Path: utzoo!utgpu!watmath!iuvax!rutgers!usc!venera.isi.edu!lmiller
From: lmiller@venera.isi.edu (Larry Miller)
Newsgroups: comp.lang.pascal
Subject: Re: file windows (was Re: Standard Pascal)
Summary: C Uses ungetc--which is uglier?
Message-ID: <9069@venera.isi.edu>
Date: 26 Jul 89 18:07:01 GMT
References: <8616@pyr.gatech.EDU> <18965@paris.ics.uci.edu> <SCHWARTZ.89Jul12023301@shire.cs.psu.edu> <4623@freja.diku.dk> <2929@virginia.acc.virginia.edu>
Reply-To: lmiller@venera.isi.edu.UUCP (Larry Miller)
Organization: Information Sciences Institute, Univ. of So. California
Lines: 76

In article <2929@virginia.acc.virginia.edu> scl@virginia.acc.Virginia.EDU (Steve Losen) writes:
>In article <4623@freja.diku.dk> dat0@rimfaxe.diku.dk (Dat-0 undervisningsassistent) writes:
>>schwartz@shire.cs.psu.edu (Scott Schwartz) writes:
>>>Alastair Milne writes:
>>>| >	while (input^ in [space, tab, newline, cr, formfeed]) do
>>>| >		get (input)
>>>| 
>>>|     How does this constitute not commiting to read the file variable?  You are
>>>|     doing exactly that when you dereference the file variable to get its
>>>|     window.
>
>Here's a good example of how convenient the file buffer variable is:
>
>Suppose you want to allow the user to input a bunch of integers, separated
>by any sequence of non-numeric characters.  For example, the following
>atrocity is legal input:
>
>,,100 ,20,
>,
>
>   30
>40,, 5050
>,,,
>
>The program should process the integers 100, 20, 30, 40, and 5050.
>Here is a trivial piece of Standard Pascal that does the job correctly.
>
>while not eof do begin
>	if (input^ >= '0') and (input^ <= '9') then begin
>		read(number);
>		process(number);
>	end
>	else
>		get(input);
>end
>
>I admit that for most programming languages it is possible to come up
>with simple and elegant solutions to selected bizarre problems.  Too
>bad there isn't one language that is universally applicable.

I've been of the school that Pascal is ugly in this sense: it seems that
the language is lacking because you don't say what you want to do.  Here is
the same code in C, more or less

	#include <stdio.h>
	#include <ctype.h>

	main()
	{
	  int	c = 0;
	  int	val;

	  while (1)
	  {
	    while (!isdigit(c = getchar()))
	      if (c == EOF)
		return 0;
	    ungetc(c, stdin);		/* read one too many character */
	    scanf("%d", &val);
	    printf("%d\n", val);
	  }
	}

In C, we can't surreptitiously read a character (Pascal: input^).  We have
to really and truly read it using getchar().  Once we discover that we've
read one too many characters, we "push it back" into the input stream:
ungetc(c, stdin).  Of course, the input stream must be buffered.

The advantage of C's method is that we don't get obscure behavior: if we
want to read a character, we read it (with getchar).  If we want to "unread"
it, we call ungetc.

Larry Miller				lmiller@venera.isi.edu (no uucp)
USC/ISI					213-822-1511
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