The C code generated by YACC can be altered before compiling and executing. The generated parser "rules the show" in that it calls the function yylex() when it needs a token. Figure 3 shows a similar picture to that of LEX in Figure 3 of Chapter 3.
the input to the executable a.out program is again the source program because the a.out file contains the included scanner from LEX or the user-written scanner, yylex().
YACC produces a LALR(1) parser. The generated parser produces no output when presented with a correct source program unless the user-written actions contain ouput statements.
