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I'm making a program to evaluate conditional proposition (~ or and -> <->). As the users input propositional variables and truth values (true, false) ,and proposition; the program will go through the inputs and return the truth value for the whole proposition.

  • For ex: if i set p = true, q = true, r = false and input: p or q and r.

  • Is the anyway I can cut it into q and r first, then process and put it back to result (which is false), then process the next bit (p or false) ??. And it has to keep cutting out bits (In proper order of Precedence) and putting them back until I have left is a single true or false.

    • And what I'm I supposed to use to hold user input (array, string) ???

    • Any help would be appreciated ! Thanks.

Bryan Vuong
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    You hold the user input in a string (should be obvious, that's what the user enters). But as you *parse* that string you might want to transform the input into some other data structure. Data structures that represent the syntactic structure of the input (including precedence) are called *abstract syntax trees* (AST) so you might want to research that. Not sure for something as simple as this that a full blown AST is needed, but that's something you'll decide as you code. You might also want to look into any of the many parsing tools available that will help you solve this problem. – john Feb 23 '19 at 08:14
  • I don't think we need AST concept to solve this problem tho since it is too complex and it's just a starter lab from my discrete structure class. I will look up and research more on parsing tools. Thanks for your advice by the way!! – Bryan Vuong Feb 23 '19 at 08:31

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Tasks like this are usually split into two phases, lexical analysis and syntactic analysis.

Lexical analysis splits the input into a stream of tokens. In your case the tokens would be the operators ~, or, and, ->, <->, variables and the values true, false. You didn't mention them but I'd imagine you also want to include brackets as tokens in your language. Your language is simple enough that you could just write the lexical analyser yourself but tools such as flex or ragel might help you.

Synyactic analysis is where you tease out the syntactic structure of your input and perform whatever actions you need (evaluate the preposition in your case). Syntactic analysis is more complex than lexical analysys. You could write a recursive descent parser for this task, or you could use a parser generator to write the code for you. The traditional tool for this is called bison, but it's a bit clunky. I like another simple tool called the lemon parser generator although it's more C orientated than C++.

john
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