CS 631, Fall 2009


Assignment #9: 10 points

Date Due: Monday, 11/16/09.


	Read Sections 4.8-4.9 of Aho and yacc documentation.

(3)  1.	Rewrite the C version of the infix to postfix expression translator 
	(see HW3, problem #1) to call yylex() in place of the code for lexan()
	in the file lexer.c.  Use lex to create the yylex() function so that 
	lexan() now merely calls yylex(), which recognizes NUMs, IDs, operators,
	skips whitespace and counts lines.  Only NUMs, IDs, whitespace and 
	newlines require regular expressions with actions.  All other single 
	characters can be returned as the character itself.

	The code to convert the value of a NUM and to enter an ID in the symbol
	table goes in the lex actions for these tokens.  The lexbuf[] array is 
	no longer needed since lex has its own input buffer and returns the 
	matched strings in the global yytext, which has length yylen.  Test
	the translator thoroughly to verify its correctness and hand in the
	well-commented source code and output from your tests.**

(1)  2.	Compile and test the yacc calculator program in Figure 4.58.
	The source file is available on the course website in /aho/Fig4.58.y.
	Show that addition and multiplication have the correct precedences
	and associativities.**

(3)  3.	Modify the expression grammar for the yacc calculator program in 
	Figure 4.58 to include subtraction (-) and division (/) operators. 
	The source file is available on the course website in /aho/Fig4.58.y.
	Modify the yylex() function to return a NUMBER token for integers
	up to 7 digits in length, as in Figure 4.59, using the grammar of
	Figure 4.58.  Test your calculator to show that the operators work 
	correctly and have the correct precedences and aassociativities.**

(3)  4. Use yacc to create a parser for the "dangling else" grammar (4.67) 
	on page 281.  Use precedence and associativity declarations
	to produce a parser with no parse table conflicts.  Print the
	parse table produced by yacc -v. Compare with Figure 4.51 on 
	page 282 and explain any differences. Note: It isn't necessary to 
	create a yylex() function because you don't have to run the parser.**

      **For full credit, turn in well commented program listings and the
        output of the computer runs for this problem.