Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!cs.utexas.edu!uunet!yale!haveraaen-magne
From: haveraaen-magne@CS.YALE.EDU (Magne Haveraaen)
Newsgroups: comp.lang.pascal
Subject: Re: Standard Pascal
Message-ID: <66375@yale-celray.yale.UUCP>
Date: 13 Jul 89 21:19:49 GMT
References: <SCHWARTZ.89Jul6030200@shire.cs.psu.edu><20239@adm.BRL.MIL>
Sender: root@yale.UUCP
Reply-To: haveraaen-magne@CS.YALE.EDU (Magne Haveraaen)
Organization: Yale University Computer Science Dept, New Haven CT  06520-2158
Lines: 79

In article <20239@adm.BRL.MIL> RDK%vm.temple.edu@cunyvm.cuny.edu (Robert Keiser) writes:
>[...]  Then to read anything, you
>first would have to issue a blank readln followed by your read.  The code
>would look something like this:
>              .
>              .
>              .
>     WRITELN('WHAT IS YOUR NAME');
>     READLN; (* FORCE A GET *)
>     I := 1;
>     WHILE NOT EOLN DO
>      BEGIN
>       READ(NAME[I]);
>       I := I + 1;
>     END;
>        .
>        .
>        .
>Sorry about all caps but the Cyber doesn't have upper/lower case (normally)
>[...]
>As far as the I/O being one of Pascal's strengths, try explaining the above
>example to a beginner.  I've tried, the response is usually a quizzical look
>and a shaking head.

Explaining this programming style isn't so difficult, unless your
student has a background from ForTran, or, which is sadly a very common
case, the Pascal textbook author has a background in Fortran, and thus
gives all kinds of misleading guidelines on how to program dialogues in
Pascal. In Pascal 'readln' does not read-a-line, but
reads-to-the-next-line. It acts as a synchronization utility,
synchronizing the writeln with the next input line, removing all
unwanted trailing characters from the preceding input line.  That is
why you should not supply the 'readln' procedure with any parameters,
as these parameters will be given values from the previous inputline,
before the synchronization takes place. But some dialects of Pascal did
not conform to this, and it seems that most textbook writers have
tested their programs on compilers that have non-standard input
handling, making the teaching of standard Pascal quite difficult.

Over to the more general question of get/put vs. read/write. The
example skip-blanks is a good one:

	procedure skipBlanks (var input : text);
	(* note input is a file variable parameter, not the textfile input *)
	(* routine advances file pointer over blanks to the point where
	   something interesting may be read	*)
	begin
	while (input^ in [space, tab, newline, cr, formfeed]) do
                get (input)
	end;

So let us use this in a program:

	program niceDialogues (input, output) ;
	:
	var age : integer;
	:
	begin
	:
	writeln('How old are you (type ''N'' if you don''t want to tell)? ');
	skipBlanks(input);
	if input^ in ['0'..'9','N','n']
	then case input^ of
		'0'..'9'	: begin read(age); ... end;
		'N','n'		: writeln('OK - just keep it a secret!')
	     end
	else writeln('I assume you don''t want to tell me anyway.');
	:
	end.

Note how I can rely on the built-in 'read' procedure to convert the
text-string to integers when reading 'age', without having problems
with the non-numeric reply alternative. To do the equivalent without
get/put would require 'skipBlanks' to return the character it stopped
reading (as noted by Scott Schwartz <schwartz@shire.cs.psu.edu>), and
would force me to write my own
'readIntegerFromACharacterVariableAndTextFile' procedure.

Magne