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Lex Theory Practice

Yacc Theory Practice I Practice II

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More Lex Strings Reserved Debugging

More Yacc Recursion If-Else Errors Attributes Actions Debugging

If your program has a large collection of reserved words it is more efficient to let lex simply match a string and determine in your own code whether it is a variable or reserved word. For example, instead of coding

"if"            return IF;
"then"          return THEN;
"else"          return ELSE;

{letter}({letter}|{digit})*  {
         yylval.id = symLookup(yytext);
         return IDENTIFIER;
     }

where symLookup returns an index into the symbol table, it is better to detect reserved words and identifiers simultaneously, as follows:

{letter}({letter}|{digit})*  {
         int i;

         if ((i = resWord(yytext)) != 0)
             return (i);
         yylval.id = symLookup(yytext);
         return (IDENTIFIER);
     }

This technique significantly reduces the number of states required, and results in smaller scanner tables.