Thread: Need help with my POP method!!!

I have LinkedStack class, what I simply want to do is make a delimiter match alogorithm, I also have the working algorithm for it but the problem is implementing the algorithm . I'm just stuck in my POP method and which will fix just one line of algorithm currently showing incompatible types.

I have a abstract DataElement class, A CharElement class extending DataElement and a LinkedStack class. My LinkedStacked class has all the push, pop and peek methods. I need help in making the pop method a char type.

Below is my LinkedStack Class, just before the class ends, I have the pop method, so you can scroll all the way down and see the method.
Here is what I had previously, this method below worked perfectly when I tried to create a stack, and then peek and pop and then push again; but not with the delimiter algorithm method in my tester.

public char pop() throws StackException
    {
        if (stackTop == null)
            throw new StackException();
        stackTop= stackTop.link
    }

public class LinkedStack {
 
        protected class StackNode extends CharElement
        {
 
           public DataElement info;
           public StackNode link;        
 
        public StackNode()
        {
            info = null;
            link = null;
        }
 
        public StackNode(DataElement item, StackNode pointer)
        {
            info = item;
            link = pointer;
        }
 
        public String toString()
        {
            return info.toString();
        }
 }
 
    private StackNode stackTop;
 
 
    public LinkedStack()
    {
        stackTop = null;
    }
 
    public void initializeStack()
    {
        stackTop = null;
    }
 
    public boolean isEmptyStack()
    {
        return (stackTop == null);
    }
 
    public boolean isFullStack()
    {
        return false;
    }
 
    public void push(DataElement newItem)
    {
        StackNode newTop;
        newTop  = new StackNode(newItem, stackTop );
        stackTop = newTop;
    }
 
    public DataElement peek() throws StackException
    {
        if (stackTop == null)
            throw new StackException();
        return stackTop.info;
    }
 
 
 
    public char pop() throws StackException   <-----------------------------------------------------------------------HERE IS THE POP METHOD, HELP ME CODE It, i need a return!!
    {
        if (stackTop == null)
            throw new StackException();
 
 
    }
 
   }

here is my tester,
there is still stuff to write but having an error on one line of the method 'i'sdelimitermatching', I have marked it with arrows!

public class Tester {
    public static void main(String[] args) {
 
    LinkedStack stack1 = new LinkedStack();
    stack1.push(new CharElement('a'));
    stack1.push(new CharElement('b'));
    stack1.push(new CharElement('c'));
    stack1.push(new CharElement('d'));
    stack1.push(new CharElement('e'));
 
 
 
    System.out.println("Top of the Stack = " + stack1.peek());  
 
    while (!stack1.isEmptyStack())
    {
        System.out.println(stack1.peek());
 
        stack1.pop();
 
    }
 
    } 
 
    public boolean isDelimiterMatching(String inputExpr) {
 
 
        int stackSize = inputExpr.length();
        LinkedStack theStack = new LinkedStack();
        for (int j = 0; j < inputExpr.length(); j++) {
            char ch = inputExpr.charAt(j);
            switch (ch) {
            case '{':
            case '[':
            case '(':
                    String temp = Character.toString(ch);
                    theStack.pop();     <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<Here is it, gotta fix it by fixing the pop method!
                    break;
            case '}':
            case ']':
            case ')':
                    if (!theStack.isEmptyStack()) {
                        char stackContent;
                       stackContent = theStack.pop();
                        if ((ch == '}' && stackContent!= '{') 
                                || (ch == ']' && stackContent != '[')
                                || (ch == ')' && stackContent != '(')){
                            System.out.println("Mismatch found: " + ch + " at " + j);
                            return false;
                        }
                    } else {
                        System.out.println("Mismatch found: " + ch + " at " + j);
                        return false;
                    }
                    break;
            default: break;
            }
        }
        if (!theStack.isEmptyStack()){
            System.out.println("Error: missing right delimiter");
            return false;
        }
        return true;
    }

Well I have the major part of the code but the problem is the Pop method, I need it to cut the head of my stack, i mean remove the top and at the same time return me the top that it just took off so that I can store in the char variable in my "isDelimittermatching" method(Marked with arrows above).
Please help me out write the pop method! Have been trying for ours now!

Originally Posted by Norm

Comment: should the code save what is returned by the pop() method?

Where can the pop() method get the data it is supposed to return? How can the pop() method remove the top of the stack so that the second node in the stack becomes the top of the stack?

should the code save what is returned by the pop() method?
Yes, it should save whatever its taking off, at least that is what I'm guessing because see on my tester below, I want to store whatever it is popping in the char variable, I'm sorry the arrow mark in my tester was in the wrong position. I've corrected it now. The method pop was first void and then I made it to char to work with the code below, but I need the pop method to pop and also return what it popped.

 char stackContent;
                       stackContent = theStack.pop();

Where can the pop() method get the data it is supposed to return?
from stackTop I guess, which is in LinkedStack class.
How can the pop() method remove the top of the stack so that the second node in the stack becomes the top of the stack?
In the void method that I have written at the very top of my post, it is kinda removing it, more like changing the reference. But If you could help in removing and also at the same time return whatever it is removing that would be great!

Originally Posted by Norm

If you know where the data exists that pop() is supposed to return, how can pop() get that data?

To remove the top/first of a linked list, get a pointer to the first node in the list from the top-of-the-list pointer and copy that node's connecting link to the top-of-the-list pointer.

This what I did, it says incomaptible data types, the real questions is how do I convert? I do have a getCopy method in my char element class. So far thw wrong code is below,

 public char pop() throws StackException
    {
        if (stackTop == null)
            throw new StackException();
 
        char entry = this.stackTop.getCopy();
    }

and then I could do stackTop = stackTop.link;
and return entry. But how do I convert from DataElement to char. (CharElement extends from DataElement)

--- Update ---

I tried casting it, still error on return statement help me out, its due today! just one method and I'm done!
public char pop() throws StackException

    {
        if (stackTop == null)
            throw new StackException();
 
            DataElement entry = this.stackTop.getCopy();
            stackTop = stackTop.link;
            return (new DataElement(entry));
    }

Here is my code, all the classes are in here, you can skip stackexception and strelement(does nothing - to be deleted) classes: https://compilr.com/xemkruz/linkedst...nkedStack.java

How can i make pop opposite of push, its so different! i have 4 more hours to submit this! Just one method!

Originally Posted by Norm

push() adds a DataElement object to the stack,
peek() returns the DataElement that is at the top of the stack
pop should return a DataElement object from the top of the stack like peek() does

or push() and peek() should be changed so they work with a char like you want pop() to do.

Aint I close? I jst need the return statement to work below, tried casting charElement into return entry since it is char type !, nope doesn't work! But the code makes sense?

public char pop() throws StackException
{
if (stackTop == null)
throw new StackException();

DataElement entry = this.stackTop.getCopy();
stackTop = stackTop.link;
return (new CharElement(entry); <<<< Error!!
}

Originally Posted by Norm

I think the design is wrong.
If push() puts a DataElement on the stack
and peek() shows what's on the top of the stack as a DataElement
then pop() should return a DataElement

all three should work with the same data type.

yeah, I can do that, but if I do that,
char stackContent shows me error,

char stackContent;
                       stackContent = theStack.pop();

how can i put whatever is in pop which is dataelement into a char?