Write a Java program to implement stack by creating user defined push method for

Question

Write a Java program to implement stack by creating user defined push method for entering { and (, and pop method for retrieving { and ( from the stack. This program is for expression evaluation and syntax parsing. This program should display appropriate error message if input expression has wrong.

This is Code Example:

int howMany=5;
String[] myStatement= new String[howMany];
char[] myStack= new char[40];
int stackTop;

// Main method
myStatement[0]="( 1 + 3) * { 2 - 1 )";

...

myStatement[4]="( 1 + 3) * ((2 - 1 )";

for (int i=0;i stackTop=-1;
for (int j=0;j char c= myStatement[i].charAt(j);
if (c=='{' || c=='(') {
myPush(c);
}
if (c=='}') {
char popedChar=myPop();
// check correct or not
}
if (c==')') {
char popedChar=myPop();
// check correct or not
}
}

public static void myPush(char c){
// push operation
}

public static char myPop(){
// pop operation
return c;
}

The output is

( 1 + 3) * { 2 - 1 )
^ error: '}' expected

Explanation / Answer

Hi,

Basically this question is for determining the balanced paranthesis inside string .

So i am writing the method in which you can pass the myStatement[] array and get the desired result

/**
In case the expression is balanced then this method returns null else it returns the parenthesis which is required to fix error
**/

private static String isExpressionBalanced(String input){
       Stack<String> stack = new Stack<String>();
       String isBalanced = null;

       for(int i=0; i < input.length(); i++){
           String str = ""+input.charAt(i);       //store characters as String

           //if opening bracket then push into stack
           if(str.equals("(") || str.equals("[") || str.equals("{")){
               stack.push(str);
           }

           //if closing bracket, pop bracket and compare if its a pair
           if(str.equals(")") || str.equals("]") || str.equals("}")){
               //if stack becomes empty in between then also its not balanced
               if(stack.isEmpty()){
                   return str;
               }
               String opening = stack.peek();
               if(str.equals(")") && opening.equals("(")){
                   stack.pop();
               }
               if(str.equals("]") && opening.equals("[")){
                   stack.pop();
               }
               if(str.equals("}") && opening.equals("{")){
                   stack.pop();
               }
           }          
       }
       //if stack is empty at end, then its balanced
       if(input.length() > 0 && stack.isEmpty()){
           isBalanced = null;
       }

       return isBalanced;
   }
  
/**
   main method in which the string array will be passed
**/
  
public static void checkExpression(String[] myStatement) {
       for ( String stmt : myStatement) {
       if (isExpressionBalanced(stmt) != null) {
           System.out.println("The expression: " + stmt + " is unbalanced by paranthesis: " + isExpressionBalanced(stmt))
           }
       }
}

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