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From: es2a+@andrew.cmu.edu (Eric Stuyvesant)
Newsgroups: comp.sys.handhelds
Subject: HP 48: Weird IFERR | THEN END behavior
Message-ID: <YbKxdFy00WBLM1o15J@andrew.cmu.edu>
Date: 4 Dec 90 17:18:09 GMT
Organization: Class of '91, Carnegie Mellon, Pittsburgh, PA
Lines: 65


     I hate to call this by the b** word, but it at least seems like
incorrect documentation:

     In order to evaluate an expression with certain definite numbers
substituted for names that occur in the expression, HP has provided |. 
Given an expression in level 2 and a list of name-value pairs in level
1, | is supposed to substitute the values for the names in the
expression.

    In order to provide run-time error trapping, HP has provided the
IFERR trap-clause THEN error-clause {ELSE normal-clause} END structure.

From the HP 48 Programmer's Reference Manual, p. 174 (IFERR):

"   If an error occurs during execution of the trap clause:
        1.  The error is ignored.

        [Non-relevant points]

        5.  If Last Arguments is enabled [Flag -55 is clear], the arguments
            to the command that caused the error are returned to the stack.
        6.  Program execution jumps to the error clause.
"  [End Quote]


Now try:


-55 SF Enter                               @ Last Arguments disabled
'INV(X)' Enter
{ X 0 } Enter
\<< IFERR | THEN END \>> Enter             @ | is Alpha-Blue-J
EVAL

This returns nothing (as advertised).

Next, try:

-55 CF Enter                               @ Last Arguments enabled
'INV(X)' Enter
{ X 0 } Enter
\<< IFERR | THEN END \>> Enter             @ | is Alpha-Blue-J
EVAL

This returns 0.

    This behavior is evident on both a REV-A machine and a REV-D machine.

    I presume this is happening because the LASTARG command is returning
the last argument it has seen, in this case, an argument to some
not-user-visible function called by |.  Can anyone shed any light on the
situation?

Thanks in Advance.

P.S.  Am I the only one who thinks HP should include an ML version of
EXCO in their next calculator?
P.P.S. Why can't you single-step through IFERR structures?



-Eric Stuyvesant
es2a+@andrew.cmu.edu
Go MEMS!