Re: While or if: function to get next operator or operand

From	James Kuyper <jameskuyper@verizon.net>
Newsgroups	comp.lang.c
Subject	Re: While or if: function to get next operator or operand
Date	2015-11-20 12:02 -0500
Organization	Self
Message-ID	<564F5220.6090604@verizon.net> (permalink)
References	<ca6f25b9-c067-4945-b4dd-2ba0a97ec136@googlegroups.com> <n2nhag$hv4$1@dont-email.me> <beead803-e6f9-487d-a3da-d2777d5ce30a@googlegroups.com>

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On 11/20/2015 11:39 AM, Alla _ wrote:
> On Friday, November 20, 2015 at 8:24:14 PM UTC+4, Eric Sosman wrote:
>> On 11/20/2015 11:06 AM, Alla _ wrote:
>>> Hello!
>>>
>>> Please, take a look at the function below. It is from K&R.
>>>
>>> I dare to wonder why they use a while loop if the condition
>>> considers only one item in the list. Why not to use if statement?
>>> What is the purpose and the logic behind the while loop in this case?
>>> [...]
>>> /* the below while loop */
>>>      while ((s[0] = c = getch()) == ' ' || c == '\t')
>>>          ;
>>
>>      It's not looking at "only one item," it's looking at as many
>> leading space or tab characters as getch() supplies.  The loop
>> terminates when the supplied character is something other than a
>> space or tab, and that non-space/tab character is stored in s[0].
>>
> No. It looks if the input is equal to a space or a tab, and if yes, it
> assigns a space or a tab to s[0]. ...

Not quite. First of all, it always assigns the value of c to s[0],
regardless of the value of c. It then checks whether that value is ' '
or '\t'. The key question you should ask yourself is what is the value
the controlling expression if c==' '? Also, what is the value of the
controlling expression if c=='\t'? What is the value of the controlling
expression if c isn't equal to either ' ' or '\t'?
Are any of those three values non-zero? The answer is "yes" - if you
disagree, you've made a mistake. Please explain why you think the value
is always zero, and I'll try to explain what your mistake is. The loop
will continue executing as long as the value of the controlling
expression is non-zero.

> ... After that, s[1] = '\0', and the 
> array is complete. 

That doesn't happen until the loop exits. In other words, it doesn't
happen until the controlling expression has a value of zero.

Thread

While or if: function to get next operator or operand Alla _ <modelling.data@gmail.com> - 2015-11-20 08:06 -0800
  Re: While or if: function to get next operator or operand Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-20 08:17 -0800
    Re: While or if: function to get next operator or operand Richard Heathfield <rjh@cpax.org.uk> - 2015-11-20 16:44 +0000
  Re: While or if: function to get next operator or operand Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-11-20 11:23 -0500
    Re: While or if: function to get next operator or operand Alla _ <modelling.data@gmail.com> - 2015-11-20 08:39 -0800
      Re: While or if: function to get next operator or operand Richard Heathfield <rjh@cpax.org.uk> - 2015-11-20 16:47 +0000
      Re: While or if: function to get next operator or operand Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-11-20 11:54 -0500
        Re: While or if: function to get next operator or operand Alla _ <modelling.data@gmail.com> - 2015-11-20 09:13 -0800
        Re: While or if: function to get next operator or operand David Thompson <dave.thompson2@verizon.net> - 2015-12-06 08:04 -0500
      Re: While or if: function to get next operator or operand James Kuyper <jameskuyper@verizon.net> - 2015-11-20 12:02 -0500
  Re: While or if: function to get next operator or operand Richard Heathfield <rjh@cpax.org.uk> - 2015-11-20 16:41 +0000
    Re: While or if: function to get next operator or operand Alla _ <modelling.data@gmail.com> - 2015-11-20 09:11 -0800
      Re: While or if: function to get next operator or operand Richard Heathfield <rjh@cpax.org.uk> - 2015-11-20 17:39 +0000
        Re: While or if: function to get next operator or operand Alla _ <modelling.data@gmail.com> - 2015-11-20 10:42 -0800
        Re: While or if: function to get next operator or operand Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2015-11-21 14:58 -0500
          Re: While or if: function to get next operator or operand Richard Heathfield <rjh@cpax.org.uk> - 2015-11-23 12:21 +0000
        Re: While or if: function to get next operator or operand Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-21 21:29 +0000
          Re: While or if: function to get next operator or operand BartC <bc@freeuk.com> - 2015-11-21 22:31 +0000
            Re: While or if: function to get next operator or operand BartC <bc@freeuk.com> - 2015-11-22 10:57 +0000
              Re: While or if: function to get next operator or operand Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-22 13:45 +0000
          Re: While or if: function to get next operator or operand Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2015-11-22 01:31 -0500
          Re: While or if: function to get next operator or operand Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-23 04:33 -0800
            Re: While or if: function to get next operator or operand Philip Lantz <prl@canterey.us> - 2015-11-23 23:44 -0800
        Re: While or if: function to get next operator or operand Rosario19 <Ros@invalid.invalid> - 2015-11-22 09:39 +0100
          Re: While or if: function to get next operator or operand Rosario19 <Ros@invalid.invalid> - 2015-11-22 09:49 +0100
            Re: While or if: function to get next operator or operand asetofsymbols@gmail.com - 2015-11-22 02:27 -0800
              Re: While or if: function to get next operator or operand asetofsymbols@gmail.com - 2015-11-22 02:32 -0800
        Re: While or if: function to get next operator or operand Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2015-11-22 10:02 -0500
          Re: While or if: function to get next operator or operand Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-23 17:23 +0000
            Re: While or if: function to get next operator or operand Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2015-11-23 12:31 -0500
              Re: While or if: function to get next operator or operand Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2015-11-23 13:02 -0500
        Re: While or if: function to get next operator or operand Alla _ <modelling.data@gmail.com> - 2015-11-22 07:06 -0800
          Re: While or if: function to get next operator or operand Barry Schwarz <schwarzb@dqel.com> - 2015-11-22 11:43 -0800
            Re: While or if: function to get next operator or operand Alla _ <modelling.data@gmail.com> - 2015-11-24 02:03 -0800
            Re: While or if: function to get next operator or operand Alla _ <modelling.data@gmail.com> - 2015-11-24 02:11 -0800
              Re: While or if: function to get next operator or operand James Kuyper <jameskuyper@verizon.net> - 2015-11-24 07:09 -0500
          Re: While or if: function to get next operator or operand Richard Heathfield <rjh@cpax.org.uk> - 2015-11-23 12:26 +0000
            Re: While or if: function to get next operator or operand raltbos@xs4all.nl (Richard Bos) - 2015-11-23 13:26 +0000
        Re: While or if: function to get next operator or operand Alla _ <modelling.data@gmail.com> - 2015-11-22 07:17 -0800
  Re: While or if: function to get next operator or operand James Kuyper <jameskuyper@verizon.net> - 2015-11-20 12:15 -0500
    Re: While or if: function to get next operator or operand Alla _ <modelling.data@gmail.com> - 2015-11-20 10:44 -0800
    Re: While or if: function to get next operator or operand Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-20 12:31 -0800
      Re: While or if: function to get next operator or operand Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2015-11-20 15:38 -0500
        Re: While or if: function to get next operator or operand James Kuyper <jameskuyper@verizon.net> - 2015-11-20 16:14 -0500
          Re: While or if: function to get next operator or operand Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2015-11-20 16:55 -0500
            Re: While or if: function to get next operator or operand Geoff <geoff@invalid.invalid> - 2015-11-20 15:04 -0800
        Re: While or if: function to get next operator or operand Barry Schwarz <schwarzb@dqel.com> - 2015-11-20 20:59 -0800
        Re: While or if: function to get next operator or operand Alla _ <modelling.data@gmail.com> - 2015-11-20 23:50 -0800
          Re: While or if: function to get next operator or operand Alla _ <modelling.data@gmail.com> - 2015-11-21 00:18 -0800
        Re: While or if: function to get next operator or operand Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-22 14:10 -0800
      Re: While or if: function to get next operator or operand Philip Lantz <prl@canterey.us> - 2015-11-22 19:00 -0800
        Re: While or if: function to get next operator or operand James Kuyper <jameskuyper@verizon.net> - 2015-11-23 06:57 -0500
          Re: While or if: function to get next operator or operand Philip Lantz <prl@canterey.us> - 2015-11-23 23:49 -0800
            Re: While or if: function to get next operator or operand James Kuyper <jameskuyper@verizon.net> - 2015-11-24 07:13 -0500
        Re: While or if: function to get next operator or operand Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-23 07:55 -0800
          Re: While or if: function to get next operator or operand Philip Lantz <prl@canterey.us> - 2015-11-23 23:36 -0800
            Re: While or if: function to get next operator or operand Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-27 13:32 -0800
    Re: While or if: function to get next operator or operand Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-11-20 16:13 -0500
      Re: While or if: function to get next operator or operand Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-22 13:54 -0800
  Re: While or if: function to get next operator or operand Geoff <geoff@invalid.invalid> - 2015-11-20 12:19 -0800
  Re: While or if: function to get next operator or operand Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-20 15:32 -0800
    Re: While or if: function to get next operator or operand Chad <cdalten@gmail.com> - 2015-11-20 18:38 -0800
      Re: While or if: function to get next operator or operand Keith Thompson <kst-u@mib.org> - 2015-11-20 19:45 -0800
        Re: While or if: function to get next operator or operand Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-20 20:12 -0800
      Re: While or if: function to get next operator or operand Alla _ <modelling.data@gmail.com> - 2015-11-20 20:45 -0800
        Re: While or if: function to get next operator or operand Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-20 21:04 -0800
      Re: While or if: function to get next operator or operand Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-22 13:57 -0800
    Re: While or if: function to get next operator or operand Alla _ <modelling.data@gmail.com> - 2015-11-20 20:43 -0800

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