Reverse Polish notation (RPN), also known as reverse Łukasiewicz notation, Polish postfix notation or simply postfix notation, is a mathematical notation in which operators follow their operands, in contrast to prefix or Polish notation (PN), in which operators precede their operands. The notation does not need any parentheses for as long as each operator has a fixed number of operands.
A promotional Hewlett-Packard "No Equals" hat from the 1980s – both a boast and a reference to RPN
Stack (abstract data type)
In computer science, a stack is an abstract data type that serves as a collection of elements with two main operations:Push, which adds an element to the collection, and
Pop, which removes the most recently added element.
Similarly to a stack of plates, adding or removing is only practical at the top.
The programmable pocket calculator HP-42S from 1988 had, like nearly all of the company's calculators of that time, a 4-level-stack and could display two of four values of the stack registers X, Y, Z, and T at the same time due to its two-line display, here X and Y. In later models like the HP-48, the number of levels was increased to be only limited by memory size.
