Expression-Operator Ordering

KermMartian · Posted: 07 Nov 2011 11:24:10 pm Post subject: Expression-Operator Ordering

Are you a die-hard RPN fan? Can you not live without LISP? Do those both disgust you? Let's duke it out, tell us what you like (and why! The why is very important. No one-word posts).

If you're not familiar with these concepts, read up on reverse Polish notation (RPN) (postfix notation), Polish notation (prefix notation), and infix notation.
_________________

Ashbad · Posted: 07 Nov 2011 11:28:21 pm Post subject: Re: Expression-Operator Ordering

I can't wrap my mind around RPN well, but prefix notation seems fun and intuitive, in a sense, because I feel it's thinking more in terms of robust functions than just plain old operators. Of course, I'm best with infix notation, because I of course, learned it first, like virtually everyone else has. However, when you think about it, if you weren't used to it, it would seem a bit strange to put an operator between two numbers, rather than before or in a logical order of operations.

Qwerty.55 · Expert Joined: 08 Dec 2010 Posts: 613

As much as I hate actually using RPN in real life, almost all of my programs involving equation parsing use RPN for the internal representations of functions. It minimizes the total characters in the representation string while remaining unambiguous, and ridiculously easy to convert to a syntax tree. For computers, it is the best expression ordering syntax.
_________________
∂²Ψ -(2m(V(x)-E)Ψ
----- = -------------
∂x² ℏ²Ψ

_player1537 · Guru-in-Training Joined: 25 Nov 2009 Posts: 2958

Prefix, of course. I don't have any reasons except: it makes you think, and when you come back to it later, it's easier to tell what you meant. I would say that you don't have to include extra parentheses to make sure it interprets (3+2)*4 correctly, but the same statement in Scheme would be (* (+ 3 2) 4), which uses twice as many characters and even more spaces.

That said, I also enjoy RPN for a similar reason, it really pushes your mind to behave like a stack. When I was working on a stack-based programming language, just trying to wrap my head around code that would test the interpreter was extremely difficult.

Of course, infix notation is the easy one, but I find that I still add many spaces to make sure it looks good, and that I'll be able to read it later.
_________________
http://tanner.myserverathome.com

christop · Posted: 22 Nov 2011 12:42:36 am Post subject:

Ummm, RPN is Postfix notation (though you already know that, I'm sure).

RPN is indeed very handy with asm programming: push operands onto the stack (possibly treating one or more registers as a virtual top of the stack) and then perform operations on the values on the top.

Prefix notation is also pretty common in many (if not most) languages, such as when calling functions: