Loop invariant for a while loop

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Java

algorithm

Hi,

I am writing A Post Fix Calculator program, but having trouble identifying the loop invariant. I have tried looking for examples on the internet and on Youtube but most of the examples are for easy functions that usually do not involve a lot of if-statements.

The algorithm pushes some number taken from the user into the stack and then pops the last two numbers in the stack if the user enters an arithmic operator and pushes the result back into the stack.

I really appreciated any hints on what the loop invariant should be for this algorithm. Also, any resources you know (internet, books, ebooks) that provide an explanation for the loop invariant as well as examples for more complicated algorithms that involve if-statements and taking input from user is appreciated!

public static void main(String [] args){
		
		Scanner keyboard = new Scanner(System.in);//to read input from the user
		Stack stack = new Stack();                //to push user input into the stack if it is a number
		String input = "";                        //to save user input into a variable
		int tempVar = 0;                          //to save a poped number for calculation
		
		System.out.println("\nWelcome To The Postfix Calculator!");//printing message for the user
		
		do{
			//ask user to enter a value
			System.out.print("\nEnter a number or an arthmetic operator, or 0 to display the result: ");
			
			input = keyboard.nextLine(); //put user input into a variable
			
			/*If input is "+" the last two elements in the stack are poped and their addition 
			 * is pushed into the stack*/
			if(input.equals("+")){
				tempVar = (int)stack.pop();
				tempVar = (int)stack.pop() + tempVar;
				stack.push(tempVar);
			}
			
			/*If input is "-" the last two elements in the stack are poped and their subctraction 
			 * is pushed into the stack*/
			else if(input.equals("-")){
				tempVar = (int)stack.pop();
				tempVar = (int)stack.pop() - tempVar;
				stack.push(tempVar);
			}
			
			/*If input is "*" the last two elements in the stack are poped and their multiplication 
			 * is pushed into the stack*/
			else if(input.equals("*")){
				tempVar = (int)stack.pop();
				tempVar = (int)stack.pop() * tempVar;
				stack.push(tempVar);
			}
			
			/*If input is "/" the last two elements in the stack are poped and their division 
			 * is pushed into the stack*/
			else if(input.equals("/")){
				tempVar = (int)stack.pop();
				tempVar = (int)stack.pop() / tempVar;
				stack.push(tempVar);
			}
			
			/*If input is none of these and not "0" then the number is parsed and pushed into the
			 * stack*/
			else if(!input.equals("0"))
				stack.push(Integer.parseInt(input));
			
		}
		while(!input.equals("0"));
		
		if(stack.empty())
			System.out.println("\nYou haven't entered any value, please try again.");
		else
			System.out.println("\nFinal Value: " +  stack.pop());
		
	}

What I have tried:

I have tried looking into some examples on the loop invariant, but couldn't find examples for more complicated algorithms