Tech Erudition

/**
* Basic Calculator Application
* Author: Prasad.A
*/
using System;
using System.Collections;
namespace ConsoleCalculator {
 /* Basic Calculator Class */
 public class BaseCalculator {
   /* Dictionary to store variables and its values */
   private IDictionary _dictionary = null;
   /* Stack to carry out the operations */
   private Stack operationStack = null;   
   /* Default Constructor */
   public BaseCalculator() {
     this._dictionary = new Hashtable();
     Console.WriteLine("Basic Calculator Application");
     Console.WriteLine("Type 'quit' to terminate.");
   }   
   /* Get the substring of the string */
   private String GetSubString(String expression,int start,int end) {
     return expression.Substring(start,(end-start));
   }   
   /* Push element on to operation stack */
   private void Push(Object element) {
     operationStack.Push(element);
   }       
   /* Pop element from the operation stack */
   private Object Pop() {
     if(operationStack.Count > 0) return operationStack.Pop();
     return null;
   }   
   /* Peek element from the operation stack */
   private Object Peek() {
     if(operationStack.Count > 0) return operationStack.Peek();
     return null;
   }         
   /* Evaluate the expression within '(' and ')' */
   private void EvaluateSubExpression() {     
     Object element = Pop(); /* This is just behind ')' */
     while(element != null && element.ToString().CompareTo("(") != 0) {
       /* Get result from binary evaluation */
       Object binaryEvaluation = BinaryEvaluate(element);
       element = Pop(); /* This will be the operator for next iteration */
       /* Did we get start of sub expression */
       if(element.ToString().CompareTo("(")==0) {
         /* Push the result and break out of loop */
         Push(binaryEvaluation);
         break;
       }
       /* Push back the operator that was popped */
       Push(element);
       /* Now one of the operand is the result itself */
       element = binaryEvaluation;
     }
   }   
   /* Evaluate the expression */
   private Object EvaluateExpression() {
     Object result = null;
     Object element = Pop(); /* This is operand two */
     while(element!=null) {
       /* Get result from binary evaluation */
       result = BinaryEvaluate(element);
       /* If we have reached end of stack, break */
       if(Peek() == null) break;
       /* Opearand two is result itself */
       element = result;
     }
     return result;
   }   
   /* Evaluate the binary expression, the input parameter is operand2,
    * Get the operator element and operand1 from stack. If operator element
    * or operand1 is not present do a unary operation else if operator element
    * is = then do a assignment operation else do a binary evaluation.
    */ 
   private Object BinaryEvaluate(Object operandTwo) {
     Object evalBinary = new Object();
     Object operatorElement = null;
     Object operandOne = null;           
     if(Peek() != null &&
        Peek().ToString().CompareTo("(")!=0)
       operatorElement = Pop();        
     if(operatorElement != null &&
        Peek() != null &&
        Peek().ToString().CompareTo("(")!=0)
       operandOne = Pop();        
     if(operatorElement!=null &&
        operatorElement.ToString().CompareTo("=")!=0) {
       operandTwo = GetObjectValue(operandTwo);
       operandOne = GetObjectValue(operandOne);
     }
     if(operatorElement==null)
       evalBinary = GetObjectValue(operandTwo);
     else if(operandOne==null) {
       if(operatorElement.ToString().CompareTo("+")==0)
         evalBinary = (+1.0)*double.Parse(operandTwo.ToString());
       else if(operatorElement.ToString().CompareTo("-")==0)
         evalBinary = (-1.0)*double.Parse(operandOne.ToString());
     }
     else {
       switch(operatorElement.ToString()) {
         case "+":
           evalBinary = double.Parse(operandOne.ToString())+
           double.Parse(operandTwo.ToString()); break;
         case "-":
           evalBinary = double.Parse(operandOne.ToString())-
           double.Parse(operandTwo.ToString()); break;
         case "*":
           evalBinary = double.Parse(operandOne.ToString())*
           double.Parse(operandTwo.ToString()); break;
         case "/":
           evalBinary = double.Parse(operandOne.ToString())/
           double.Parse(operandTwo.ToString()); break;
         case "=":
           SetIdValue(operandOne.ToString(),operandTwo.ToString());
           evalBinary = null; break;
       }
     }
     return evalBinary;
   }   
   /* Parse the input string and evaluate as and when required */
   private Object parse(String expression) {
     operationStack = new Stack();
     int parsePointer = 0; /* Points to beginning of token */     
     bool inIdentifier = false,inNumber = false;
     for(int index=0; index < expression.Length; ++index) {
       char ch=expression[index];
       /* We got a identifier or number delimiter */
       if(ch == ' ' || ch == '\t') {
         if(inIdentifier) {
           Push(GetSubString(expression,parsePointer,index));
           parsePointer = index+1;
           inIdentifier = false;
         }
         if(inNumber) {
           Push(GetSubString(expression,parsePointer,index));
           parsePointer = index+1;
           inNumber = false;
         }
       }
       /* We got start of sub expression */
       else if(ch=='(') {
         Push(ch.ToString());
         parsePointer = index+1;
       }
       /* We got end of sub expression */
       else if(ch == ')') {
         if(inIdentifier) {
           Push(GetSubString(expression,parsePointer,index));
           parsePointer = index+1;
           inIdentifier = false;
         }
         if(inNumber) {
           Push(GetSubString(expression,parsePointer,index));
           parsePointer = index+1;
           inNumber = false;
         }
         EvaluateSubExpression();         
       }
       /* We got assignment operator */
       else if(ch=='=') {
         if(inIdentifier) {
           Push(GetSubString(expression,parsePointer,index));           
           inIdentifier = false;
         }
         Push(ch.ToString());
         parsePointer = index+1;
       }
       /* We got arithmetic operator */
       else if(isOperator(ch)) {
         if(inIdentifier) {
           Push(GetSubString(expression,parsePointer,index));           
           inIdentifier = false;
         }
         if(inNumber) {
           Push(GetSubString(expression,parsePointer,index));
           inNumber = false;
         }
         Push(ch.ToString());         
         parsePointer = index+1;
       }
       /* We got a letter and we are not in identifier */
       else if(Char.IsLetter(ch) && !inIdentifier) {
         inIdentifier = true;
       }
       /* We got a digit and we are not in number */
       else if(Char.IsDigit(ch) && !inNumber) {
         inNumber = true;
       }
       /* We got a decimal point */
       else if(ch=='.') {
         if(index < expression.Length-1 &&
            !Char.IsDigit(expression[index+1])) {
           Push(GetSubString(expression,parsePointer,index));
           parsePointer = index+1;
           inNumber = false;
         }
       }
       /* We are in identifier and we got a identifier delimiter */
       else if(inIdentifier && isIdDelimiter(ch)) {
         Push(GetSubString(expression,parsePointer,index));
         parsePointer = index+1;
         inIdentifier = false;
       }
       /* We are in number and we got a number delimiter */
       else if(inNumber && isNumberDelimiter(ch)) {
         Push(GetSubString(expression,parsePointer,index));
         parsePointer = index+1;
         inNumber = false;
       }
     }
     /* Are we still in identifier */
     if(inIdentifier)
       Push(GetSubString(expression,parsePointer,expression.Length));
     /* Are we still in number */
     else if(inNumber)
       Push(GetSubString(expression,parsePointer,expression.Length));
     /* Let us return the result of expression */
     return EvaluateExpression();
   }   
   /* Decide on the identifier delimiter */
   private bool isIdDelimiter(char delim) {
     if(delim=='.'||delim=='('||delim==')') return true;
     return isOperator(delim);
   }
   /* Decide on the nubmer delimiter */
   private bool isNumberDelimiter(char delim) {
     if(delim=='('||delim==')') return true;
     return Char.IsLetter(delim);
   }
   /* Decide on the operator */
   private bool isOperator(char delim) {
     if(delim=='+'||delim=='-'||delim=='*'||delim=='/'||delim=='=')
       return true;
     return false;
   }   
   /* Get the value of input object
    * return either number or variable value)
    */
   private Object GetObjectValue(Object inputObject) {
     Object valueObject = null;
     try {
       valueObject = GetNumberObject(inputObject.ToString());
     } catch(FormatException fe) {
       valueObject = GetIdValue(inputObject.ToString());
     }
     return valueObject;
   }   
   /* Return the numeric representation of input string */
   private Object GetNumberObject(String inputString) {
     Object numberObject = null;
     try {
       numberObject = int.Parse(inputString);
     } catch(FormatException fei) {
       try {
         numberObject = double.Parse(inputString);
       } catch(FormatException fed) {
         throw new FormatException("Error! Illegal number format");
       }
     }
     return numberObject;
   }   
   /* Set the variable and its value in _dictionary */

   private void SetIdValue(String id,String val) {
     Object number = null;
     try {
       number = GetNumberObject(val);
     } catch(FormatException fe) {
       number = GetIdValue(val);
     }
     if(number != null) _dictionary[id] = number.ToString();
   }   
   /* Get the value of the variable from _dictionary */
   private Object GetIdValue(String id) {
     Object idValue = _dictionary[id];
     if(idValue != null) return idValue;
     throw new Exception("Error! Variable '"+id+"' not yet defined");
   }   
   /* Core function to keep the interaction alive */
   public void RunCalculator() {
     while(true) {
       Console.Write(">>> ");
       String input = Console.ReadLine();
       if(input.CompareTo("quit")==0)
         break;
       try {
         Object result = parse(input);
         if(result != null)
           Console.WriteLine(result.ToString());
       } catch(Exception ex) {
         Console.WriteLine(ex.Message);
       }
     }
   }
 } 
 /* A user class that contains the main function
  * To give life to calculator */
 public class UserCalculator {
   public static void Main(string[] args) {
     BaseCalculator bc = new BaseCalculator();
     bc.RunCalculator();
   }
 }
}

Basic Calculator Application
Type 'quit' to terminate.
>>> a=10.5
>>> b
Error! Variable 'b' not yet defined
>>> b=20
>>> b
20
>>> c=a+b
>>> c
30.5
>>> d=10.1+(b*c)/a
>>> d
68.1952380952381
>>> quit