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From: schwartz@shire.cs.psu.edu (Scott Schwartz)
Newsgroups: comp.lang.pascal
Subject: I/O (was Re: Standard Pascal)
Message-ID: <SCHWARTZ.89Jul13231501@shire.cs.psu.edu>
Date: 14 Jul 89 03:15:01 GMT
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Sender: news@psuvax1.cs.psu.edu
Organization: Pennsylvania State University, computer science
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In-reply-to: RDK%vm.temple.edu@cunyvm.cuny.edu's message of 13 Jul 89


Robert Keiser writes:

|      WRITELN('WHAT IS YOUR NAME');
|      READLN; (* FORCE A GET *)
|      I := 1;
|      WHILE NOT EOLN DO
|       BEGIN
|        READ(NAME[I]);
|        I := I + 1;
|      END;

| I mean, it would seem that to follow the way Pascal is supposed to
| work with the "look ahead" buffer it would require something special
| to avoid the above example.

A pure implementation of Pascal I/O would have the program block just
after it began executing while it waited for the INPUT buffer to get
filled. 

The most common technique for making interactive I/O work sensibly is
called "lazy I/O", and arranges not to block for input until necessary.

In your example, the "READLN (* FORCE A GET *)" doesn't seem to be
what you really want to do, since it could potentially discard valid
input in addition to filling the buffer.

Beyond that, I would have expected the EOLN to force the (buffer
filling) GET, since that's when you actually require the data.
Similarly, if you had some thing like "foo := input^" as the first use
of "input", the buffer would be filled at that point.  (Of course, the
runtime system should only do the inplicit GET once, to initialize the
input stream, not each time you do that statement.  Some pascal to C
translators screw this up.)

A quick test with Sun's Pascal compiler shows that omitting the READLN
results in a working program, as expected.

--
Scott Schwartz		<schwartz@shire.cs.psu.edu>