C++ || Simple Tokenizer Lexer Using A Finite State Machine
The following is sample code which demonstrates the implementation of a simple Lexer using a table driven Finite State Machine.
In its simplest form, a Finite State Machine is a procedure that can: (1) store the status of an event, (2) can operate on new (or existing) input to change the status of an event, and (3) can cause an action to take place (based on the input change) for the particular event being examined.
State machines usually adhere to the following basic principles:
• Has an initial state or record of something stored someplace
• Has a set of possible input events
• Has a set of new states that may result from the input
• Has a set of possible actions or output events that result from a new state
Finite state machines usually have a limited or finite number of possible states. The Finite State Machine implemented on this page has 6 transition states. Those transition states has the ability to isolate tokens from a given input string, based on characteristics defined in the transition state table.
The machine was implemented with compiler construction in mind, resulting in the grammar for the 6 transition states to resemble that of many popular programming languages.
The transition table defined in this program can be further fine tuned by allowing the states to accept or reject different parameters, and/or by adding more transition states. Tuning the parameters in the state transition table allows you to change the behavior of the state machine without having the need to alter the Finite State Machine Algorithm (the “Lexer” function) much at all.
NOTE: The example on this page uses a sample input file. Click here to download the file.
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// ============================================================================ // Author: Kenneth Perkins // Date: Jan 29, 2014 // Taken From: http://programmingnotes.org/ // File: FSM.cpp // Description: The following demonstrates the implementation of a simple // Lexer using a table driven Finite State Machine. // ============================================================================ #include <iostream> #include <cstdlib> #include <cctype> #include <string> #include <vector> #include <fstream> using namespace std; /* This holds the transition states for our Finite State Machine -- They are placed in numerical order for easy understanding within the FSM array, which is located below */ enum FSM_TRANSITIONS { REJECT = 0, INTEGER, REAL, OPERATOR, STRING, UNKNOWN, SPACE }; /* This is the Finite State Machine Table -- The zero represents a place holder, so the row in the array starts on row 1 instead of 0. -- You can tune this table to make the states accept or reject different parameters, thus changing its behavior. More states can be added to this table. integer, real, operator, string, unknown, space */ int stateTable[][7] = {{0, INTEGER, REAL, OPERATOR, STRING, UNKNOWN, SPACE}, /* STATE 1 */ {INTEGER, INTEGER, REAL, REJECT, REJECT, REJECT, REJECT}, /* STATE 2 */ {REAL, REAL, UNKNOWN, REJECT, REJECT, REJECT, REJECT}, /* STATE 3 */ {OPERATOR, REJECT, REJECT, REJECT, STRING, REJECT, REJECT}, /* STATE 4 */ {STRING, STRING, REJECT, STRING, STRING, REJECT, REJECT}, /* STATE 5 */ {UNKNOWN, UNKNOWN, UNKNOWN, UNKNOWN, UNKNOWN, UNKNOWN, REJECT}, /* STATE 6 */ {SPACE, REJECT, REJECT, REJECT, REJECT, REJECT, REJECT}}; // struct to hold token information struct TokenType { string token; int lexeme; string lexemeName; }; // function prototypes vector<TokenType> Lexer(string expression); int Get_FSM_Col(char currentChar); string GetLexemeName(int lexeme); int main() { // declare variables ifstream infile; string fileName = ""; string expression = ""; vector<TokenType> tokens; // get data from user cout<<"\nPlease enter the name of the file: "; getline(cin, fileName); infile.open(fileName.c_str()); if(infile.fail()) { cout<<"\n** ERROR - the file \""<<fileName<<"\" cannot be found!\n\n"; exit(1); } // use a loop to scan each line in the file while(getline(infile, expression)) { // use the "Lexer" function to isolate integer, real, operator, // string, and unknown tokens tokens = Lexer(expression); // display the tokens to the screen for(unsigned x = 0; x < tokens.size(); ++x) { cout<<tokens[x].lexemeName<<" \t" <<tokens[x].token<<endl; } } infile.close(); return 0; }// end of main /** * FUNCTION: Lexer * USE: Parses the "expression" string using the Finite State Machine to * isolate each individual token and lexeme name in the expression. * @param expression - A std::string containing text. * @return - Returns a vector containing the tokens found in the string */ vector<TokenType> Lexer(string expression) { TokenType access; vector<TokenType> tokens; char currentChar = ' '; int col = REJECT; int currentState = REJECT; int prevState = REJECT; string currentToken = ""; // use an FSM to parse the expression for(unsigned x = 0; x < expression.length();) { currentChar = expression[x]; // get the column number for the current character col = Get_FSM_Col(currentChar); /* ======================================================== THIS IS WHERE WE CHECK THE FINITE STATE MACHINE TABLE USING THE "col" VARIABLE FROM ABOVE ^ ========================================================= */ // get the current state of the expression currentState = stateTable[currentState][col]; /* =================================================== THIS IS WHERE WE CHECK FOR A SUCESSFUL PARSE - If the current state of the expression == REJECT (the starting state), then we have sucessfully parsed a token. - ELSE we continue trying to find a sucessful token =================================================== */ if(currentState == REJECT) { if(prevState != SPACE) // we dont care about whitespace { access.token = currentToken; access.lexeme = prevState; access.lexemeName = GetLexemeName(access.lexeme); tokens.push_back(access); } currentToken = ""; } else { currentToken += currentChar; ++x; } prevState = currentState; } // this ensures the last token gets saved when // we reach the end of the loop (if a valid token exists) if(currentState != SPACE && currentToken != "") {// ^^ we dont care about whitespace access.token = currentToken; access.lexeme = currentState; access.lexemeName = GetLexemeName(access.lexeme); tokens.push_back(access); } return tokens; }// end of Lexer /** * FUNCTION: Get_FSM_Col * USE: Determines the state of the type of character being examined. * @param currentChar - A character. * @return - Returns the state of the type of character being examined. */ int Get_FSM_Col(char currentChar) { // check for whitespace if(isspace(currentChar)) { return SPACE; } // check for integer numbers else if(isdigit(currentChar)) { return INTEGER; } // check for real numbers else if(currentChar == '.') { return REAL; } // check for characters else if(isalpha(currentChar)) { return STRING; } // check for operators else if(ispunct(currentChar)) { return OPERATOR; } return UNKNOWN; }// end of Get_FSM_Col /** * FUNCTION: GetLexemeName * USE: Returns the string equivalent of an integer lexeme token type. * @param lexeme - An integer lexeme token type. * @return - An std::string string representing the name of the integer * lexeme token type. */ string GetLexemeName(int lexeme) { switch(lexeme) { case INTEGER: return "INTEGER"; break; case REAL: return "REAL "; break; case OPERATOR: return "OPERATOR"; break; case STRING: return "STRING"; break; case UNKNOWN: return "UNKNOWN"; break; case SPACE: return "SPACE"; break; default: return "ERROR"; break; } }// http://programmingnotes.org/ |
QUICK NOTES:
The highlighted lines are sections of interest to look out for.
The code is heavily commented, so no further insight is necessary. If you have any questions, feel free to leave a comment below.
NOTE: The example on this page uses a sample input file. Click here to download the file.
The following is sample output.
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Please enter the name of the file: FiniteStateMachine_programmingnotes_freeweq_com.txt STRING Rat12F STRING Function STRING function STRING Identifier OPERATOR ( OPERATOR ) OPERATOR : OPERATOR : OPERATOR ; OPERATOR , OPERATOR { OPERATOR } OPERATOR = OPERATOR [ OPERATOR ] STRING int STRING boolean STRING real STRING If STRING if STRING else STRING fi STRING while STRING Return STRING return OPERATOR ; STRING printf STRING scanf OPERATOR ( STRING IDs OPERATOR ) OPERATOR ; OPERATOR = OPERATOR = OPERATOR ! OPERATOR = OPERATOR > OPERATOR > OPERATOR < OPERATOR = OPERATOR > OPERATOR < OPERATOR = OPERATOR + OPERATOR - OPERATOR * OPERATOR / OPERATOR / OPERATOR - STRING true STRING false INTEGER 123 INTEGER 0000 REAL 13.456 REAL .567 UNKNOWN 12.45.67. STRING a_c__4t STRING a____t STRING x STRING x56 STRING _fgt OPERATOR % OPERATOR & OPERATOR / OPERATOR * STRING Middle OPERATOR * OPERATOR / OPERATOR / OPERATOR * STRING end OPERATOR * OPERATOR / INTEGER 67 STRING yu |
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