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

2. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 13 Program Demo Stack.java Stack.java