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Re: Program to cipher a text, new thread, fresh one

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Richard Heathfield <rjh@cpax.org.uk> writes:
[...]
> On 01/11/15 10:42, Ben Bacarisse wrote:
>> Richard Heathfield <rjh@cpax.org.uk> writes:
[...]
>>>    while((ch = getchar()) != EOF)
>>>    {
>>>      if(isalpha((unsigned char)ch))
>>
>> You don't need the cast here...
>
> Actually, rather to my surprise, you're right. This is because ch comes 
> from getchar(), which is defined in terms of getc, which is defined in 
> terms of fgetc, which is defined to return a character as an unsigned 
> char converted to an int. That's yet another cast I can consign to the 
> dustbin (although only because of the particular knowledge we have of 
> the origin of ch).
[...]

A key insight is that ch is of type int.

If ch were of type char, we'd have to assume that it might contain
a negative value, particularly if it came from user input or from
a command-line argument.  If a character value is stored in an int
object *and* we haven't done anything silly, we can assume it's
a non-negative value within the range of unsigned char (or EOF,
but we've already checked that).  (Copying a char value to an int
object would qualify as doing something silly.)

A consistent way to handle this is to follow these rules:

- A char argument to is*() or to*() must be cast to unsigned int.
- An int argument need not be cast.
- Important: If you store a character value in an int, it must be in the
  range 0..UCHAR_MAX, for example if it's the result of calling
  getchar().  Don't copy a character from a string into an int object.

Oddly, this would have been clearer to me if it had been named "c"
rather than "ch", since the common idiom is

    while ((c = getchar()) != EOF) ...

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Program to cipher a text, new thread, fresh one Alla _ <modelling.data@gmail.com> - 2015-10-30 08:23 -0700
  Re: Program to cipher a text, new thread, fresh one Bartc <bc@freeuk.com> - 2015-10-30 15:39 +0000
    Re: Program to cipher a text, new thread, fresh one Alla _ <modelling.data@gmail.com> - 2015-10-30 09:33 -0700
    Re: Program to cipher a text, new thread, fresh one Richard Heathfield <rjh@cpax.org.uk> - 2015-11-01 09:33 +0000
  Re: Program to cipher a text, new thread, fresh one Mark Storkamp <mstorkamp@yahoo.com> - 2015-10-30 11:17 -0500
    Re: Program to cipher a text, new thread, fresh one Alla _ <modelling.data@gmail.com> - 2015-10-30 09:32 -0700
      Re: Program to cipher a text, new thread, fresh one Richard Heathfield <rjh@cpax.org.uk> - 2015-11-01 09:38 +0000
  Re: Program to cipher a text, new thread, fresh one "Osmium" <r124c4u102@comcast.net> - 2015-10-30 10:55 -0600
    Re: Program to cipher a text, new thread, fresh one Richard Heathfield <rjh@cpax.org.uk> - 2015-11-01 10:07 +0000
      Re: Program to cipher a text, new thread, fresh one Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-01 10:42 +0000
        Re: Program to cipher a text, new thread, fresh one Richard Heathfield <rjh@cpax.org.uk> - 2015-11-01 11:35 +0000
          Re: Program to cipher a text, new thread, fresh one James Kuyper <jameskuyper@verizon.net> - 2015-11-01 08:33 -0500
          Re: Program to cipher a text, new thread, fresh one Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-01 16:09 +0000
            Re: Program to cipher a text, new thread, fresh one Bartc <bc@freeuk.com> - 2015-11-02 12:43 +0000
              Re: Program to cipher a text, new thread, fresh one Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-02 13:25 +0000
              Re: Program to cipher a text, new thread, fresh one Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-02 06:13 -0800
          Re: Program to cipher a text, new thread, fresh one Keith Thompson <kst-u@mib.org> - 2015-11-01 12:26 -0800
      Re: Program to cipher a text, new thread, fresh one supercat@casperkitty.com - 2015-11-01 07:59 -0800
        Re: Program to cipher a text, new thread, fresh one Richard Heathfield <rjh@cpax.org.uk> - 2015-11-02 13:48 +0000
          Re: Program to cipher a text, new thread, fresh one Bartc <bc@freeuk.com> - 2015-11-02 14:06 +0000
            Re: Program to cipher a text, new thread, fresh one Richard Heathfield <rjh@cpax.org.uk> - 2015-11-02 14:51 +0000
              Re: Program to cipher a text, new thread, fresh one Bartc <bc@freeuk.com> - 2015-11-02 15:48 +0000
                Re: Program to cipher a text, new thread, fresh one Richard Heathfield <rjh@cpax.org.uk> - 2015-11-02 16:00 +0000
                Re: Program to cipher a text, new thread, fresh one asetofsymbols@gmail.com - 2015-11-03 01:40 -0800
                Re: Program to cipher a text, new thread, fresh one Richard Heathfield <rjh@cpax.org.uk> - 2015-11-03 11:16 +0000
                Re: Program to cipher a text, new thread, fresh one "Osmium" <r124c4u102@comcast.net> - 2015-11-03 05:21 -0600
                Re: Program to cipher a text, new thread, fresh one asetofsymbols@gmail.com - 2015-11-03 03:41 -0800
                Re: Program to cipher a text, new thread, fresh one Richard Heathfield <rjh@cpax.org.uk> - 2015-11-03 11:53 +0000
                Re: Program to cipher a text, new thread, fresh one "Osmium" <r124c4u102@comcast.net> - 2015-11-03 05:57 -0600
                Re: Program to cipher a text, new thread, fresh one Richard Heathfield <rjh@cpax.org.uk> - 2015-11-03 12:48 +0000
                Re: Program to cipher a text, new thread, fresh one Rosario19 <Ros@invalid.invalid> - 2015-11-04 09:09 +0100
                Re: Program to cipher a text, new thread, fresh one Rosario19 <Ros@invalid.invalid> - 2015-11-04 09:37 +0100
                Re: Program to cipher a text, new thread, fresh one Alla _ <modelling.data@gmail.com> - 2015-11-04 07:09 -0800
                Re: Program to cipher a text, new thread, fresh one "Osmium" <r124c4u102@comcast.net> - 2015-11-03 05:53 -0600
                Re: Program to cipher a text, new thread, fresh one asetofsymbols@gmail.com - 2015-11-03 04:29 -0800
                Re: Program to cipher a text, new thread, fresh one "Osmium" <r124c4u102@comcast.net> - 2015-11-03 06:42 -0600
      Re: Program to cipher a text, new thread, fresh one Geoff <geoff@invalid.invalid> - 2015-11-01 08:07 -0800
      Re: Program to cipher a text, new thread, fresh one Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-02 02:26 -0800
        Re: Program to cipher a text, new thread, fresh one Rosario19 <Ros@invalid.invalid> - 2015-11-06 07:53 +0100
          Re: Program to cipher a text, new thread, fresh one Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-07 10:01 -0800
  Re: Program to cipher a text, new thread, fresh one Alla _ <modelling.data@gmail.com> - 2015-10-30 10:50 -0700
    Re: Program to cipher a text, new thread, fresh one Alla _ <modelling.data@gmail.com> - 2015-10-30 10:53 -0700
      Re: Program to cipher a text, new thread, fresh one Barry Schwarz <schwarzb@dqel.com> - 2015-10-30 11:52 -0700
        Re: Program to cipher a text, new thread, fresh one Alla _ <modelling.data@gmail.com> - 2015-10-30 12:12 -0700
          Re: Program to cipher a text, new thread, fresh one Barry Schwarz <schwarzb@dqel.com> - 2015-10-30 21:05 -0700
            Re: Program to cipher a text, new thread, fresh one Alla _ <modelling.data@gmail.com> - 2015-10-30 23:58 -0700
              Re: Program to cipher a text, new thread, fresh one Barry Schwarz <schwarzb@dqel.com> - 2015-10-31 03:09 -0700
                Re: Program to cipher a text, new thread, fresh one Alla _ <modelling.data@gmail.com> - 2015-10-31 03:47 -0700
                Re: Program to cipher a text, new thread, fresh one Barry Schwarz <schwarzb@dqel.com> - 2015-10-31 11:42 -0700
      Re: Program to cipher a text, new thread, fresh one Barry Schwarz <schwarzb@dqel.com> - 2015-10-30 12:04 -0700
      Re: Program to cipher a text, new thread, fresh one Alla _ <modelling.data@gmail.com> - 2015-10-30 12:10 -0700
  Re: Program to cipher a text, new thread, fresh one "Osmium" <r124c4u102@comcast.net> - 2015-10-30 13:17 -0600
    Re: Program to cipher a text, new thread, fresh one Alla _ <modelling.data@gmail.com> - 2015-10-31 00:04 -0700
      Re: Program to cipher a text, new thread, fresh one Alla _ <modelling.data@gmail.com> - 2015-10-31 00:25 -0700
  Re: Program to cipher a text, new thread, fresh one Keith Thompson <kst-u@mib.org> - 2015-10-30 12:34 -0700
    Re: Program to cipher a text, new thread, fresh one Alla _ <modelling.data@gmail.com> - 2015-10-30 21:49 -0700
      Re: Program to cipher a text, new thread, fresh one Keith Thompson <kst-u@mib.org> - 2015-10-31 13:18 -0700
  Re: Program to cipher a text, new thread, fresh one Alla _ <modelling.data@gmail.com> - 2015-10-31 00:29 -0700
  Re: Program to cipher a text, new thread, fresh one Alla _ <modelling.data@gmail.com> - 2015-10-31 01:12 -0700
    Re: Program to cipher a text, new thread, fresh one Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-31 12:24 +0000
      Re: Program to cipher a text, new thread, fresh one Alla _ <modelling.data@gmail.com> - 2015-10-31 06:11 -0700
      Re: Program to cipher a text, new thread, fresh one Bartc <bc@freeuk.com> - 2015-10-31 15:58 +0000
        Re: Program to cipher a text, new thread, fresh one Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-31 17:37 +0000
          Re: Program to cipher a text, new thread, fresh one Bartc <bc@freeuk.com> - 2015-10-31 18:14 +0000
            Re: Program to cipher a text, new thread, fresh one Keith Thompson <kst-u@mib.org> - 2015-10-31 13:29 -0700
              Re: Program to cipher a text, new thread, fresh one Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-31 20:35 +0000
            Re: Program to cipher a text, new thread, fresh one Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-31 20:32 +0000
  Re: Program to cipher a text, new thread, fresh one Rosario19 <Ros@invalid.invalid> - 2015-11-04 09:50 +0100

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