Scanning & FLEX CPSC 388 Ellen Walker Hiram College

Scanning (review) Input: characters from the source code Output: Tokens –Keywords: IF, THEN, ELSE, FOR … –Symbols: PLUS, LBRACE, SEMI … –Variable tokens: ID, NUM Augment with string or numeric value

Token Class (partial) Class Token { Public: TokenType tokenval; string tokenchars; double numval; }

GetToken(): A scanning function Token *getToken(istream &sin) –Read characters from sin until a complete token is extracted, return the token –Usually called by the parser –Note: version in the book uses global variables and returns only the token type

Using GetToken (Review) Token *myToken = GetToken(cin); While (myToken != NULL){ //process the token switch (myToken->TokenType){ //cases for each token type } myToken = GetToken(cin); }

Result of GetToken (Review)

Regular Expressions for Common Tokens Special characters: (the characters) Identifier: [a-zA-Z][a-zA-Z_]* Numbers: –Int: [1-9][0-9]* –Float: [1-9][0-9]*(  |(.[0-9]*)) –Scientific: [1-9][0-9]*(  |(.[0-9]*))(E+e)(+|–|  )[1-9][0-9]*

Reg. Exp. For Comments Comment to end of line –//[^\n]* (last part: (all chars except \n)* ) /*…*/ comment –ab (~b|b~a)*b?ba <--- ab … ba –/\* (~\* | \*~/)*(\*)? \*/ <--- needs escapes! –Does not require matching of “inner” /**/

Comments in Practice Often handled by “ad-hoc” methods Scanner simply loops to ignore characters from /* to */ –If character is not ‘*’, ignore it –Else if next character is not “/”, ignore it –Else ignore “/*” and return to scanning normally

Delimiters and Ambiguity Comments are not totally ignored! –“fo/**/r” is not the keyword “for” ! Principle of longest substring (“maximal munch”) –“fork” is not “for” followed by “k” Disallow keywords as identifiers –Scan identifier, then look it up instead of including keywords explicitly in language

FORTRAN’s mistakes Ignored white space (no delimiters) –DO99I=1.2 (DO99I = 1.2) vs. –DO99I=1,2 (DO 99 I = 1, 2) No reserved words –IF(IF.EQ.0)THENTHEN=17 Result: arbitrary backtracking (or lookahead) needed!

TINY Lexemes Reserved words: if, then, else, end, repeat, until, read, write Symbols: +, -, *, /, =, <, (, ), ;, := Other: number (integer only), identifier (letters only) Comment: {…} Principle of longest substring holds

TINY DFA

Using the TINY DFA Implement DFA directly or with a table Each call to gettoken() starts at the current point of the string, scans until no transition is possible. If final state is reached, return the token determined by the link to the final state. Otherwise, report an error. Characters in [ ] are not consumed

DFA pseudocodde State = Start_state While (chars available ){ last_state = state; state = next_state(next_char, state); if state = null return (final (last_state)); } return final(last_state);

LEX (FLEX) FLEX generates a scanner automatically! –Input: description of regular expression for each token, optional additional code –Output: lex.yy.c - includes function yylex() for parsing (like gettoken)

DFA Pseudocode state = initial-state while(chars in string){ c = next char from string state = next_state[state][c] } If final[state] return ACCEPT

Parts of a LEX file Definitions –code for the top of the file, and define expressions such as “digit” –All code in %{ and %} directly copied Rules – { expression } {code when recognized} Auxiliary Routines –Define additional functions here (including main)

Predefined items yylex() - lex scanning routine (like getToken) - generated by FLEX yytext - current string (a character array, not a C++ string class) Input() - get a char from flex input ECHO - print yytext to yyout

Example: Definitions %{ /* add line numbers to text and print */ #include int lineno=1; %} line.*\n %

Example: Rules & Aux. Code {line} {cout << lineno++ <<“ “<< yytext;} % main(){ yylex(); return 0; }

Using the Scanner First, create the code –flex test.lex Next, compile the program –g++ lex.yy.c -o test -lfl Finally, scan the input file –./test < input_file