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Re: A finalized program

From Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups comp.lang.c
Subject Re: A finalized program
Date 2015-09-11 13:46 +0100
Organization A noiseless patient Spider
Message-ID <87lhcdgm3d.fsf@bsb.me.uk> (permalink)
References <d93ca264-e41a-4f82-8c09-cd87731b2eed@googlegroups.com> <76d2dbff-c516-4e0e-8420-7a7914d7a8f4@googlegroups.com>

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Alla _ <modelling.data@gmail.com> writes:

> I have checked the style of this program,
> and the result showed an error message:
>
> "expected new line before ";"
>  
> while (!(scanf ("%16[0-9]", card_number) == 1 && getchar() == '\n'))
>     {
>         printf ("This is not a card number; please, try again\n");
>         while (getchar() != '\n'); /* here, before this ";"*/
>     }

There are situation where that won't work or will confuse the user, but
I don't think the details matter here.

I've said it before but I feel I want to have another go: don't spend
time on this.  Prompting the user and reading interactive input in a
robust and clear way is extraordinarily difficult and you will never do
it again once you start "real" programming.  I have not written a
program that does this in the last 35 years -- it's a complex skill that
is of no value.

Read the data using " %16[0-9]" (note the space) and tell the user if
the length is wrong.  If you like, add a %c to read the next character
to check if it's '\n'.  That's it.  Check that you've got data of the
right length and process it.  No loops and no trying to clear down the
input buffer.  Make it simple because that will mean you can move on to
more complex program that do interesting things rather than wrestle with
a problem this is so transient.  Write more programs, rather then
spending so long on this one aspect!

Richard has said don't use scanf, but that's bad advice!!  The correct
advice is don't use scanf for complex things -- it *will* drive you mad.
But for simple input tasks in beginners' programs it is ideal.  He
re-wrote your example using fgets and saying how easy it was.  He just
had to write write a dozen or so lines of code to do what %[0-9] does!
And he has to use cast, which he hates!  What's more, fgets has it's own
problems which can confuse users in interactive programs.

In short, there are loads of interesting things to lean that you will
use time and time again (command-line argument processing is one) but
this isn't one of them!

<snip>
-- 
Ben.

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Thread

A finalized program Alla _ <modelling.data@gmail.com> - 2015-09-08 04:13 -0700
  Re: A finalized program Alla _ <modelling.data@gmail.com> - 2015-09-08 04:19 -0700
    Re: A finalized program fir <profesor.fir@gmail.com> - 2015-09-08 04:30 -0700
    Re: A finalized program Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-09-08 14:45 +0100
      Re: A finalized program Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-09-08 08:29 -0700
      Re: A finalized program Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-09-08 08:52 -0700
        Re: A finalized program Martin Shobe <martin.shobe@yahoo.com> - 2015-09-08 13:01 -0500
        Re: A finalized program Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-09-08 21:00 +0100
          Re: A finalized program Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-09-08 13:33 -0700
            Re: A finalized program Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-09-09 00:28 +0100
  Re: A finalized program Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-09-08 14:26 +0100
    Re: A finalized program Alla _ <modelling.data@gmail.com> - 2015-09-08 06:42 -0700
      Re: A finalized program Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-09-08 20:50 +0100
        Re: A finalized program Alla _ <modelling.data@gmail.com> - 2015-09-11 01:47 -0700
  Re: A finalized program jt@toerring.de (Jens Thoms Toerring) - 2015-09-08 13:41 +0000
    Re: A finalized program Alla _ <modelling.data@gmail.com> - 2015-09-10 01:31 -0700
      Re: A finalized program jt@toerring.de (Jens Thoms Toerring) - 2015-09-10 10:39 +0000
        Re: A finalized program David Thompson <dave.thompson2@verizon.net> - 2015-09-20 18:35 -0400
    Re: A finalized program Alla _ <modelling.data@gmail.com> - 2015-09-10 01:33 -0700
    Re: A finalized program Alla _ <modelling.data@gmail.com> - 2015-09-10 10:16 -0700
      Re: A finalized program bartekltg <bartekltg@gmail.com> - 2015-09-10 20:08 +0200
        Re: A finalized program Alla _ <modelling.data@gmail.com> - 2015-09-11 02:05 -0700
          Re: A finalized program Richard Heathfield <rjh@cpax.org.uk> - 2015-09-11 10:17 +0100
            Re: A finalized program Richard Heathfield <rjh@cpax.org.uk> - 2015-09-11 10:19 +0100
            Re: A finalized program Alla _ <modelling.data@gmail.com> - 2015-09-11 02:28 -0700
            Re: A finalized program Alla _ <modelling.data@gmail.com> - 2015-09-11 02:29 -0700
        Re: A finalized program Alla _ <modelling.data@gmail.com> - 2015-09-11 02:19 -0700
          Re: A finalized program Richard Heathfield <rjh@cpax.org.uk> - 2015-09-11 10:23 +0100
            Re: A finalized program Alla _ <modelling.data@gmail.com> - 2015-09-11 02:33 -0700
              Re: A finalized program Keith Thompson <kst-u@mib.org> - 2015-09-11 14:11 -0700
                Re: A finalized program jt@toerring.de (Jens Thoms Toerring) - 2015-09-11 21:39 +0000
          Re: A finalized program bartekltg <bartekltg@gmail.com> - 2015-09-11 12:49 +0200
      Re: A finalized program Richard Heathfield <rjh@cpax.org.uk> - 2015-09-10 23:31 +0100
        Re: A finalized program Alla _ <modelling.data@gmail.com> - 2015-09-11 02:15 -0700
      Re: A finalized program jt@toerring.de (Jens Thoms Toerring) - 2015-09-10 22:37 +0000
        Re: A finalized program Richard Heathfield <rjh@cpax.org.uk> - 2015-09-10 23:47 +0100
          Re: A finalized program jt@toerring.de (Jens Thoms Toerring) - 2015-09-11 10:18 +0000
        Re: A finalized program Alla _ <modelling.data@gmail.com> - 2015-09-11 01:07 -0700
        Re: A finalized program Alla _ <modelling.data@gmail.com> - 2015-09-11 02:14 -0700
          Re: A finalized program jt@toerring.de (Jens Thoms Toerring) - 2015-09-11 10:10 +0000
            Re: A finalized program Alla _ <modelling.data@gmail.com> - 2015-09-11 04:19 -0700
  Re: A finalized program Barry Schwarz <schwarzb@dqel.com> - 2015-09-08 15:53 -0700
  Re: A finalized program Alla _ <modelling.data@gmail.com> - 2015-09-11 04:27 -0700
    Re: A finalized program bartekltg <bartekltg@gmail.com> - 2015-09-11 14:05 +0200
  Re: A finalized program Alla _ <modelling.data@gmail.com> - 2015-09-11 04:42 -0700
    Re: A finalized program bartekltg <bartekltg@gmail.com> - 2015-09-11 14:15 +0200
      Re: A finalized program Alla _ <modelling.data@gmail.com> - 2015-09-11 05:50 -0700
        Re: A finalized program bartekltg <bartekltg@gmail.com> - 2015-09-11 15:00 +0200
      Re: A finalized program Alla _ <modelling.data@gmail.com> - 2015-09-11 05:50 -0700
    Re: A finalized program Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-09-11 13:46 +0100
      Re: A finalized program Richard Heathfield <rjh@cpax.org.uk> - 2015-09-11 14:19 +0100

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