Recursive Descent Exercise1

CSCI 400

Recursive Descent Exercise #1

Consider the recursive descent code from the textbook, repeated below:

/* Function factor 

Parses strings in the language generated by the rule:  
<factor> -> id  |  (<ex)  
*/ 

void factor() {
 // Determine which RHS */
   if (nextToken) == ID_CODE)
     lex(); // For the RHS id, just call lex

 
  /* If the RHS is ( – call lex to pass over the left parenthesis, 
     call expr, then check for the right parenthesis */

   else if (nextToken == LEFT_PAREN_CODE) 
   {
           lex();
           expr();
         if (nextToken == RIGHT_PAREN_CODE)
           lex();
         else
            error();
   }  /* End of else if (nextToken == ...  */
   else error(); /* Neither RHS matches */
 }

/* Function term

   Parses strings in the language generated by the rule: 
    <term> -> <factor> {(* | /) <factor> }  
*/

 void term() {

 /* Parse the first factor */

   factor();
   while (nextToken == AST_CODE || nextToken == SLASH_CODE) 
   {
     lex();
     factor();
   }
}

/* Function expr 

Parses strings in the language generated by the rule: 
<expr> → <term> { (+ | -) <term> }     

*/

void expr() {

// Parse the first term 
 
  term(); 

  /* As long as the next token is + or -, call lex to get the next token, 
     and parse the next term */

   while (nextToken == PLUS_CODE || nextToken == MINUS_CODE)
   {
     lex();
     term();  
   }
}

Assume the symbols a, b and c will return ID_CODE. Use the above functions to do a top-down parse of the statement:

a + b * c

Do a trace similar to slide #36 in Chapter 4 ppt.