Editing Reverse Polish notation (section)

==== {{anchor|HP}}Hewlett-Packard ====
{{main|HP calculators}}
[[File:Hewlett-Packard No Equals hat.jpg|thumb|A promotional Hewlett-Packard "No Equals" hat from the 1980s – both a boast and a reference to RPN]]
{{anchor|Three-level RPN}}[[Hewlett-Packard]] engineers designed the [[Hewlett-Packard 9100A|9100A Desktop Calculator]] in 1968 with reverse Polish notation<ref name="Osborne_1994"/> with only three stack levels with working registers ''X'' ("keyboard"), ''Y'' ("accumulate") and visible storage register ''Z'' ("temporary"),<ref name="Monnier_1968"/><ref name="HP9100A_1968"/> a reverse Polish notation variant later referred to as ''three-level RPN''.<ref name="HP9100A_1998"/> This calculator popularized reverse Polish notation among the scientific and engineering communities.
{{anchor|Four-level RPN|Classical RPN|Automatic Memory Stack}}The [[HP-35]], the world's first handheld scientific [[calculator]],<ref name="Osborne_1994"/> introduced the classical ''four-level RPN'' with its specific ruleset of the so-called ''operational (memory) stack''<ref name="HP35_UM"/><ref group="nb" name="NB_Operational_memory_stack"/> (later also called ''automatic memory stack''<ref name="HP42_OM"/><ref name="HP15C_2011"/><ref group="nb" name="NB_Operational_memory_stack"/>) in 1972.<ref name="Laporte_2014"/> In this scheme, the {{key press|Enter {{larger|↑}}}} key duplicates values into Y under certain conditions (''automatic stack lift'' with ''temporary stack lift disable''), and the top register ''T'' ("top") gets duplicated on drops (''top copy on pop'' aka ''top stack level repetition'') in order to ease some calculations and to save keystrokes.<ref name="HP15C_2011"/> HP used reverse Polish notation on every handheld calculator it sold, whether scientific, financial, or programmable, until it introduced the [[Hewlett-Packard HP-10|HP-10]] [[adding machine]] calculator in 1977. By this time, HP was the leading manufacturer of calculators for professionals, including engineers and accountants.

Later calculators with LCDs in the early 1980s, such as the [[HP-10C]], [[HP-11C]], [[HP-15C]], [[HP-16C]], and the financial [[HP-12C]] calculator also used reverse Polish notation. In 1988, Hewlett-Packard introduced a business calculator, the [[HP-19B]], without reverse Polish notation, but its 1990 successor, the [[HP-19BII]], gave users the option of using algebraic or reverse Polish notation again.

{{anchor|Dynamic stack}}In 1986,<ref name="Wickes_1987"/><ref name="RPLMAN"/> HP introduced [[RPL (programming language)|RPL]], an object-oriented successor to reverse Polish notation. It deviates from classical reverse Polish notation by using a dynamic stack only limited by the amount of available memory (instead of three or four fixed levels) and which could hold all kinds of data objects (including symbols, strings, lists, matrices, graphics, programs, etc.) instead of just numbers. The system would display an error message when running out of memory instead of just dropping values off the stack on overflow as with fixed-sized stacks.<ref name="Wessman_2016"/> It also changed the behaviour of the stack to no longer duplicate the top register on drops (since in an unlimited stack there is no longer a top register) and the behaviour of the {{key press|Enter {{larger|↑}}}} key so that it no longer duplicated values into Y, which had shown to sometimes cause confusion among users not familiar with the specific properties of the ''automatic memory stack''. From 1990 to 2003, HP manufactured the [[HP-48 series]] of graphing RPL calculators, followed by the [[HP-49 series]] between 1999 and 2008. The last RPL calculator was named [[HP 50g]], introduced in 2006 and discontinued in 2015. However, there are several community efforts like [[newRPL]] or [[DB48X]] to recreate RPL on modern calculators.

As of 2011, Hewlett-Packard was offering the calculator models 12C, 12C Platinum, [[HP 17bII+|17bII+]], [[HP 20b|20b]], [[HP 30b|30b]], [[HP 33s|33s]], [[HP 35s|35s]], [[HP 48gII|48gII]] (RPL) and 50g (RPL) which support reverse Polish notation.<ref name="HP_Calc"/>

{{anchor|Entry RPN}}While calculators emulating classical models continued to support classical reverse Polish notation, new reverse Polish notation models feature a variant of reverse Polish notation, where the {{key press|Enter {{larger|↑}}}} key behaves as in RPL. This latter variant is sometimes known as ''entry RPN''.<ref name="HP_Evolves"/>

{{anchor|128-level RPN|Advanced RPN}}In 2013, the [[HP Prime]] introduced a ''128-level'' form of entry RPN called ''advanced RPN''. In contrast to RPL with its dynamic stack, it just drops values off the stack on overflow like other fixed-sized stacks do.<ref name="Wessman_2016"/> However, like RPL, it does not emulate the behaviour of a classical operational RPN stack to duplicate the top register on drops.

In late 2017, the list of active models supporting reverse Polish notation included only the 12C, 12C Platinum, 17bii+, 35s, and Prime. By July 2023, only the 12C, 12C Platinum, the freshly released [[HP 15C Collector's Edition]], and the Prime remain active models supporting RPN.
{{See also|#Community-developed hardware-based calculators{{!}}HP-related community-developed calculators}}