main.cpp

/*
  Simple integer arithmetic calculator according to the following BNF
  exps      --> exp | exp NEWLINE exps
  exp       --> term {addop term}
  addop     --> + | -
  term      --> factor {mulop factor}
  mulop     --> * | /
  factor    --> ( exp ) | INT
*/

#include <iostream>
#include <iomanip>
#include <fstream>
#include <sstream>
#include <string>
#include "tokens.h"
#include "FlexLexer.h"

using namespace std;

string toknames[] = {
"INT", "LPAREN", "RPAREN", "PLUS", "MINUS", "TIMES", "DIVIDE", "NEWLINE"
};

string tokname(int tok) {
  return tok<257 || tok>264 ? "BAD_TOKEN" : toknames[tok-257];
}

yyFlexLexer         lexer;
YYSTYPE             yylval;

int     nextToken;  //global variable stores the token to be processed

void readNextToken( void ); //read the next token into the variable nextToken

void exps( void );  //process all expressions in the input
int  exp( void );   //returns the integer value of an expression
int term ( void );  //returns the integer value of an term
int factor( void ); //returns the integer value of an factor

//If the next token matches expectedToken, read the next token and return true
//otherwise, print an error message and return false
bool match( int expectedToken );

//print the error message
void error( string errorMsg );

//skip the rest of the line
void skipline( void );

int main(int argc, char **argv) {
    ifstream    ifs;

    if (argc!=2)
    {
        cerr << "usage: " << argv[0] << " filename" << endl;
        return 1;
    }
    ifs.open( argv[1] );
    if( !ifs )
    {
        cerr << "Input file cannot be opened.\n";
        return 0;
    }
    cout << "Lexcial Analysis of the file " << argv[1] << endl;

    lexer.switch_streams(&ifs, NULL);

    readNextToken();

    exps();

    return 0;
}

//print the error message, and
//terminate the program
void error( string errorMsg )
{
    cout << errorMsg << endl;
    exit(1);
}

//skip the rest of the line
void skipline( void )
{
    while( nextToken != NEWLINE && nextToken != 0 )
        readNextToken();
}


//read the next token into the variable nextToken
void readNextToken( void )
{
    nextToken = lexer.yylex();
}

//If the next token is expectedToken, call readNextToken and return true,
//otherwise, print an error message and return false
bool match( int expectedToken )
{
    if ( expectedToken == nextToken )
    {
        readNextToken();
        return true;
    }
    else
        return false;
}

void exps( void )
{
    int     count = 1;
    int     value;

    do
    {
        try {
            value = exp();
            cout << "expression " << count << " : " << value << endl;
        } catch(runtime_error e) {
            skipline();
            cout << "expression " << count << " :    wrong syntax -- " << e.what() << endl;
        }
        count ++;
    } while ( match(NEWLINE) );
}

//returns the integer value of an expression
int exp( void )
{
    string token;
    token = tokname(nextToken);
    readNextToken();
    int temp = term();
    while ((token == "PLUS") || (token == "MINUS"))
    {
        if (token == "PLUS")
        {
            temp += term();
        }
        else if (token == "MINUS")
        {
            temp -= term();
        }

    }
    return (temp);
    //PUT YOUR IMPLEMENTATION HERE
}
//returns int value of a term
int term ( void )
{
    string token;
    token = tokname(nextToken);
    readNextToken();
    int temp = factor();
    while ((token == "TIMES") || (token == "DIVIDE"))
    {
        if (token == "TIMES")
        {
            temp *= factor();
        }
        else if (token == "DIVIDE")
        {
            temp = temp / factor();
        }

    }
    return(temp);
    //PUT YOUR IMPLEMENTATION HERE
}
//returns int value of factor
int factor( void )
{
    string token;
    token = tokname(nextToken);
    readNextToken();
    int temp;

    if (token == "INT")
    {
        temp = yylval.ival;
        return(temp);
    }
    else if (token == "LPAREN")
    {
        readNextToken();

        temp = exp();

        readNextToken();

    }
    else
    {
        throw runtime_error("Syntax error");
    }


}