
> > if you want use isalpha to give you some idea of
> > whether or not putchar(c) is going to output an alpha,
> > then isalpha((unsigned char)c) is the way to do it,
> > because the value returned by putchar(c)
> > compares equal to ((unsigned char)c).
> 
> I don't find this argument persuasive, because in essence it's
> no more than the tautology that 'isalpha( (unsigned char) c )'
> is equal to 'isalpha( (unsigned char) c )'.  It is just as true
> that 'isalpha( *(unsigned char*) &c )' will be equal to
> 'isalpha( *(unsigned char*) &c )' (and similarly for putting
> the value through the fputc()/fget() step).  That doesn't prove
> anything.


isalpha((unsigned char)c) will tell you 
if the output of putchar(c) is an alpha 
because putchar(c) is equal to putchar((unsigned char)c).


-- 
pete

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Thread

A very simple parser with scanf & C pozz <pozzugno@gmail.com> - 2011-09-23 07:03 -0700
  Re: A very simple parser with scanf & C Nobody <nobody@nowhere.com> - 2011-09-23 18:28 +0100
  Re: A very simple parser with scanf & C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-23 18:29 +0100
    Re: A very simple parser with scanf & C pozz <pozzugno@gmail.com> - 2011-09-23 23:16 +0200
      Re: A very simple parser with scanf & C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-24 00:09 +0100
        Re: A very simple parser with scanf & C pozz <pozzugno@gmail.com> - 2011-09-24 09:32 +0200
  Re: A very simple parser with scanf & C "BartC" <bc@freeuk.com> - 2011-09-23 21:24 +0100
    Re: A very simple parser with scanf & C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-23 22:00 +0100
      Re: A very simple parser with scanf & C "BartC" <bc@freeuk.com> - 2011-09-24 00:04 +0100
        Re: A very simple parser with scanf & C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-24 00:34 +0100
          Re: A very simple parser with scanf & C "BartC" <bc@freeuk.com> - 2011-09-24 01:00 +0100
            Re: A very simple parser with scanf & C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-24 02:42 +0100
              Re: A very simple parser with scanf & C "BartC" <bc@freeuk.com> - 2011-09-24 12:13 +0100
                Re: A very simple parser with scanf & C "BartC" <bc@freeuk.com> - 2011-09-24 12:25 +0100
                Re: A very simple parser with scanf & C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-24 15:05 +0100
                Re: A very simple parser with scanf & C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-24 14:45 +0100
                Re: A very simple parser with scanf & C "BartC" <bc@freeuk.com> - 2011-09-24 17:45 +0100
                Re: A very simple parser with scanf & C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-24 22:21 +0100
                Re: A very simple parser with scanf & C "BartC" <bc@freeuk.com> - 2011-09-25 00:52 +0100
                Re: A very simple parser with scanf & C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-25 01:54 +0100
                Re: A very simple parser with scanf & C "BartC" <bc@freeuk.com> - 2011-09-25 02:51 +0100
                Re: A very simple parser with scanf & C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-25 12:22 +0100
                Re: A very simple parser with scanf & C Gene <gene.ressler@gmail.com> - 2011-09-25 01:35 -0700
                Re: A very simple parser with scanf & C pete <pfiland@mindspring.com> - 2011-09-25 08:41 -0400
                Re: A very simple parser with scanf & C Gene <gene.ressler@gmail.com> - 2011-09-25 11:23 -0700
                Re: A very simple parser with scanf & C Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-09-25 19:41 +0300
                Re: A very simple parser with scanf & C "BartC" <bc@freeuk.com> - 2011-09-25 19:45 +0100
                Re: A very simple parser with scanf & C Keith Thompson <kst-u@mib.org> - 2011-09-25 12:10 -0700
                Re: A very simple parser with scanf & C "BartC" <bc@freeuk.com> - 2011-09-25 20:43 +0100
                Re: A very simple parser with scanf & C James Kuyper <jameskuyper@verizon.net> - 2011-09-25 15:55 -0400
                Re: A very simple parser with scanf & C "BartC" <bc@freeuk.com> - 2011-09-26 11:07 +0100
                Re: A very simple parser with scanf & C Richard Damon <news.x.richarddamon@xoxy.net> - 2011-09-26 11:40 -0400
                Re: A very simple parser with scanf & C Keith Thompson <kst-u@mib.org> - 2011-09-26 12:22 -0700
                Re: A very simple parser with scanf & C pacman@kosh.dhis.org (Alan Curry) - 2011-09-26 21:19 +0000
                Re: A very simple parser with scanf & C Keith Thompson <kst-u@mib.org> - 2011-09-26 16:11 -0700
                Re: A very simple parser with scanf & C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-26 23:26 +0100
                Re: A very simple parser with scanf & C Keith Thompson <kst-u@mib.org> - 2011-09-26 12:18 -0700
                Re: A very simple parser with scanf & C Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-09-27 02:27 +0300
                Re: A very simple parser with scanf & C Keith Thompson <kst-u@mib.org> - 2011-09-27 10:06 -0700
                Re: A very simple parser with scanf & C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-27 21:10 +0100
                Re: A very simple parser with scanf & C Seebs <usenet-nospam@seebs.net> - 2011-09-26 22:59 +0000
                Re: A very simple parser with scanf & C Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-09-30 05:18 -0700
                Re: A very simple parser with scanf & C "BartC" <bc@freeuk.com> - 2011-09-30 17:06 +0100
                Re: A very simple parser with scanf & C Richard Damon <news.x.richarddamon@xoxy.net> - 2011-09-25 18:05 -0400
                Re: A very simple parser with scanf & C Keith Thompson <kst-u@mib.org> - 2011-09-25 15:30 -0700
                Re: A very simple parser with scanf & C Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-24 14:01 -0800
                Re: A very simple parser with scanf & C pete <pfiland@mindspring.com> - 2012-01-25 11:05 -0500
                Re: A very simple parser with scanf & C Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-31 16:01 -0800
                Re: A very simple parser with scanf & C pete <pfiland@mindspring.com> - 2012-01-31 23:47 -0500
                Re: A very simple parser with scanf & C Tim Rentsch <txr@alumni.caltech.edu> - 2012-03-08 09:21 -0800
                Re: A very simple parser with scanf & C pete <pfiland@mindspring.com> - 2012-03-10 15:18 -0500
                Re: A very simple parser with scanf & C Tim Rentsch <txr@alumni.caltech.edu> - 2012-03-13 15:49 -0700
                Re: A very simple parser with scanf & C pete <pfiland@mindspring.com> - 2012-03-14 18:56 -0500
                Re: A very simple parser with scanf & C Tim Rentsch <txr@alumni.caltech.edu> - 2012-03-15 17:37 -0700
                Re: A very simple parser with scanf & C pete <pfiland@mindspring.com> - 2012-03-17 00:24 -0500
                Re: A very simple parser with scanf & C Tim Rentsch <txr@alumni.caltech.edu> - 2012-03-21 05:21 -0700
    Re: A very simple parser with scanf & C pozz <pozzugno@gmail.com> - 2011-09-23 23:23 +0200
      Re: A very simple parser with scanf & C "BartC" <bc@freeuk.com> - 2011-09-23 23:53 +0100
  Re: A very simple parser with scanf & C Rui Maciel <rui.maciel@gmail.com> - 2011-09-25 14:07 +0100

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