Stacks

1. Stacks Ellen Walker CPSC 201 Data Structures Hiram College

2. Stack • Collect data • Access only “most recently added” item • When “most recently added” item is removed, the former 2nd most recently added item becomes available! • Why is it called a stack? • Stack of plates in the cafeteria • Stack of books…

3. Operations on a Stack • Create an empty stack (constructor) • Is the stack empty? (isEmpty) • Add a new item to the stack. (push) • Remove the item that was most recently added (pop) • Retrieve the item that was most recently added (peek)

4. Example • Create an empty stack • Push “A” • Push “B” • Peek (returns B) • Pop (returns B) • Push “C” • Pop (returns C) • Pop (returns A) • Stack is now empty

5. LIFO vs. FIFO • A stack is a LIFO (last in, first out) data structure • New addition makes older items inaccessible • Models interruptions, like call waiting • Alternative is FIFO (first in, first out) • We will see this later, in Queue data structure • Queues are fair when someone has to wait

6. Stack Interface (see p. 151) Public interface StackInt<E> { E push(E obj); //returns object inserted E peek(); //returns top object (no change) E pop(); //returns & removes top object boolean empty(); //is it empty? }

7. Interpreting Backspaces with a Stack Stack<Character> readAndCorrect(String input){ Stack<Character> myStack = new Stack<Character>(); for(int j=0;j<input.length();j++){ if(input.charAt(j) != ‘\08’){ //backspace is ‘\08’ myStack.push(input.charAt(j)); else myStack.pop(); //pop last character } return myStack; }

8. Printing the Line //Print the line (as it was in the stack) void printStack(Stack<char> st){ while !st.empty(){ System.out.print(st.pop()); } System.out.println(“”); }

9. But we have a problem… • The most recent entry to the stack is the last character on the line! • Reverse the values using two stacks Stack<Character> reverse (Stack<Character> stack1){ stack2 = new Stack<Character>(); while(!stack1.empty()){ stack2.push(stack1.pop()); } return stack2; }

10. Putting it all together • //program to read a file with backspaces and • //print each line as corrected public static void main(String[] args){ BufferedReader in = new BufferedReader (new FileReader (args[1])); String line = null; while (line = in.readLine()) != null){ Stack<Character> st1 = readAndCorrect(line); Stack<Character> st2 = reverse(st1); printStack(st2); }}

11. Recognizing a Palindrome • A Palindrome reads the same forward and backward • Madam I’m Adam • Able was I ere I saw Elba • Solution: • Push every character of the string, excluding spaces and punctuation onto a stack • Build the reversed string by popping each element off the stack (using StringBuilder) • Compare the original string (excluding punctuation) to the reversed string

12. Another Example: Balancing Parens • A very useful application for Lisp or Scheme! • Algorithm • For each character: • If it’s not a paren, ignore it • If it’s a left paren, push it • If it’s a right paren • If the stack is empty, return false (too few left) • else pop the stack • If the stack is not empty, return false (too few right) • Return true (balanced parentheses)

13. Balancing Examples • (defun v(x y) (or (and x y) x y)) • (cond ((null x) nil) ((t nil)) • (cons (car (cdr x)) y))