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From: edp@jareth.enet.dec.com (Eric Postpischil (Always mount a scratch monkey.))
Newsgroups: comp.sys.handhelds
Subject: Re: OBGET and EXGET
Message-ID: <12069@shlump.nac.dec.com>
Date: 30 May 90 21:16:21 GMT
References: <3ahdinw01@ULKYVX.bitnet>
Sender: newsdaemon@shlump.nac.dec.com
Reply-To: edp@jareth.enet.dec.com (Eric Postpischil (Always mount a scratch monkey.))
Organization: Digital Equipment Corporation
Lines: 36

I don't think OBGET and EXGET are in the HP-48.  They are replaced by
OBJ-> which, for an algebraic input, returns to the stack the
"outermost" function (e.g., "+" in '3+4*5'), the number of arguments of
that function, and the arguments.  Then you can further decompose the
arguments by using OBJ-> again, so you can completely take apart and
reconstruct an algebraic.  This operation is one of the things I (and
probably others) asked HP for; it is a natural way to work with
algebraic expressions.

The OBGET and EXGET operations had a shortcoming in that you could not
use them to take apart and reconstruct algebraics without additional
information.  Viz., I think there exist expressions for which EXGET and
other HP-28 operations cannot be used to determine which subexpressions
are arguments of which functions unless your program has knowledge of
which functions are infix or prefix and how many arguments they take --
so you would need something like built-in tables of functions in order
to write a general algebraic manipulation program.

You can get TRG* in a program with { 'EXP(&A)' 'COS(&A/i)+SIN(&A/i)*i' }
^MATCH.  This will look for EXP applied to any expression and will
convert it to the trigonometric definition.  I think it may perform
multiple expansions if the expressions appear disjointly in the
original; you can defeat that by taking apart the expression first and
presenting only pieces to ^MATCH.  On the other hand, if you want to
replace all occurrences, you need to call ^MATCH repeatedly until it
indicates there are no more matches.

You can use the RULES menu to find the appropriate match pattern.  Just
create an expression with &A in it, such as 'SIN(&A)', or &B . . . if
there are more than one different subexpression.  Then invoke RULES and
transform it as you please.  ->DEF changes the above to
'(EXP(&A*i)-EXP(-(&A*i)))/(2*i)'.  Incorporate the result in a list with
the original expression, and use ^MATCH.


				-- edp