Hey guys!
I'm trying to build a program that will evaluate boolean expressions. For example:
a>(b>(a^b)) or maybe ¬a>(a^b).
It doesn't really matter if you know boolean logic or not since i'm ok doing the calculations. The only part i'm struggling with is parsing the formula. I need to read in a formula entered into the command line and then split it into it's subformals. For the first example, the subformals would be:
b>(a^b)
a^b
a
b
These values, including the original formula, will then be stored in an array but for the life of me i can't get it to work. I've played around with stringTokenizer but very quickly realized that wasn't going to be of much use and i've also had a go with stream Tokenizer and attempted recursive descent parsing but to no avail. Any help will be much appreciated.
What problems are you running into? It sounds to me like some kind of recursion is the way to go. What's not working?
Well, when i tried using string and stream tokenizer i just hit a bit of a brick wall. I realised stringtokenizer was way too basic since all i could really get it to do was use brackets as delimiters but this obviously didn't produce all the subformals that were required. For example, on the example i used in my original post, the subformal b>(a^b) was left out of the result set. streamTokenizer didn't seem much more useful and then when i played around with recursive descent parsing i just got kinda lost to be honest. I'm not really the best of coders and i'm not too sure how to proceed with the program. I feel like I may be missing something really obvious.
Kermit, here's a bit to get you started. First there are two kinds of operators unary and binary. Unary just means it effects a single expression and binary means it works on two.
So ¬ is unary
and > and ^ are binary
Then you have expressions which are either a single item like "a" or "b" or compound like
(a > b)
Any time you come to a compound expression you need to evaluate the smaller parts (note that a compound expression can be made up of other compound expressions. So this is a compound expression:
(a > b)
and here's another that's a compound expression made up of two compound expressions:
(a > b) > (b > c)
You'll need to keep track of parenthesis to tell whether it's compound or not. I think going character by character and keeping track of open and closed parentheses (to see whether you're inside a compound expression) is the way to go.
I hope that gets you started...
Thanks for the advice! I was actually just thinking about doing this the way you just suggested. It's just turning the idea into working code... I'll have a mess around and see what I can come up with. Thanks again
You might find that stacks are pretty handy for keeping track of those compound expressions.
Hey guys, i've been playing around with this for a quite a while now and i'm still stuck. I've managed to get the program to split simple compound formulas like (a^b) v (b) but i can't get more complex compounds to work such as (a(b(a^b). It's the brackets that are doing it cause i can't get it to keep track of multiple opening brackets and have it know where the correct closing bracket is. *sigh*
Are you using stacks to keep track of how deep into a compound expression your in?
Post up some code and I'll take a look...
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