I'm looking to write a Truth Table Generator as a personal project.
There are several web-based online ones here and here.
(Example screenshot of an existing Truth Table Generator)
I have the following questions:
How should I go about parsing expressions like: ((P => Q) & (Q => R)) => (P => R)
Should I use a parser generator like ANTLr or YACC, or use straight regular expressions?
Once I have the expression parsed, how should I go about generating the truth table? Each section of the expression needs to be divided up into its smallest components and re-built from the left side of the table to the right. How would I evaluate something like that?
Can anyone provide me with tips concerning the parsing of these arbitrary expressions and eventually evaluating the parsed expression?
This sounds like a great personal project. You'll learn a lot about how the basic parts of a compiler work. I would skip trying to use a parser generator; if this is for your own edification, you'll learn more by doing it all from scratch.
The way such systems work is a formalization of how we understand natural languages. If I give you a sentence: "The dog, Rover, ate his food.", the first thing you do is break it up into words and punctuation. "The", "SPACE", "dog", "COMMA", "SPACE", "Rover", ... That's "tokenizing" or "lexing".
The next thing you do is analyze the token stream to see if the sentence is grammatical. The grammar of English is extremely complicated, but this sentence is pretty straightforward. SUBJECT-APPOSITIVE-VERB-OBJECT. This is "parsing".
Once you know that the sentence is grammatical, you can then analyze the sentence to actually get meaning out of it. For instance, you can see that there are three parts of this sentence -- the subject, the appositive, and the "his" in the object -- that all refer to the same entity, namely, the dog. You can figure out that the dog is the thing doing the eating, and the food is the thing being eaten. This is the semantic analysis phase.
Compilers then have a fourth phase that humans do not, which is they generate code that represents the actions described in the language.
So, do all that. Start by defining what the tokens of your language are, define a base class Token and a bunch of derived classes for each. (IdentifierToken, OrToken, AndToken, ImpliesToken, RightParenToken...). Then write a method that takes a string and returns an IEnumerable'. That's your lexer.
Second, figure out what the grammar of your language is, and write a recursive descent parser that breaks up an IEnumerable into an abstract syntax tree that represents grammatical entities in your language.
Then write an analyzer that looks at that tree and figures stuff out, like "how many distinct free variables do I have?"
Then write a code generator that spits out the code necessary to evaluate the truth tables. Spitting IL seems like overkill, but if you wanted to be really buff, you could. It might be easier to let the expression tree library do that for you; you can transform your parse tree into an expression tree, and then turn the expression tree into a delegate, and evaluate the delegate.
Good luck!
I think a parser generator is an overkill. You could use the idea of converting an expression to postfix and evaluating postfix expressions (or directly building an expression tree out of the infix expression and using that to generate the truth table) to solve this problem.
As Mehrdad mentions you should be able to hand roll the parsing in the same time as it would take to learn the syntax of a lexer/parser. The end result you want is some Abstract Syntax Tree (AST) of the expression you have been given.
You then need to build some input generator that creates the input combinations for the symbols defined in the expression.
Then iterate across the input set, generating the results for each input combo, given the rules (AST) you parsed in the first step.
How I would do it:
I could imagine using lambda functions to express the AST/rules as you parse the tree, and building a symbol table as you parse, you then could build the input set, parsing the symbol table to the lambda expression tree, to calculate the results.
If your goal is processing boolean expressions, a parser generator and all the machinery that go with is a waste of time, unless you want to learn how they work (then any of them would be fine).
But it is easy to build a recursive-descent parser by hand for boolean expressions, that computes and returns the results of "evaluating" the expression. Such a parser could be used on a first pass to determine the number of unique variables, where "evaluation" means "couunt 1 for each new variable name".
Writing a generator to produce all possible truth values for N variables is trivial; for each set of values, simply call the parser again and use it to evaluate the expression, where evaluate means "combine the values of the subexpressions according to the operator".
You need a grammar:
formula = disjunction ;
disjunction = conjunction
| disjunction "or" conjunction ;
conjunction = term
| conjunction "and" term ;
term = variable
| "not" term
| "(" formula ")" ;
Yours can be more complicated, but for boolean expressions it can't be that much more complicated.
For each grammar rule, write 1 subroutine that uses a global "scan" index into the string being parsed:
int disjunction()
// returns "-1"==> "not a disjunction"
// in mode 1:
// returns "0" if disjunction is false
// return "1" if disjunction is true
{ skipblanks(); // advance scan past blanks (duh)
temp1=conjunction();
if (temp1==-1) return -1; // syntax error
while (true)
{ skipblanks();
if (matchinput("or")==false) return temp1;
temp2= conjunction();
if (temp2==-1) return temp1;
temp1=temp1 or temp2;
}
end
int term()
{ skipblanks();
if (inputmatchesvariablename())
{ variablename = getvariablenamefrominput();
if unique(variablename) then += numberofvariables;
return lookupvariablename(variablename); // get truthtable value for name
}
...
}
Each of your parse routines will be about this complicated. Seriously.
You can get source code of pyttgen program at http://code.google.com/p/pyttgen/source/browse/#hg/src It generates truth tables for logical expressions. Code based on ply library, so its very simple :)
Related
I am new to c#. I have a question about parsing a string. If i have a file that contains dome lines such as PC: SWITCH_A == ON or a string like PC: defined(SWITCH_B) && SWITCH_C == OFF. All the operators(==, &&, defined) are string here and all the switch names(SWITCH_A) and their values are identifiers(OFF). How do i parse these kind of string? Do i first have to tokenize them split them by new lines or white spaces and then make an abstract syntax tree for parsing them? Also do i need to store all the identifiers in a dictionary first? I have no idea about parsing can anyone help? an tell me with an example how to do it what should be the methods and classes that should be included? Thanks.
Unfortunately, Yes. You have to tokenize them if the syntax that you are parsing is something custom and not a standard syntax where a compiler already exists for parsing the source.
You could take advantage of Expression Trees. They are there in the .NET Framework for building and evaluating dynamic languages.
To start parsing the syntax you have to have a grammar document that describes all the possible cases of the syntax in each line. After that, you can start parsing the lines and building your expression tree.
Parsing any source code typically goes a character at a time since each character might change the entire semantics of the piece that is being parsed.
So, i suggest you start with a grammar document for the syntax that you have and then start writing your parser.
Make sure that there isn't anything already out there for the syntax you are trying to parse as these kind of projects tend to be error-prone and time consuming
Now since your high-level grammar is
Expression ::= Identifier | IntegerValue | BooleanExpression
Identifier and IntegerValue are constant literals in the source, so you need to start looking for a BooleanExpression.
To find a BooleanExpression you need to look for either BooleanBinaryExpression, BooleanUnaryExpression, TrueExpression or FalseExpression.
You can detect a BooleanBinaryExpression by look for the && or == operators and then taking the left and right operands.
To detect a BooleanUnaryExpression you need to look for the word defined and then parse the identifier in the parantheses.
And so on...
Notice that your grammar supports recursion in the syntax, look at the definition of the AndExpression or EqualsExpression, they point back to Expression
AndExpression ::= Expression '&&' Expression
EqualsExpression ::= Expression '==' Expression
You got a bunch of methods in the String Class in the .NET Framework to assist you in detecting and parsing your grammar.
Another alternative is that you can look for a parser generator that targets c#. For example, see ANTLR
I have a search criteria stored in a string:
string Searchstr = "(r.Value.Contains("PwC") || (r.Value.Contains("Canadian") && r.Value.Contains("thrive"))) || (r.Value.Contains("Banana") && r.Value.Contains("Gayle"))"
I want to use this in a If statement to check the values:
if(searchstr)
{
then do this....
}
but the if should have a searchstr as boolean.
How to convert this to boolean?
EDIT: The requirement is to give search criteria dynamically in a text box in the following format - "PwC OR (Canadian AND thrive)".
Which will be used to search an XML file.
Therefore I have loaded an XML file and want to have a Where condition in LINQ for which I need to use Dynamic LINQ but string is not allowed in that and also I have some braces to deal with.
Thinking of that I have taken the resultset from the XML(The tag value which i need to search)
var selectedBook = from r in document.Root.Descendants("Archives").Elements("Headline").Elements("Para")
select r;
and would ideally like to try something like:
var query=selectedbook.Where(searchstr)
OR
if(searchstr){....then do this}
You will need to do a bit of work to make this happen, but it is possible.
You should have a look at the dynamic LINQ library. This allows you to specify LINQ conditions (and other clauses) as strings and execute them just like LINQ operators.
Start with the explanation on ScottGu's blog and follow the links:
http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx
I'm assuming the string is going to reference only a specific set of objects (r or r.Value in this case, for example - or anything else you want, as long as you know it beforehand). If this is the case, then:
Create a delegate that takes the objects (that may be referenced) as parameters
and returns a bool, as you want.
Programmatically write a small C# source file in memory that defines the query
as the body of a method (with a fixed name, preferably) that conforms to the delegate specified above.
Use the CSharpCodeProvider class to compile an assembly
with your custom function that returns the bool you want.
Run the dynamically written and compiled code from your main program.
Well as you may guess it is not going to be straight forward but at the same time it is not as hard a problem as it seems
You can perform a few steps to get what you want:
Get the search expression as input (for e.g. "PwC OR (Canadian AND thrive)")
Write an extension method on XElement that returns true and takes the search criteria as input. You will then be able to use
var selectedBook = from r in
document.Root.Descendants("Archives").Elements("Headline").Elements("Para")
where r.SatisfiesCriteria(searchCriteria)
select r;
Write a parser class that parses searchCritera and stores it in parsed format. (for e.g. you can convert it into postfix notation). This is quite easy and you can use standard algorithm for this. For your purpose OR, AND will be operators and PwC etc. will be operands. Parenthesis will get removed as part of parsing.
Now simply invoke this parser from with in your extension method and then evaluate the postfix expression you get. This again can be done through standard stack based evaluation. Infact it would be better if you parse the criteria once and then only evaluate in where. While evaluating you need to replace the operands with r.Value.Contains
It seems like a good scenario for http://scriptcs.net/
I have a database here with certain rules I need to apply to a a bunch of Strings, they're expressions that can occur within the Strings. They are expressed like
(word1 AND word2) OR (word3)
I can't hardcode those (because they may be changed in the database), so I thought about programmatically turning those expressions into Regex patterns.
Has anybody done such a task yet or has an idea on how to do this the best way?
I'm not wuite sure about how to deal with more complex expressions, how to take them apart and so on.
Edit: I'm using C# in VisualStudio / .NET.
The data is basically directory paths, a customer wants to get their documents organized, so the String I'm having are paths, the expressions in the DB could look like:
(office OR headquarter) AND (official OR confidential)
So if the file's directory path contains office and confidential, it should match.
Hope this makes it clearer.
EDIT2:
Heres some dummy examples:
The paths could look like:
c:\documents\official\johnmeyer\court\out\letter.doc
c:\documents\internal\appointments\court\in\september.doc
c:\documents\official\stevemiller\meeting\in\letter.doc
And the expressions like:
(meyer or miller) AND (court OR jail)
So this expression would match the 1st path/ file, but not the 2nd and 3rd one.
No answer, but a good hint:
The expressions you have are actual trees constructed by the parentheses. You need a stack machine to parse the text into a (binary) tree structure, where each node is an AND or OR element and the leaves are the words.
Afterwards, you can simply construct your regex in whatever language you need by walking the tree using depth first search and adding prefix and suffix data as needed before/after reading the subtree.
Consider an abstract class TreeNode having a method GenerateExpression(StringBuilder result).
Each actual TreeNode item will be either an CombinationTreeNode (with a CombinationMode And/Or) or an SearchTextTreeNode (with an SearchText property).
GenerateExpression(StringBuilder result) for CombinationTreeNode will look similar like that:
result.Append("(");
rightSubTree.GenerateExpression(result);
result.Append(") " + this.CombinationMode.ToString() + " (");
rightSubTree.GenerateExpression(result);
result.Append(")");
GenerateExpression(StringBuilder result) for SearchTextTreeNode is much easier:
result.Append(this.SearchText);
Of course, your code will produce a regular expression instead of the input text, as mine does.
I have a string, which contains a custom expression, I have to parse and evaluate:
For example:
(FUNCTION_A(5,4,5) UNION FUNCTION_B(3,3))
INTERSECT (FUNCTION_C(5,4,5) UNION FUNCTION_D(3,3))
FUNCTION_X represent functions, which are implemented in C# and return ILists.
UNION or INTERSECT are custom functions which should be applied to the lists, which are returned from those functions.
Union and intersect are implemented via Enumerable.Intersect/Enumerable.Union.
How can the parsing and evaluating be implemented in an elegant and expandable manner?
It depends on how complex your expressions will become, how many different operators are going to be available, and a whole number of different variables. Whichever way you do it, you will probably need to first determine a grammar for your mini-language.
For simple grammars, you can just write a custom parser. In the case of many calculators and similar applications, a recursive descent parser is expressive enough to handle the grammar and is intuitive to write. The linked Wikipedia page gives a sample grammar and the implementation of a C parser for it. Eric White also has a blog post on building recursive descent parsers in C#.
For more complex grammars, you will likely want to skip the work of creating this yourself and use a lex/yacc-type lexer and parser toolset. Normally you give as input to these a grammar in EBNF or similar syntax, and they will produce the code necessary to parse the input for you. The parser will typically return a syntax tree which you can traverse, allowing you to apply logic for each token in the input stream (each node in the tree). For C#, I have worked with GPLex and GPPG, but others such as ANTLR are also available.
Basic Parsing Concepts
In general, you want to be able to split each item in the input into a meaningful token, and build a tree based on those tokens. Once the tree is built, you can traverse the tree and perform the necessary action at each node. A syntax tree for FUNCTION_A(5,4,5) UNION FUNCTION_B(3,3) might look like this, where the node types are in capital letters and their values are in parenthesis:
PROGRAM
|
|
UNION
|
------------------------------
| |
FUNCTION (FUNCTION_A) FUNCTION(FUNCTION_B)
| |
------------- ----------
| | | | |
INT(5) INT(4) INT(5) INT(3) INT(3)
The parser needs to be smart enough to know that when a UNION is found, it needs to be supplied with two items to union, etc. Given this tree, you would start at the root (PROGRAM) and do a depth-first traversal. At the UNION node, the action would be to first visit all children, and then union the results together. At a FUNCTION node, the action would be to first visit all of the children, find their values, and use those values as parameters to the function, and secondly to evaluate the function on those inputs and return the value.
This would continue for all tokens, for any expression you can come up with. In this way, if you spend the time to get the parser to produce the right tree and each node knows how to perform whatever action it needs to, your design is very extensible and can handle any input that matches the grammar it was designed for.
I need to parse and split C and C++ functions into the main components (return type, function name/class and method, parameters, etc).
I'm working from either headers or a list where the signatures take the form:
public: void __thiscall myClass::method(int, class myOtherClass * )
I have the following regex, which works for most functions:
(?<expo>public\:|protected\:|private\:) (?<ret>(const )*(void|int|unsigned int|long|unsigned long|float|double|(class .*)|(enum .*))) (?<decl>__thiscall|__cdecl|__stdcall|__fastcall|__clrcall) (?<ns>.*)\:\:(?<class>(.*)((<.*>)*))\:\:(?<method>(.*)((<.*>)*))\((?<params>((.*(<.*>)?)(,)?)*)\)
There are a few functions that it doesn't like to parse, but appear to match the pattern. I'm not worried about matching functions that aren't members of a class at the moment (can handle that later). The expression is used in a C# program, so the <label>s are for easily retrieving the groups.
I'm wondering if there is a standard regex to parse all functions, or how to improve mine to handle the odd exceptions?
C++ is notoriously hard to parse; it is impossible to write a regex that catches all cases. For example, there can be an unlimited number of nested parentheses, which shows that even this subset of the C++ language is not regular.
But it seems that you're going for practicality, not theoretical correctness. Just keep improving your regex until it catches the cases it needs to catch, and try to make it as stringent as possible so you don't get any false matches.
Without knowing the "odd exceptions" that it doesn't catch, it's hard to say how to improve the regex.
Take a look at Boost.Spirit, it is a boost library that allows the implementation of recursive descent parsers using only C++ code and no preprocessors. You have to specify a BNF Grammar, and then pass a string for it to parse. You can even generate an Abstract-Syntax Tree (AST), which is useful to process the parsed data.
The BNF specification looks like for a list of integers or words separated might look like :
using spirit::alpha_p;
using spirit::digit_p;
using spirit::anychar_p;
using spirit::end_p;
using spirit::space_p;
// Inside the definition...
integer = +digit_p; // One or more digits.
word = +alpha_p; // One or more letters.
token = integer | word; // An integer or a word.
token_list = token >> *(+space_p >> token) // A token, followed by 0 or more tokens.
For more information refer to the documentation, the library is a bit complex at the beginning, but then it gets easier to use (and more powerful).
No. Even function prototypes can have arbitrary levels of nesting, so cannot be expressed with a single regular expression.
If you really are restricting yourself to things very close to your example (exactly 2 arguments, etc.), then could you provide an example of something that doesn't match?