C# - Evaluate Postfix Expression

C# - Evaluate Postfix Expression

CODE

using System; using System.Collections.Generic; class Program { public static void Main() { try { //Provide the postfix expression string postFixExpression = "2 3 1 * + 9 -"; //initialise the class and call the Evaluate() method PostFixStack ps = new PostFixStack(); double answer = ps.Evaluate(postFixExpression); //print the result Console.WriteLine("{0} = {1}", postFixExpression, answer); //OUTPUT //2 3 1 * + 9 - = 4 } catch (Exception ex) { Console.WriteLine("Error: " + ex.Message); } Console.ReadKey(); } } class PostFixStack { private int top = -1; private List<string> items = new List<string>(); //Returns true if the Stack is empty public bool IsEmpty() { if (top == -1) return true; else return false; } //insert the data to the top of the stack //the top variable is incremented before inserting the element public bool Push(string data) { top++; items.Add(data); return true; } //removes and returns the topmost element from the stack //the top variable is decremented by 1 public string Pop() { if (IsEmpty()) { Console.WriteLine("The Stack is empty."); return ""; } else { string value = items[top]; items.RemoveAt(top); top--; return value; } } public double Evaluate(string postFixExpression) { //The operators and operands are expected to be space separated //split the expression based on space string[] expressionParts = postFixExpression.Split(' '); //loop through the splitted parts foreach (string expressionPart in expressionParts) { string expressionPartModified = expressionPart.Trim(); if (expressionPart == "") continue; double result; bool isNumber = double.TryParse(expressionPartModified, out result); //if it is a number, push it to the stack if (isNumber) Push(expressionPartModified); else { //if it is an operator, pop the last 2 items from the stack which will be operands double val1 = Convert.ToDouble(Pop()); double val2 = Convert.ToDouble(Pop()); ///apply the current operator to the last popped values string currentOperator = expressionPartModified; switch (currentOperator) { case "+": Push((val1 + val2).ToString()); break; case "-": Push((val1 - val2).ToString()); break; case "*": Push((val1 * val2).ToString()); break; case "/": Push((val1 / val2).ToString()); break; default: throw new Exception("Operator '" + currentOperator + "' not supported"); } } } //pop the last available value from the stack which will be the result. return Convert.ToDouble(Pop()); } }