World: r3wp

Hello

Hello, I am struggling to build a parser of equation. The idea is 
a function who would take sth like "SQRT(1-x^2)/(1-x)^3" and return 
a block of rebol code: [divide square-root subtract 1 power x 2 power 
subtract 1 x 3].
Anyone has done it already?

I know Gabriele has done something along those lines, and I'm sure 
others have as well. I don't know of any that return REBOL code. 
Can you be more specific about what problems you're having?

Geocachi, see

http://www.fm.tul.cz/~ladislav/rebol/evaluate.r

(can return Rebol code)

Thanks ladislav... but I end up with my own implementation...  http://www.rebol.org/view-script.r?script=parse-equation.r
But I will check the your implementation to see how it differs :)

Hello, I put a new version of my parse-equation.r script on rebol.org: 
the parser engine is now completed and finalized. I improved the 
recursive implementation to make the code much more readable, understandable 
and elegant. Future improvements should be now limited to adding 
new math functions and a syntax error handling to return usefull 
error messages to the user when she/he makes a syntax error in the 
equation.  http://www.rebol.org/view-script.r?script=parse-equation.r

very nice work !

Thanks... just uploaded a new file on rebol.org... I mistakenly put 
a old one :)

this does not work with signed numbers as in "1 + -2"

indeed, not yet implemented... in the meantime use "1 - 2" instead

And i am not sure how to implement signed numbers... Usinfg the minus 
sign introduces a level of complexiy I want to avaoid in the code. 
I might decide to force the use of a function like neg()... I am 
still thinking about it...

actually, i found a quite straightforward way to handle signed numbers 
:) i will post later today (for now, I am going to ski... shusss!!!!)

Hello sqlab... Signed numbers and signed sub-expression are now properly 
handled :)
>> parse-equation "+(1+-x)**-sqrt(-1--x)"

== [power add 1.0 negate x negate square-root subtract -1.0 negate 
x]

Thanks for pointing my attention to this issue.

http://www.rebol.org/view-script.r?script=parse-equation.r

Very nice script. ^ character can also be added as power operator 
just like **. As in some languages ^ is power operator.

Oh and English comments in the source code please, if possible :)

Just a terminological note: the strings you convert to REBOL expressions 
are not "mathematical equations". They are just mathematical expressions.

an equation has an = sign in the expression

BTW, there is an error in associativity:

>> parse-equation "3-3-3"
== [subtract 3.0 subtract 3.0 3.0]

(you need to take into account, that some operations are left-associative, 
while other operations may be right-associative)

The same error here:

>> parse-equation "3-3+3"
== [subtract 3.0 add 3.0 3.0]

Did you see http://www.rebol.org/view-script.r?script=supercalculator.r&sid=f4jz
(script start in the just last 20 lines)
instead of parse you can use "replace/all" and work evene better

Ladislav, thanks for your comments... I will have a closer look  
on this associativity issue.

MaxV. Yes, I have seen supercalculator but it crashes on simple expression 
like sqrt(1-3)/(1+4)...

MaxV sorry, my mistake... normal it crashes :)

But when i tried it, it fails on an expression... I do not remember 
which one...

MaxV: supercaclulator fails on 1/-(3+2)

regarding the supercalculator - it has wrong associativity as well

like parse-equation.r did yesterday btw :)

Ladislav... sorry, what should be the code returned?

in which case?

3-3+3

add subtract 3 3 3

Do you have an example where this would make a difference?

>> subtract 3.0 add 3.0 3.0
== -3.0

>> add subtract 3.0 3.0 3.0
== 3.0

I think, that counts as a difference

oups :)

Do you think that parsing reversal would fix the problem

not completely

you need to realize, that the ** operator is right-associative

I should review some basic math stuff :))

Ladislav... according to wiki, addition, subtraction, multiplication 
and division are left-assiciative... http://en.wikipedia.org/wiki/Operator_associativity

yes

Now that I take a paper and a pen... left-associativity is indeed 
natural :)

So natural that i forgot about the associativity when thinking on 
how to split an expression i successive blocks! So stupid!

:-D

Thanks for pointing this. Hopefully, I have been clever not to mark 
this version as 1.0.0 :)

I put my %evaluate.r script processing operators with different priorities 
and associativities to the

http://www.rebol.org/view-script.r?script=evaluate.r


as well, since I think there it will be more accessible to the public

Your script is so much more complicated :)... maybe why it works... 
Good idea to put on rebol.org!

A quick doc to show how to use it?

Much more complicated

 - do not forget it processes three different rule sets, each of which 
 uses different priorities and associativities.