# HP -28S Manual Page 304

• HDirect formula entry" calculators, and BASIC language computers
that have an immediate-execute mode, allow you to key in an en-
tire expression in its ordinary algebraic form, then compute the
result when you press a termination key (variously labeled
I
ENTER
I,
I
ENDLINE
I, I
RETURN
I,
and so on). This approach has the advantage
of preserving the correspondence between written expressions and
keystrokes, but usually the disadvantage of providing no intermedi-
ate results. (The HP-71B CALC mode is an exception.) You have to
know the full form of an expression before you start to enter it-it
is difficult to Hwork your way through a problem," varying the cal-
culation according to intermediate results.
Getting Used to the HP-28S
HP-28S operating logic is based on a mathematical logic known as
HPolish Notation," developed by the Polish logician Jan
~ukasiewicz
(Wookashye'veech)
(1878-1956). Conventional algebraic notation
places arithmetic operators
between
the relevant numbers or variables
when evaluating algebraic expressions.
~ukasiewicz's
notation speci-
fies the operators
before
the variables. A variation of this logic
specifies the operators
after
the variables-this is termed HReverse Pol-
ish Notation," or HRPN" for short.
The basic idea of RPN is that you enter numbers or other objects into
the calculator first, then execute a command that acts on those entries
(called Harguments"). The Hstack" is just the sequence of objects wait-
ing to be used. Most commands return their results to the stack,
where they can then be used as arguments for subsequent operations.
The HP-28S uses an RPN stack interface because it provides the nec-
essary flexibility to support the wide variety of HP-28S mathematical
capabilities in a uniform manner. All calculator operations, including
those that can not be expressed as algebraic expressions, are per-
formed in the same manner-arguments from the stack, results to the
stack.
C: Notes for Algebraic Calculator Users
303