RE: Evaluate complex conditions

[Richard Casey <casey_r@xxxxxxxxxxxxxxxx>]

Hmm. Sounds like fun! I remember a college project to parse an arithmetic 
expression (with parentheses) and return the result; I imagine similar 
logic would work for you. Let me see if I can remember! I don't know how 
you are currently parsing your conditional statement; I just assume you 
wind up with TRUE or FALSE at the end.

To start off, enclose your original conditional statement in parentheses. 
That way you don't have to treat the beginning and end of the statement as 
a special case. As you process the conditional statement, whenever you hit 
a left parenthesis, save your current status and begin processing as though 
you were examining a new statement. You'll need an array to keep track of 
your status so you can handle multiple levels of parentheses. When you hit 
a right parenthesis, take the value of the statement evaluated so far (TRUE 
or FALSE) and jump back to the previous level. The TRUE/FALSE value would 
replace the part of the statement that was in parentheses. Continue until 
you get to the end of your conditional statement. You'll need to monitor 
your array counter. Ideally, the array will be empty when you're done! 
Otherwise, you have too many left or right parentheses.

Let's take a look at your example statement

        a=b and (c=d or c=e or c=f)

Using the above logic, the program would evaluate "(c=d or c=e or c=f)" as 
an individual statement.
Let's assume that evaluates as TRUE. Then the program would "see" the 
statement as

        a=b and TRUE

To illustrate multiple levels of parentheses, let's try

        a=b and (c=d and (e=f or e=g))

The innermost statement "(e=f or e=g)" would be evaluated as an individual 
statement (let's assume TRUE).
The program would then "see"

        a=b and (c=d and TRUE)

The new innermost statement "(c=d and TRUE)" would be evaluated as an 
individual statement (let's assume FALSE).
The program would then "see"

        a=b and FALSE


Hopefully this will give you some ideas to work with. Let me know if I need 
to clarify.

Richard Casey


-----Original Message-----
From:   Terry Grider [SMTP:tgrider@arkansas.net]
Sent:   Monday, October 23, 2000 12:44 PM
To:     MIDRANGE-L@midrange.com
Subject:        Re: Evaluate complex conditions

Sorry guys,  I didn't do a very good job of explaining what I was looking 
for.
I don't mean I want code that is an example of testing for complex 
conditions, I
mean I'm looking for code that will evaluate statements like the one in my
original example and determine the truth of the statement.  You see, I'm 
writing
a query tool and I want the user to be able to provide selection criteria 
in an
english style IF format similar to cobol or other query tools I've seen.  I 
can
currently handle simple criteria with "and/or" connectors, but I can't seem 
to
come up with a good method for handling multiple levels of parenthesis.

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