Typo correction:

In the 2nd sentence of my post, I meant to say:

RPN has at least one useful advantage over FPN.

Michael Ossipoff

Either one requires a stack, but an operation consumes less space than a number, so the FPN stack takes fewer resources to implement the worst case stack.

I could be wrong about this.

That's a weak advantage. It takes more instructions (calculator key presses) to code with FPN.
RPN 3,15+ (where comma is <enter>)
FPN +3,15, <-- extra comma

Thanks for pointing out the possible worst-case-memory-use advantage of FPN.

Maybe in the earlier days of computers, such an advantage could have been crucial.

And thanks also for pointing out the fewer-instructions/keypresses advantage of RPN.

I hadn't looked at it closely enough to notice that.

It adds to RPN's other advantage--the convenience of entering each operation as it's encountered among the operands.(Instead of first initially going through and entering all the operations in reverse order of occurrence among the operands).

Thanks again for pointing those things out.

Michael Ossipoff