RPN and Calculator Modes: Expression, Algebraic, Direct Algebraic, Normal and RPN

Not every calculator expects you to type the same way. Calc Pro gives you five entry modes, from simple left-to-right math to the stack-based Reverse Polish Notation (RPN) that engineers love. This guide explains each one, with the same examples, so you can pick the mode that suits you.

Choosing a Mode

Open Options → Calculator → Calculator mode and pick one:

Mode	How it works	2 + 3 × 4
Normal	Calculates left to right as you go	20
Algebraic	Order of operations; number first, then function	14
Direct Algebraic	Order of operations; functions as written	14
Expression	Type the whole calculation, then press =	14
RPN	Numbers first, then the operation	14

Normal: Left to Right

Normal mode works like a basic pocket calculator: each operation happens as soon as you press the next one.

Example: 2 + 3 × 4

1. Press 2 + 3
2. Press ×: the display shows 5, because 2 + 3 has already been calculated
3. Press 4 =: the result is 20

Normal mode is great for running totals, like adding up a shopping bill, but it ignores the usual order of operations.

Algebraic: Order of Operations

Algebraic mode follows standard math rules, doing × and ÷ before + and −, and it supports brackets.

Example: 2 + 3 × 4

1. Press 2 + 3 × 4 =
2. The result is 14

Functions act on the number you've already entered:

Example: sin(30°)

1. Make sure the display shows DEG
2. Press 30, then sin
3. The result is 0.5

Direct Algebraic: Write It as You Read It

Direct Algebraic also follows the order of operations, but you press a function before its number, the way it's written on paper.

Example: sin(30°)

1. Press sin, then 30
2. Press =: the result is 0.5

Example: √16 + 2

1. Press √, then 16
2. Press + 2 =: the result is 6

This mode is popular with students because calculations match the textbook.

Expression: See the Whole Calculation

In Expression mode you type the entire calculation, see all of it on the display, and press = when you're ready.

Example: area of a half circle with radius 7.5

1. Type π × 7.5 x² ÷ 2
2. The display shows π×7.5²÷2
3. Press =: the result is 88.35729338221

Because you can see everything you've typed, it's easy to spot a mistake before you press =. Every expression is also saved to the tape.

RPN: Reverse Polish Notation

RPN is the stack-based system made famous by HP calculators. You enter the numbers first, then the operation, so you never need brackets or an = key.

How the Stack Works

Numbers wait on a stack. With the default 4-level stack, the levels are called X (the one on the display), Y, Z and T. You can change the stack size to 2, 4, 5, 10 or 50 in Options → Calculator → RPN stack size.

• Enter (which replaces the = key) pushes the number on the display up the stack, ready for the next one
• An operation like + or × combines X and Y and leaves the result in X
• A one-number function like x² or √ works on X straight away

RPN Keys

Key	What it does
Enter	Push the displayed number onto the stack
X<>Y	Swap the bottom two values
R Up	Rotate the stack up
Roll	Rotate the stack down
Drop	Remove the bottom value

Example 1: 2 + 3 × 4

1. Press 3 Enter 4 ×: the display shows 12
2. Press 2 +: the result is 14

Example 2: (3 + 4) × (5 + 6)

1. Press 3 Enter 4 +: the display shows 7
2. Press 5 Enter 6 +: the display shows 11, and 7 waits on the stack
3. Press ×: the result is 77

No brackets needed: the stack remembers the 7 while you work out the 11.

Example 3: √(3² + 4²)

1. Press 3 x²: the display shows 9
2. Press 4 x²: the display shows 16
3. Press +: the display shows 25
4. Press √: the result is 5

Which Mode Should I Use?

• Quick totals and everyday math: Normal
• Students: Direct Algebraic or Expression, to match the textbook
• Used to a classic scientific calculator: Algebraic
• Engineers and HP fans: RPN

Switching is free and instant, so try a couple. For the rest of the settings, see Customizing Calc Pro.

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