Calculator: Support Reverse Polish Notation (RPN)

[tigerhawkvok]

Problem Statement

Reverse Polish Notation / RPN / Postfix mode is an alternate calculator entry mode that is very powerful (leveraging a stack does this!) and should be simple enough to implement.

This behaviour can be seen in physical HP scientific calculators, or the calc mode in Emacs.

Evidence or User Insights

It's a simple addition with minimal overhead that would please some potential users that right now need to seek other solutions.

Goals

Users can toggle an RPN mode of entry
Low-Fidelity Concept

Simple psuedocode:

#! python3

# Or a function list, or something to check if input is "function" or "input"
functionMap = { 
    "+": lambda x, y: x + y,
    # ....
    "mod": lambda x, y: x % y
}

stack = []
while True:
    userInput = getInput()
    # If it's not a function, add it to the stack
    if userInput not in functionMap.keys():
        stack.append(userInput)
    else:
        # Evaluate the last two items on the stack.
        # In real usage, the last stack item is often implicit, eg, 
        # 27 [enter] 3 / 
        # rather than
        # 27 [enter] 3 [enter] /
        b = stack.pop()
        a = stack.pop()
        stack.append(functionMap[userInput](a, b))

[MicrosoftIssueBot]

This is your friendly Microsoft Issue Bot. I've seen this issue come in and have gone to tell a human about it.

[grochocki]

Thanks for the feature suggestion! This is really well thought-out, and we really appreciate it. Let's keep this idea open for discussion so the community has the chance to provide feedback.

[rbeesley]

Shunting-yard algorithm? The first time I considered this, I thought it would make sense to reuse the history capability as a stack, and this would have the added benefit of being able to swap or move results around the stack by drag and drop. But the calculator engine is highly fixed how it works. I now think it might be easier and less disruptive to have a mode (like scientific) which would maintain its own stack and just use a shunting-yard to convert to Infix for the calculator engine. The equals key (=) would just become Enter, and you'd loose the need for the parenthesis. Postfix (RPN) to Infix is pretty straight forward and actually easier to implement than Infix to Postfix, but it'd also be handy to have an operation which converts expressions back and forth without evaluating until the Enter/Equals key is pressed.

(1+2)/3 could be written Infix and pressing the RPN key would transform it to 1 2 + 3 / for the user and swapping the mode at the same time. Pressing Enter/Equal at this time would evaluate. Where this would be unusual is initially being in RPN mode where normally the operator carries out an evaluation, so it would normally convert like 1 2 + → 3 → 33 / → 1, where the values in bold are what are added to the stack/output.

I've been waiting for this mode for 20 years.

[AtomicMegaNerd]

This is the one feature I wish that Windows calculator had. Once you get used to RPN it is really hard to go back. The calculator on macOS offers this option and every other device I use has some kind of RPN calculator setup on it. I really like the Fluent design interface on Windows calculator and this would really make me want to use this more if it offered this as an option!

[dgua]

Calculator is USELESS without RPN mode.

[grochocki]

We reviewed the pitch and would love to explore this idea further! I think this pitch is a great start, but there are still some open questions. Moving this into planning to iron out some of the details. Keep in mind that not all ideas that make it into the planning phase are guaranteed to make it to release.

A couple top-of-mind open questions:

• This pitch could be strengthened with more evidence/user insights. RPN surely has historical usage, but is it still actively taught/used? If so, in which fields?
• How can we ensure that users do not accidentally get into RPN mode? This is _extremely_ important to prevent confusion for typical/casual users.
• How does this affect display output and history items?

I created calculator-specs/rpn to track progress. For more information on next steps, check out our spec workflow.

[arigit]

huge +1 on this request. This feature is overdue by some decades now.
MacOS calculator has included RPN support for a very long time. Take a look at: https://support.apple.com/en-in/guide/calculator/welcome/mac for inspiration on a reasonably well done implementation.

[michaelglasson]

RPN is the best way I know to do nested calculations without the need for parentheses. I strongly support this proposal.

[Artoriuz]

Leaving this comment here as a form of appreciation. RPN mode would be amazing to have and a huge time saver for anyone used to using RPN calculators.

[gustavoavellar]

+1

[bazzol]

+1 this is really needed for many people!