The Stack Class

1. The Stack Class Final Review Fall 2005 CS 101 Aaron Bloomfield

2. Motivation • Same as for Vectors • We want an easy way to store elements in a object without having to worry about manipulating arrays

3. Properties of our Stack class • It needs to have an array to hold the values • That array will be fixed at size 1000 • We’ll call it ‘array’ • Thus, we also need a value to hold the number of elements in the stack • We’ll call it ‘size’ • Our Stack class so far: public class Stack { int array[] = new int[1000]; int size = 0;

4. About stacks • With a stack, you can only add and remove elements from ONE end • Both occur from the same end • Think of a stack of papers – you are always adding a new paper to the top or removing it from the top • Adding an element is called pushing an element • Removing an element is called popping an element

5. Methods in our Stack class • Create a Stack object • Insert and remove elements into/from the Stack • Get the top element • Find if the stack is empty • Print it out to the screen • Search the stack for a value

6. The Stack Constructor • Ready? public Stack () { } • Rather boring… • As we initialized the variables earlier, we don’t need to do so here • But we could have done here just as well

7. Pushing an element onto the Stack • To add an element onto the Stack, we want to do two things • Insert it into the array at the proper position • Increment the size of the array • The code: public void push (int value) { array[size++] = value; } • We could have done this in two lines as well: public void push (int value) { array[size] = value; size = size + 1; }

8. Popping an element from the Stack • To add an element onto the Stack, we want to do two things • Find (and return) the top element in the Stack • Decrement the size of the array • The code: public int pop () { return array[--size]; } • We could have done this in two lines as well: public int pop () { size = size – 1; return array[size]; }

9. peek() and empty() • peek() is a quickie: public int peek () { return array[size-1]; } • empty() is also a quickie: public boolean empty() { return size == 0; }

10. Searching the Stack • Note that we want to search up to the value in size, not the entire (1,000 element) array • The method: public boolean search (int forwhat) { for ( int i = 0; i < size; i++ ) if ( array[i] == size ) return true; return false; }

11. Printing the Stack • The method: public String toString() { String ret = "Stack["; for ( int i = 0; i < size; i++ ) { ret += array[i]; if ( i != size-1 ) ret += ", "; } ret += "]"; return ret; } • This is just so that the method doesn’t print a comma after the last element • Our for loop body could also have been: ret += array[i] + ", ";

12. Using our Stack public static void main (String args[]) { Stack stack = new Stack(); System.out.println ("pushing elements 5 through 10 onto the stack"); for ( int i = 5; i <= 10; i++ ) stack.push(i); System.out.println (stack); System.out.println ("peek() returned: " + stack.peek()); System.out.println ("search(9) returned: " + stack.search(9)); int ret = stack.pop(); System.out.println ("pop() returned: " + ret); System.out.println (stack); System.out.println ("peek() returned: " + stack.peek()); ret = stack.pop(); System.out.println ("pop() returned: " + ret); ret = stack.pop(); System.out.println ("pop() returned: " + ret); System.out.println (stack); System.out.println ("peek() returned: " + stack.peek()); System.out.println ("search(9) returned: " + stack.search(9)); }

13. Program Demo • Stack.java