3/9/15 CS 280 Programming Language Concepts Design Notes Programming Assignment 3 1 3/9/15 High Level Questions •  Do I understand the requirements? •  Do I understand how to meet the requirements? Observations •  You will be given an implementation of a lexical analyzer: a header file, a getToken() function, and a pushback token function •  The lexical analyzer will keep track of line numbers for you •  The key to the assignment is deciding how to represent the data and how to structure the functions 2 3/9/15 Recursive Descent Parsing •  Production rules will need to be implemented in functions •  Functions for production rules will call getToken() or other functions for other rules •  Functions will return a parse tree – so you need to come up with some kind of class definition for parse tree Grammar Rules Program ::= StmtList StmtList ::= Stmt | Stmt StmtList Stmt ::= PRINT Expr SC | SET ID Expr SC Expr ::= Expr PLUS Term | Expr MINUS Term | Term Term ::= Term STAR Primary | Term SLASH Primary | Primary Primary ::= ID | INT | STRING •  You need a function per rule •  The functions take a pointer to the input stream to read •  The functions return a pointer to a parse tree 3 3/9/15 Getting Started •  Decide how you will represent the result of a parse •  Declare all your functions, then define them •  You will probably need a global error counter to keep track of the number of errors •  Think about the idea of an error strategy: if I find an error, what should I do besides just printing out an error message? Parse Tree •  Common members in a PTree base class –  type –  left and right child •  Common methods in base class –  getType(), getValue(), traversals •  Probably want to think about a class inheritance hierarchy. Several classes that are subclasses of Ptree –  –  –  –  Class for “list of statements” Class for “print” and “set” Classes for different expressions Class for identifier, integer, string 4 3/9/15 If I were you I would reuse this main int main( int argc, char *argv[] ) { istream *br; ifstream infile; // check args and open the file if( argc == 1 ) br = &cin; else if( argc != 2 ) { return 1; // print an error msg } else { infile.open(argv[1]); if( infile.is_open() ) br = &infile; else { cout << argv[1] << " can't be opened" << endl; return 1; } } // pass br as the first argument to Program() 5