![]() In the latter cases, the analyzer must also specify the exact value of the token: what the identifier is, whether the constant is, say, 9 or 888, whether the operator is + or *, and whether the relational operator is = or >. The parser, called yyparse() by yacc, then resumes control and makes another call to the lexical analyzer to get another token.In a compiler, the different values of the token indicate what, if any, reserved word of the language has been found or whether an identifier, constant, arithmetic operator, or relational operator has been found. This name is convenient because a yacc-generated parser calls its lexical analyzer with this name.To use lex to create the lexical analyzer for a compiler, end each lex action with the statement return token, where token is a defined term with an integer value.The integer value of the token returned indicates to the parser what the lexical analyzer has found. ![]() lex and yacc often work well together for developing compilers.As noted, a program uses the lex- generated scanner by repeatedly calling the function yylex(). yacc generates parsers, programs that analyze input to insure that it is syntactically correct. If you work on a compiler project or develop a program to check the validity of an input language, you might want to use the system tool yacc. Scientific Calculator using LEX & YACC tools
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