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From: info-vax@ucbvax.ARPA
Newsgroups: fa.info-vax
Subject: Re: Possible VAX/VMS PL/I bug
Message-ID: <4884@ucbvax.ARPA>
Date: Sat, 16-Feb-85 19:24:37 EST
Article-I.D.: ucbvax.4884
Posted: Sat Feb 16 19:24:37 1985
Date-Received: Mon, 18-Feb-85 05:41:24 EST
Sender: daemon@ucbvax.ARPA
Organization: University of California at Berkeley
Lines: 33

From: hplabs!sdcrdcf!darrelj@BERKELEY (Darrel VanBuer)

PL/I will make lots of implicit conversions for you (probably too many), and
I much prefer to be warned on them, since they may indicate errors like a 
misspelled variable.  A returns declaration guarantees what type is the
result of any such conversions (the same is true in a more limited way in
Fortran).  The following fragment will usually "work" in PL/I the way you'd
expect, but really ought to draw several warnings, just as the RETURN does.
 DCL ANS CHAR(20) VARYING;
 ANS = SQRT('101011'B * 4.5E1);
The way you get rid of the warnings is thru uses of the pseudo functions FIX
and FLOAT for numbers, and PUT STRING(ANS) ... for the numeric to string
conversion.  Just because PL/I is willing to and has rules for all kinds of
strange implicit conversions doesn't mean it isn't bad form.  Some of the
conversions can be fairly expensive.  Better to at least SAY your knowingly
doing something funny.  Some instances of the conversions also give
unexpected results.  The expression 1/3 + 25 results in 5.33333333333333 in
IBM 360 PL/I when you follow all the (explicitly given) coercion rules (it
also generates an OVERFLOW exception if enabled.  The reason is that all
those constants are decimal (not binary, and makes no difference if you
assign result to a binary variable), the rules say the result of 1/3 will be
DECIMAL(15,14) precision and for the add will also be (15,14)--it ought to
be 16,14 but 15 is the limit (and even with a different limit, the 1/3 will
still be (n,n-1) and will still overflow the ADD.

Darrel J. Van Buer, PhD
System Development Corp.
2500 Colorado Ave
Santa Monica, CA 90406
(213)820-4111 x5449
...{allegra,burdvax,cbosgd,hplabs,ihnp4,orstcs,sdcsvax,ucla-cs,akgua}
                                                            !sdcrdcf!darrelj
VANBUER@USC-ECL.ARPA