HP 20B manual Reverse Polish Notation RPN Mode, RPN Stack

Mathematical Calculations

For example, key in 1+ 2 x 5 nPr22 in Algebraic mode by pressing:

1+2*5:b2:m=. The result is 241.

Note: the calculator is limited to 12 pending operations. An operation is pending when it is waiting for the input of a number or the result of an operation of higher priority.

Reverse Polish Notation (RPN) Mode

To set the calculator in RPN mode, refer to the section titled, The Mode Menu: Setting Preferences in Chapter 1. In RPN mode, numbers are entered first, separated by pressing Ior =, followed by an operation key.

Note: pressing Ior =is optional after entering a number, if the next key pressed is an operation.

Each time you press an operation or function key in RPN, the answer is calculated immediately and displayed. For example, suppose you wanted to add two numbers in RPN, 1 and 2. Press 1I2+. The result, 3.00, is calculated and displayed immediately on the bottom line along with the (+) symbol on the top line.

Note: in RPN mode, when you are in a menu for which Ior =are valid, pressing these keys enters the number, but it also performs the action associated with the key for the menu item, which is generally saving the number in the variable or calculating the item's value.

The RPN Stack

RPN works by placing numbers in storage registers called the stack. The RPN stack has four levels numbered 1-4. The levels are stacked on top of one another. See Figure 3.

Stack Level 4

-15

Stack Level 3

12

Stack Level 2

41

Stack Level 1

23

Figure 3 The RPN Stack

In Figure 3, the stack contains four numbers, 23, 41, 12, and -15. Each level (1-4) contains one number. When a number is typed and entered into the stack by pressing I, this new number is "pushed" into level one of the stack, and each number already in the stack moves up one level. The number in Level 4, -15, is pushed out and is lost.

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