Stack

Question

You can use a Stack to check if a programming statement or a formula has balanced parentheses. Write a Windows application that provides a text box for the user to enter an expression with parenthesis. Provide a Check Parens button that when clicked runs a program that checks the number of parentheses in the expression and highlights a parenthesis that is unbalanced.

Vulpes · Answer

Here's the logic for this program and some test expressions in the form of a console application: using System; using System.Collections.Generic; struct CharIndex { public readonly char Character; public readonly int Index; public CharIndex(char c int i) { Character = c; Index = i; } } class Test { static void Main() { string [] expressions = { a[b] - (c + d) {(a + (b + c)) + (d - e[f])} {(a + (b + c)) + (d - e[f]) a + ((b + c[d]) a[b] + (c - d)} (a + b)) - (c[d] + e) }; foreach(string expression in expressions) { Console.WriteLine(expression); bool balanced = CheckParentheses(expression); if(!balanced) Console.WriteLine(The parentheses are balanced); Console.WriteLine(); } Console.ReadKey(); } static bool CheckParentheses(string expression) { var s = new Stack (); bool unbalanced = false; for(int i = 0; i { char c = expression[i]; if (c == '(' || c == '[' || c =='{') // push opening parentheses onto stack { s.Push(new CharIndex(c i)); } else if (c == ')' || c ==']' || c =='}') // check for matching opening parenthesis { if (s.Count > 0) // if there are any opening parentheses on the stack { CharIndex ci = s.Pop(); if ((c == ')' && ci.Character != '(') || (c == ']' && ci.Character != '[') || (c == '}' && ci.Character != '{')) { Console.WriteLine('{0}' is unbalanced at index {1} ci.Character ci.Index); unbalanced = true; } } else // it's an unbalanced closing parenthesis { Console.WriteLine('{0}' is unbalanced at index {1} c i); unbalanced = true; } } } while(s.Count > 0) // any remaining opening parentheses on stack must be unbalanced { CharIndex ci = s.Pop(); Console.WriteLine('{0}' is unbalanced at index {1} ci.Character ci.Index); unbalanced = true; } return unbalanced; } } The output should be: a[b] - (c + d) The parentheses are balanced {(a + (b + c)) + (d - e[f])} The parentheses are balanced {(a + (b + c)) + (d - e[f]) '{' is unbalanced at index 0 a + ((b + c[d]) '(' is unbalanced at index 4 a[b] + (c - d)} '}' is unbalanced at index 14 (a + b)) - (c[d] + e) ')' is unbalanced at index 7 I leave it to you to incorporate the logic into a windows forms application. If you need to highlight something then you'll need to use a RichTextBox rather than an ordinary TextBox for the user to enter an expression.