HP 20B - Manual (Page 18)

Mathematical Calculations
18
For example, key in 1+ 2 x 5 nPr2 2 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
I
or
=
, followed by an operation key.
Note: pressing
I
or
=
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
I
or
=
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
Stack Level 1
Stack Level 3
-15
12
Stack Level 2 41
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.