Console calculator Part 1 : The Scanner

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleCalculator
{
    class Scanner
    {
        /// <summary>
        /// Scanner class, main routine here is get_token.
        /// get_token returns the next token_value to the parser,
        /// and sets some variables. It is up to the parser to decide
        /// what to do next.
        /// </summary>
       
        public enum token_value
        {
            NAME, NUMBER, END, LIST, HELP, SQRT,
            PLUS = '+', MINUS = '-',
            MUL = '*', DIV = '/',
            PRINT = ';', ASSIGN = '=',
            LPAREN = '(', RPAREN = ')',
        };

        public token_value curr_tok;    //current token value found
        public double number_value;     //if the token was a number the value is held here
        public string name_string;      //if the token was a variable name, the name can be found here

        private char[] StringToParse;   //the "inputstream"
        private int charpos = 0;        //position of the next character to read from the inputstream
        private StringBuilder numberSB = new StringBuilder();
        private StringBuilder nameSB = new StringBuilder();

        
        /// <summary>
        /// Two generic Dictionary's that use hashing to store string keys and values for
        /// variables, commands and reserved words.
        /// SymbolTable is public, the parser likes this table too
        /// </summary>
        public Dictionary<string, double> SymbolTable = new Dictionary<string, double>();
        private Dictionary<string, token_value> ReservedWords = new Dictionary<string, token_value>();

        /// <summary>
        /// Scanner default custom constructor, defines a small set of reseved words,
        /// used to give special commands to the calculator.
        /// </summary>
        public Scanner()
        {
            ReservedWords.Add("END", token_value.END);  //Quit the application.
            ReservedWords.Add("LIST", token_value.LIST);//Show a list of all variables in the calculator.
            ReservedWords.Add("?", token_value.HELP);   //Display help info
            ReservedWords.Add("SQRT", token_value.SQRT); //Function to take the square root
        }

        //property to pass the inputline to the scanner
        //---------------------------------------------
        public string InputLine
        {
            set 
            {
                string line = value + "\n";
                StringToParse = line.ToCharArray();
            }
        }

        //for functions we must first find a left parentesis
        //--------------------------------------------------
        public bool FindLeftParentesis()
        {
            char ch;
            try
            {
                do
                {
                    ch = GetChar();
                }
                while (!ch.Equals('('));
                return true;
            }
            catch
            {
                Error("( expected"); return false;
            }
        }

        //find a character in the "inputstream"
        //-------------------------------------
        private char GetChar()
        {
            char ch = StringToParse[charpos];
            charpos++;
            return ch;
        }

        public void PutBackChar()
        {
            charpos--;
        }

        public void InitScan()
        {
            charpos = 0;
            number_value = 0.0;
            name_string = "";
        }

        public void Error(string message)
        {
            //minor error handling, but sufficient for the moment
            Console.ForegroundColor = ConsoleColor.Black;
            Console.BackgroundColor = ConsoleColor.Cyan;
            Console.WriteLine(message);
            Console.ResetColor();
        }

        /// <summary>
        /// Main function of the scanner returns the current token and
        /// sets the variables name_string and number_value when appropriate.
        /// </summary>
        /// <returns></returns>
        public token_value get_token()
        {
            char ch;

            do          //skip whitespace except '\n'
            {
                ch = GetChar();
            }
            while (ch != '\n' && char.IsWhiteSpace(ch));
            switch (ch)
            {
                case ';': case '\n':
                    curr_tok = token_value.PRINT;
                    return curr_tok;
                case '*':
                case '/':
                case '+':
                case '-':
                case '(':
                case ')':
                case '=':
                    curr_tok = (token_value)(ch);
                    return curr_tok;
                case '0':
                case '1':
                case '2':
                case '3':
                case '4':
                case '5':
                case '6':
                case '7':
                case '8':
                case '9':
                case '.':
                case ',':
                    if (ch == '.') ch = ',';
                    numberSB.Append(ch);
                    ch = GetChar();
                    if (ch == '.') ch = ',';
                    while (char.IsDigit(ch) || ch == ',')
                    {
                        numberSB.Append(ch);
                        ch = GetChar();
                    }
                    PutBackChar();
                    try
                    {
                        number_value = Convert.ToDouble(numberSB.ToString());
                        numberSB.Remove(0, numberSB.Length);
                    }
                    catch
                    {
                        Error("Fout in formaat van getal.");
                    }
                    return token_value.NUMBER;
                default:
                    if (char.IsLetter(ch) | ch.Equals('?'))
                    {
                        nameSB.Append(ch);
                        ch = GetChar();
                        while (char.IsLetterOrDigit(ch))
                        {
                            nameSB.Append(ch);
                            ch = GetChar();
                        }
                        PutBackChar();
                        name_string = nameSB.ToString();
                        nameSB.Remove(0, nameSB.Length);
                        if (ReservedWords.TryGetValue(name_string.ToUpper(), out curr_tok)) return curr_tok;
                        return token_value.NAME;
                    }
                    
                    Error("Bad token.");
                    return token_value.PRINT;
            }
        }   //end get_token    

        //print a list of all the variables defined in the symboltable
        //------------------------------------------------------------
        public void ListVars()
        {
            Console.WriteLine("+-------------------------------------------+");
            Console.WriteLine("| List of variables :                       |");
            Console.WriteLine("+-------------------------------------------+");
            Console.WriteLine();
            foreach (KeyValuePair<string, double> kvp in SymbolTable)
            {
                Console.WriteLine("Var = {0} \t = {1}", kvp.Key, kvp.Value);
            } //yeah, generics is something to get used to
            Console.WriteLine("+-------------------------------------------+");
        }
    }
}