CSCI 162 - Lecture 8 - Stacks Chapter 6, Queues Chapter 7

Instructor's Class Notes

 

1. Chapter 6 - Stacks - A stack is a data structure of ordered items such that items can be inserted and removed only at one end (called the top).
• LIFO - Stands for Last In/First Out which is appropriate in that with a stack the last item inserted is the first (or next) item that can be removed. For example, a Pez (tm) dispenser.
• Why learn Stacks? - The LIFO feature of a Stack makes it a perfect tool and ADT for helping to solve many classic Computer Science problems, including: checking for balanced parenthesis, reversing strings, expression, etc. (more on those in the book and these notes).
• LinkedStack.java - Stack ADT implemented as a Linked List. Note that a linked list is a very good data structure for implementing a stack...why? (test question alert).
  
  Source: here
  
  Documentation: here
• java.util.Stack - The Java library has a very useful generic Stack ADT.
  
  Documentation: here.
• Reverse Word - Stacks are great for reversing a word (letters get inserted in order and removed in reverse order).
  
  Source: here.
• Evaluating a Postfix Expression - See section 6.4. A postfix expression is an expression in which the pair of operands are listed first, one after the other, and the operator that acts on them comes last (instead of between the operands.).
  
  For example: 3 4 +
  
  Evaluates to: 7
• Psuedocode - The pseudocode of the Postfix Evaluator is as follows

  
      1. Initialize a stack of double numbers.
  
      2. do
              if (the next input is a number)
                  Read the next input and push it onto the stack.
              else {
                  Read the next character, which is an operation symbol.
                  Pop two numbers off the stack.
                  Combine the two numbers with the operation (using the second number
                  popped as the left operand) and push the result onto the stack.
              }
          while (there is more of the expression to read);
  
      3. At this point, the stack contains one number, which is the value of the expression.                
                      

• Token and TokenScanner classes - These classes have been provided for reading a string for tokens, where a token can be a number, an operator or a parenthesis.
  
  Token documentation here
  
  TokenScanner documentation here
  
  

 

• Chapter 7 - Queues - A queue is a data structure of ordered items such that items can be inserted only at one end (called the rear) and removed at the other end (called the front). The item at the front end of the queue is called the first item.
  
• FIFO - A queue is a first-in/first-out ADT. Items are removed from the queue in the same order in which they are inserted.
• Terminology - Enqueue is to insert an item into the queue. Dequeue is to remove an item from the queue.
• java.util.Queue - The Java library has a very useful generic Queue ADT.
  
  Documentation: here.
• Palindrome.java - A word or expression that is spelled the same forward as backward is a palindrome.. The book includes a Java program that uses a queue to determine if a string entered is a palindrome or not.
  
  Source is here.