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> I find it nearly impossible to parse this without actually maintaining the stack in my mind. You don't write out RPN in a long sequence like that. I'm a devo

by jjav 3y ago

> I find it nearly impossible to parse this without actually maintaining the stack in my mind. You don't write out RPN in a long sequence like that. I'm a devoted RPN advocate and seeing "x y + 2 ^ = x 2 ^ 2 x y * * y 2 ^ + +" is indeed difficult to parse. But that's not at all how you use RPN. You compute the problem going from smaller units to larger units getting intermediate results as you go and combining them. You'd never write out the sequence as you did and try to follow it after the fact.

tonyarkles 3y ago
I went through EE using an HP 49G+ and continue to this day using RPN in Emacs calc-mode. >> I find it nearly impossible to parse this without actually maintaining the stack in my mind. > You compute the problem going from smaller units to larger units getting intermediate results as you go and combining them That's totally it. I've tried doing infix/standard algebraic notation on a calculator and unless the screen is big enough to see all of your brackets you're almost certainly going to screw up one of the big terms in a fraction or something like that. I think a good different way of stating the "intermediate results" part is that instead of computing the solution from left to right you're computing the solution from the inside-out. By far the most satisfying part of doing computation in RPN is that moment at the end when you've got a big stack full of intermediate results and you just do that "+ + + + +" to add them all up and bam! Answer! Edit: I had two thoughts back-to-back after writing that. First thought: I do actually struggle sometimes with doing math in Lisp because prefix notation isn't a way that I'm used to doing math. Which led to "Hmmm... what if you did Lisp but with postfix notation instead of prefix notation" Second though: "Ohhh... that's Forth. And you don't need brackets at all..."
- thaumasiotes 3y ago
  > Ohhh... that's Forth. And you don't need brackets at all... You still need them if you want variadic functions. Symmetrically, you don't need parentheses in lisp if you don't want variadic functions, as long as you're willing to draw an explicit distinction between calling a function and referring to it. You might as well just make that distinction by using the parentheses, though.
samatman 3y ago
I'm convinced a large part of the benefit people see from RPN calculators is that you have to figure out the order of operations before you start. It forces a way of thinking about the equation which is less error-prone, you're more likely to notice mistakes as well. Takes a little getting used to, but the interesting thing is that analyzing an equation for the proper order of operations on an RPN calculator is not at all an error-prone process. It's a careful process, and it forces an understanding of the equation itself which just churning through it on an infix calculator does not.
xigoi 3y ago
How would you write this algebraic identity?
- jjav 3y ago
  You write equations in the usual algebraic form. You don't write RPN on paper (or screen), that's not a thing. When you're going to compute a value on the calculator, that's where you use RPN starting from the innermost computations working outwards, because you have a stack to keep all the intermediate results handy. Again for like 2+2 it doesn't make any difference, but if you have a long formula with lots of values and different operations, it is much easier to do correctly and fast with RPN.
rvba 3y ago
But this was a super easy sequence. Can you give an example how it should be written?

by jjav 3y ago

tonyarkles 3y ago

I went through EE using an HP 49G+ and continue to this day using RPN in Emacs calc-mode. >> I find it nearly impossible to parse this without actually maintaining the stack in my mind. > You compute the problem going from smaller units to larger units getting intermediate results as you go and combining them That's totally it. I've tried doing infix/standard algebraic notation on a calculator and unless the screen is big enough to see all of your brackets you're almost certainly going to screw up one of the big terms in a fraction or something like that. I think a good different way of stating the "intermediate results" part is that instead of computing the solution from left to right you're computing the solution from the inside-out. By far the most satisfying part of doing computation in RPN is that moment at the end when you've got a big stack full of intermediate results and you just do that "+ + + + +" to add them all up and bam! Answer! Edit: I had two thoughts back-to-back after writing that. First thought: I do actually struggle sometimes with doing math in Lisp because prefix notation isn't a way that I'm used to doing math. Which led to "Hmmm... what if you did Lisp but with postfix notation instead of prefix notation" Second though: "Ohhh... that's Forth. And you don't need brackets at all..."

thaumasiotes 3y ago
> Ohhh... that's Forth. And you don't need brackets at all... You still need them if you want variadic functions. Symmetrically, you don't need parentheses in lisp if you don't want variadic functions, as long as you're willing to draw an explicit distinction between calling a function and referring to it. You might as well just make that distinction by using the parentheses, though.

samatman 3y ago

I'm convinced a large part of the benefit people see from RPN calculators is that you have to figure out the order of operations before you start. It forces a way of thinking about the equation which is less error-prone, you're more likely to notice mistakes as well. Takes a little getting used to, but the interesting thing is that analyzing an equation for the proper order of operations on an RPN calculator is not at all an error-prone process. It's a careful process, and it forces an understanding of the equation itself which just churning through it on an infix calculator does not.

xigoi 3y ago

How would you write this algebraic identity?

jjav 3y ago
You write equations in the usual algebraic form. You don't write RPN on paper (or screen), that's not a thing. When you're going to compute a value on the calculator, that's where you use RPN starting from the innermost computations working outwards, because you have a stack to keep all the intermediate results handy. Again for like 2+2 it doesn't make any difference, but if you have a long formula with lots of values and different operations, it is much easier to do correctly and fast with RPN.

rvba 3y ago

But this was a super easy sequence. Can you give an example how it should be written?