C++ Recursive Descent Parser: Examining Nonterminal Classes Expr, Eprime, Term, and Tprime, Slides of Compiler Construction

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Compiler

Construction

Lecture 15

2

Recusive Descent Parser

 Let’s consider the

implementation of the C++

classes for the non-

terminal

 Start with Expr; recall the

grammar

4 1 Goal → expr 2 expr → term expr' 3 expr' → + term expr' 4 | - term expr' 5 | ε 6 term → factor term' 7 term' → (^) * factor term' 8 | ∕ factor term' 9 | ε 10 factor → number 11 | id 12 | ( expr )

5 bool Expr::isPresent() { Term* op1 = new Term(s); if(!op1->isPresent()) return false; tree = op1->AST(); Eprime* op2 = new Eprime(s, tree); if(op2->isPresent()) tree = op2->AST();

return true;

7 bool Eprime::isPresent() { int op=s->nextToken(); if(op==PLUS || op==MINUS){ s->advance(); Term* op2=new Term(s); if(!op2->isPresent()) syntaxError(s); TreeNode* t2=op2->AST(); tree = new

TreeNode(op,exprSofar,t2);

8

Eprime* op3 = new Eprime(s, tree); if(op3->isPresent()) tree = op3->AST(); return true; } else return false; }

10 bool Term::isPresent() { Factor* op1 = new Factor(s); if(!op1->isPresent()) return false; tree = op1->AST(); Tprime* op2 = new Tprime(s, tree); if(op2->isPresent()) tree = op2->AST(); return true; }

11 bool Tprime::isPresent() { int op=s->nextToken(); if(op == MUL || op == DIV){ s->advance(); Factor* op2=new Factor(s); if(!op2->isPresent()) syntaxError(s); TreeNode* t2=op2->AST(); tree = new TreeNode(op,exprSofar,t2); ......

13 1 Goal → expr 2 expr → term expr' 3 expr' → + term expr' 4 | - term expr' 5 | ε 6 term → factor term' 7 term' → (^) * factor term' 8 | ∕ factor term' 9 | ε 10 factor → number 11 | id 12 | ( expr )

14 bool Factor::isPresent() { int op=s->nextToken(); if(op == ID || op == NUM) { tree = new TreeNode(op,s->tokenValue()); s->advance(); return true; }

......