Re: [Pyparsing] Logical operations

[Eike W. <ei...@us...>]

On Tuesday 03 June 2008 09:34, happybrowndog wrote:
> Hello...
> first of all, thanks for your help in my last post on "Function
> parameters that may have brackets" - I solved the problem by going
> through the code samples as you stated and the problem is solved
> now.
>
> Now I am having problems with parsing logical operations.
>
> I have an expression:
> calcexpr = operatorPrecedence( w_operand,[(op_sign, 1,
> opAssoc.RIGHT),(op_mult, 2, opAssoc.LEFT),(op_div, 2,
> opAssoc.LEFT),(op_plus, 2, opAssoc.LEFT),(op_minus, 2,
> opAssoc.LEFT),])
>
> which parses the following correctly: "(3+6)/5".
>
> then I have:
> boolexpr = Forward()
> boolexpr << calcexpr + ("=" ^ ">" ^ "<") + calcexpr +
> Optional(("AND" ^ "OR") + boolexpr)
>
> boolexpr then parses the following correctly: "(3+6)/5 > 1 AND 2 <
> 9"
>
> but returns an error on the following: "((3+6)/5 > 1) AND (2 < 9)"
>
> ultimately, I would like to be able to parse a statement such as:
> "((3+6)/5 > 1) AND ( (2 < 9) OR ( (2+5/12 < 1) AND ( 3 < 4) ) )"
>
> but of course this would also fail.
>
> To try to get to my goal, I try adding Optional("(") and
> Optional(")") to boolexpr, such as:
>
> boolexpr << Optional("(") + calcexpr +  ("=" ^ ">" ^ "<") +
> calcexpr + Optional(")") + Optional(("AND" ^ "OR") + boolexpr)
>
> but this does not resolve the problem.  The brackets are screwing
> things up.  I think the cause may be related to the same reason I
> was having problems previously, but I can't seem to link the two.
>
> Can you please point me in the right direction?

You should should add the additional operators to operatorPrecedence. 
First define:

op_and = Keyword('AND')
op_or = Keyword('OR')
op_eq = Literal('=')
op_lt = Literal('<')
op_gt = Literal('>')

And then add the following to the call to operatorPrecedence.:

(op_and, 2, opAssoc.LEFT),

(op_or, 2, opAssoc.LEFT),

(op_eq, 2, opAssoc.LEFT),
(op_lt, 2, opAssoc.LEFT),
(op_gt, 2, opAssoc.LEFT),


The "AND" belongs to the same level like multiplication, the 'OR' 
belongs to '+' and the and the comparison operators '= < >' belong to 
the end (IMHO). (Everything completely untested.)

HTH, Eike.