Basic RPL question

[DavidM]

(02-29-2016 03:48 AM)Claudio L. Wrote:  I don't think so, algebraics are stored in RPN form, so evaluating simply means running the object as a stream of commands. There's no parsing at run time as far as I can remember.
The speed penalty may come from intermediate results being symbolic, perhaps, or some additional safety checks, I don't know.

The algebraic is stored in a sequence which resembles an RPN stream, but it is still parsed and re-evaluated when EVAL is invoked.

Case in point (at least on my 50g):

Assuming you have no globals named 'X' or 'Y', enter 'X+Y/2' onto the stack. If you're in Exact mode (system flag 105 clear), the entry will probably* get translated to:

\(X+\frac{Y}{2}\)

Press ENTER to make a duplicate for reference. Then press EVAL and watch what happens:

\(\frac{2\cdot X+Y}{2}\)

...so clearly there is some sort of parsing/re-compiling/reduction operation occurring as a result of EVAL.

*System flags are consulted, which is easy to see if you do the above steps in Approximate mode (system flag 105 set). The results will be different in that mode, and it's entirely possible that other flag settings will affect the outcome as well. I didn't go through every possible combination.