Path: utzoo!attcan!uunet!husc6!bloom-beacon!mit-eddie!bbn!uwmcsd1!csd4.milw.wisc.edu!markh
From: markh@csd4.milw.wisc.edu (Mark William Hopkins)
Newsgroups: comp.lang.pascal
Subject: Re: Number-string conversions
Summary: Verification
Message-ID: <5858@uwmcsd1.UUCP>
Date: 20 May 88 20:00:24 GMT
References: <5817@uwmcsd1.UUCP> <5484@aw.sei.cmu.edu>
Sender: daemon@uwmcsd1.UUCP
Reply-To: markh@csd4.milw.wisc.edu (Mark William Hopkins)
Organization: University of Wisconsin-Milwaukee
Lines: 167


     This is a followup from the previous article and addresses more directly
the response of R. Firth several articles back concerning the nature of 
several number-to-string procedures listed previously:

<MY INTENTIONS>:
>     While the issue is "on the table", I feel it appropriate to give a few 
>procedures to show how the conversions take place when Pascal reads & writes
>in a text file.
...
<FOLLOWED BY LISTINGS FOR ReadInt, ReadReal, WriteInt and WriteReal>
...

<THE INAPPROPRIATE CRITICISM>:
>I find it hard to express my feelings in a manner that is both clear and
>polite.  The above code is evidently rubbish; unfortunately it is typical
>of the stuff produced by amateur software writers - you can find examples
>galore, as bad or worse, in many packages distributed by major institutes
>of education.
>
>These examples would not have survived any one of: accurate specification,
>competent design review, code walkthrough, formal analysis, or thorough
>testing.  I conclude that they were subjected to none of these processes.
>
>Folks, are we surprised there is a "software crisis"?  And what can we,
>as responsible individuals, do about it?

<THE REASONS FOR BEING INAPPROPRIATE>:
(1) The procedure listings are for show only, they are not meant to be run.
    This much is obvious, since Pascal ALREADY has the facilities built-in.

(2) The procedures are meant to SIMULATE Pascal ... for better or for worse
    ... flaws and all ... in order to demonstrate just how Pascal carries
    out the special built-in read procedures and to make explicit the
    limitations and flaws of these built-in procedures.


But in any case, I feel that the criticism should be more directly addressed:

So let us scrutinize the ReadInt procedure to see if it actually does simulate
Pascal's integer read:

THE LISTINGS:
                            P A S C A L :
                     program Pascal(input, output);
                     var I : integer;
                     begin
                        read(I); write(I:1)
                     end.

			   R e a d I n t :
                  program Simulate(input, output);
                  var I : integer;

		  ... (*** Procedure ReadInt inserted here ***)

		  begin
		     ReadInt(I); write(I:1)
                  end.

where the procedure listing is:

          procedure ReadInt(var T : text; var I : integer);
          var
	     Sign : char;
          begin
	     (*** SKIP SPACES ***)
	     while T^ = ' ' do get(T);	{! may run off the end of the file}
             (*** GET THE SIGN ***)
	     if T^ in ['+','-'] then begin
	        read(T, Sign)
             end
	     else Sign := '+';
	     (*** CONVERT THE STRING TO AN INTEGER ***)
	     I := 0;
	     while T^ in ['0'..'9'] do begin
	        I := 10*I + ord(T^) - ord('0');	{! may cause false overflow
						   on '+' if number large}
  	        get(T)
             end;
	    (*** ADJUST FOR THE SIGN ***)
	    if Sign = '-' then I := -I		{! and of course fails on the
						   most negative integer}
         end;

Let us look into some of the supposed flaws of the ReadInt procedure:

                                 (1)
                          O V E R F L O W S
     In all fairness, I should mention that the way a compiler handles
overflow is implementation dependent.  The ReadInt procedure was promised to 
simulate how Pascal reads integers ... for better or for worse.

Script started on Fri May 20 14:13:34 1988
                      T H E   I N P U T   F I L E :
(csd4) 1: more In
123456789012345

 H O W   S T A N D A R D   P A S C A L   M A Y   H A N D L E   O V E R F L O W :
(csd4) 2: pix Pascal.p <In 
Execution begins...
-2045911175
Execution terminated.

           H O W   R e a d I n t   H A N D L E S   O V E R F L O W :
(csd4) 3: pix ReadInt.p <In 
Execution begins...
-2045911175
Execution terminated.

                    T H E   R E L E V A N T   C O D E :
(csd4) 4: more ReadInt.p
 ...
     procedure ReadInt(var T : text; var I : integer);
...
	I := 0;
	while T^ in ['0'..'9'] do begin           <THE MISPLACED CRITICISM>:
	   I := 10*I + ord(T^) - ord('0');	{! may cause false overflow
						   on '+' if number large}
	   get(T)
        end;
...

So much for overflow ...

  				   (2)
         O V E R R E A D I N G   T H E   E N D - O F - F I L E :
     The question asked here is whether ReadInt successfully simulates
the way Pascal reads integers when there are no integers to be read ...

Script started on Fri May 20 14:17:01 1988
                      T H E   I N P U T   F I L E :
(csd4) 1: more In
<EMPTY>

        H O W   S T A N D A R D   P A S C A L   H A N D L E S   I T : 
(csd4) 2: pix Pascal.p <In 
Execution begins...

standard input: Tried to read past end of file

                H O W   R e a d I n t   H A N D L E S   I T :
(csd4) 3: pix ReadInt.p <In 
Execution begins...

standard input: Tried to read past end of file
...
	Error in "ReadInt"+2 near line 9.
...

                    T H E   R E L E V A N T   C O D E :
(csd4) 4: more ReadInt.p
...
     procedure ReadInt(var T : text; var I : integer);
...
     begin
	(*** SKIP SPACES ***)                <THE MISPLACED CRITICISM>:
9 --->	while T^ = ' ' do get(T);	{! may run off the end of the file}
...

"Pray tell, your majesty ... excuse me, I mean 'Your Majesty' ... a simulation 
so perfect that it even simulates the flaws!"

     Now a few words about the procedures themselves.  Nobody in their right
mind would actually set up and run text read procedures that simulated what
was already built-in.  If I were actually going to write procedures to do
text I/O they would have had ERROR CORRECTION facilities built in them.