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Computer Science 1 (Studio) 07B-Queues

Computer Science 1 (Studio) 07B-Queues. What is a Data Structure?. In computer science, a data structure is a way of storing data so that it can be used efficiently by an algorithm You’ve already been exposed to several data structures in Java

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Computer Science 1 (Studio) 07B-Queues

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  1. Computer Science 1 (Studio) 07B-Queues CS1s - 07B-Queues (v1.00)

  2. What is a Data Structure? • In computer science, a data structure is a way of storing data so that it can be used efficiently by an algorithm • You’ve already been exposed to several data structures in Java • Classes are collections of attributes and behaviors which represent an object • Arrays are a contiguous collection of similar primitives/objects • The choice of data structure in a program is of primary importance because it effects difficulty of implementation and performance CS1s - 07B-Queues (v1.00)

  3. What is a Queue? • A queue is a data structure which has the unique characteristic of being a First In First Out collection • Another characteristic of a queue is that, in theory, it has infinite capacity • Regardless of how many elements are currently present in the queue, another can always be added Queue back front C G D B A H F Queue E J H G F E D C B A I Queue J I H G F E D C B A CS1s - 07B-Queues (v1.00)

  4. What is a Queue? • Examples of real life queues: • Amusement park line • Concert ticket line • Bursar’s office line • Examples of queues in computer applications • Processes or users waiting for a system resource • A printing queue for spooling print jobs • A queue of mp3’s to download off Kazaa before the RIAA shuts it down CS1s - 07B-Queues (v1.00)

  5. Queue Operations - Enque • A queue supports two fundamental operations for storing and retrieving data elements • The process of adding an element to the end of a queue is known as enqueing back front B A “Enque B” B A CS1s - 07B-Queues (v1.00)

  6. Queue Operations - Deque • The process of removing an element from the front of the queue is known as dequeing • The visual effect of dequeing is that the front element disappears and all other elements in the queue shift forward a slot back front B A “Deque A” B A CS1s - 07B-Queues (v1.00)

  7. Queue Operations • In practice, most queues are implemented with a maximum size • A queue of size 10 which currently has 9 elements stored in it: • We can ask a queue what its size is • We can ask a queue how many elements it has in it J I H G F E D C B back front CS1s - 07B-Queues (v1.00)

  8. Queue Operations • We can also ask a queue if it is full or empty • Describe a scenario where you want to know if the queue is currently full • Describe another scenario where you want to know if the queue is currently empty J I H G F E D C B back front CS1s - 07B-Queues (v1.00)

  9. Queue Operations • We should also be able to ask a queue to print out its current contents (useful for debugging) J I H G F E D C B back front Queue from Front to Back: B, C, D, E, F, G, H, I, J CS1s - 07B-Queues (v1.00)

  10. Other Queue Operations • Give me a reverse queue • Make a copy of a queue J I H G F E D C B B C D E F G H I J back front back front J I H G F E D C B J I H G F E D C B back front back front CS1s - 07B-Queues (v1.00)

  11. Other Queue Operations • Are these two queues equal? • What is the front element in the queue? • We’re peeking at it, not actually dequeing it J I H G F E D C B B C D E F G H I J back front back front Equal queues: false J I H G F E D C B Front Element: B back front CS1s - 07B-Queues (v1.00)

  12. Other Queue Operations • What is the back element in the queue? • Does the queue contain a specific element? J I H G F E D C B Back Element: J back front Queue contains F: true Queue contains X: false J I H G F E D C B back front CS1s - 07B-Queues (v1.00)

  13. Queue Caveats • A queue contains elements of the same type • A queue does not support random access • We can’t get at the fourth element from the front of the queue and change it directly J I H G F E D C B back front CS1s - 07B-Queues (v1.00)

  14. Double Ended Queues • A double ended queue (deque) adds removal from the back, and insertion at the front insert front remove back J I H G F E D C B remove front insert back back front CS1s - 07B-Queues (v1.00)

  15. Other Queues • A priority queue constantly organizes the elements in the queue based on increasing priority • A circular queue avoids shifting by repositioning the front and back locations Add 66 Remove 55 Add 55 CS1s - 07B-Queues (v1.00)

  16. Understanding How To Implement a Queue • We’re going to implement our first queue using an array as the underlying collection • The queue will be circular so that we don’t have to shift elements around • Let’s look at the mechanics of this before we dive into the implementation • Consider a queue whose maximum size is 5 0 1 2 3 4 size = 5 # elements = 0 CS1s - 07B-Queues (v1.00)

  17. Understanding How To Implement a Queue • Based on this information, how can we tell when the queue is full or empty? • How can we tell what the size of the queue is? • How can we tell how many elements are in the queue? 0 1 2 3 4 size = 5 # elements = 0 CS1s - 07B-Queues (v1.00)

  18. Understanding How To Implement a Queue • In order to track the front and the back of the queue as elements get enqued and dequed, we will rely on integer indexes • Initially the back of the queue is at index -1 and the front of the queue is at index 0 0 1 2 3 4 front = 0 size = 5 # elements = 0 back = -1 CS1s - 07B-Queues (v1.00)

  19. Understanding How To Implement a Queue • Whenever an element is enqueued, the back index is incremented • The element is then added into the array at the back index Enque: 10 0 1 2 3 4 10 front = 0 size = 5 # elements = 1 back = 0 CS1s - 07B-Queues (v1.00)

  20. Understanding How To Implement a Queue Enque: 20 0 1 2 3 4 10 20 front = 0 size = 5 # elements = 2 back = 1 CS1s - 07B-Queues (v1.00)

  21. Understanding How To Implement a Queue Enque: 30 0 1 2 3 4 10 20 30 front = 0 size = 5 # elements = 3 back = 2 CS1s - 07B-Queues (v1.00)

  22. Understanding How To Implement a Queue • Whenever an element is dequed, the element at the front index is returned • The front index is then incremented Deque: 10 0 1 2 3 4 20 30 front = 1 size = 5 # elements = 2 back = 2 CS1s - 07B-Queues (v1.00)

  23. Understanding How To Implement a Queue Deque: 20 0 1 2 3 4 30 front = 2 size = 5 # elements = 1 back = 2 CS1s - 07B-Queues (v1.00)

  24. Understanding How To Implement a Queue • The queue is now empty again Deque: 30 0 1 2 3 4 front = 3 size = 5 # elements = 0 back = 2 CS1s - 07B-Queues (v1.00)

  25. Understanding How To Implement a Queue • Continue enqueueing… Enque: 40 0 1 2 3 4 40 front = 3 size = 5 # elements = 1 back = 3 CS1s - 07B-Queues (v1.00)

  26. Understanding How To Implement a Queue Enque: 50 0 1 2 3 4 50 40 front = 3 size = 5 # elements = 2 back = 4 CS1s - 07B-Queues (v1.00)

  27. Understanding How To Implement a Queue • On the next enque, the back index needs to wrap around to the start Enqueue: 60 0 1 2 3 4 60 50 40 front = 3 size = 5 # elements = 3 back = 0 CS1s - 07B-Queues (v1.00)

  28. Understanding How To Implement a Queue Enque: 70 0 1 2 3 4 60 50 70 40 front = 3 size = 5 # elements = 4 back = 1 CS1s - 07B-Queues (v1.00)

  29. Understanding How To Implement a Queue Deque: 40 0 1 2 3 4 60 50 70 front = 4 size = 5 # elements = 3 back = 1 CS1s - 07B-Queues (v1.00)

  30. Understanding How To Implement a Queue • On the next deque, the front index needs to wrap around Deque: 50 0 1 2 3 4 60 70 front = 0 size = 5 # elements = 2 back = 1 CS1s - 07B-Queues (v1.00)

  31. Implementing a Queue - Javadoc • Look at the javadoc for the queue class we are implementing: • http://www.cs.rit.edu/~cs1s/week7/IntQueue.html • The queue will only work with integers and it will be called IntQueue • The size of the queue will be based on how IntQueue is constructed // If no size is specified, the default size is 25 IntQueue q1 = new IntQueue(); // Here the size is 10 IntQueue q2 = new IntQueue(10); CS1s - 07B-Queues (v1.00)

  32. Implementing a Queue – First Step • In Class Activity • Start by creating a class, IntQueue.java • Using the javadoc, make a compilable skeleton • Remember to return dummy values for methods that require them • Write the code for the two constructors • You will have to create several private data members to achieve this • An integer array reference variable (the actual collection) • The number of elements currently in the queue • The front index • The back index • A constant for the default size of the queue (default = 25) • You don’t need to save the size of the array. Once the constructor creates the array, you can tell its size by accesing the arrays length CS1s - 07B-Queues (v1.00)

  33. Implementing a Queue – First Step • The end result of your code so far should be to create the following diagram in memory: IntQueue myQ = new Queue (5); 0 1 2 3 4 front = 0 size = 5 # elements = 0 back = -1 CS1s - 07B-Queues (v1.00)

  34. Implementing a Queue – Accesors • In Class Activity • The next step is to implement the accessor’s and simple methods • Implement getNumberOfElements() • Implement getSize() • Implement isFull() • Implement isEmpty() • Once this is complete, write another program, TestQueue.java, which tests the creation and accessors for IntQueue CS1s - 07B-Queues (v1.00)

  35. Implementing a Queue - Enque • The final steps for implementing the queue are writing the methods for enqueing and dequeing elements • Both of these methods require a way for the indexes to wrap around when they reach the end of the array Before Enqueue: 60 0 1 2 3 4 50 40 front = 3 size = 5 # elements = 2 back = 4 CS1s - 07B-Queues (v1.00)

  36. Implementing a Queue - Enque • In this example, how can I change the back index to go from 4 to 0? • Hint: you always know the size of the array • Write down generic algorithm here: After Enque: 60 0 1 2 3 4 50 40 60 front = 3 size = 5 # elements = 2 back = 0 CS1s - 07B-Queues (v1.00)

  37. Implementing a Queue - Enque • Remember this: • Enqueue works with the back index • Dequeue works with the front index • With enque, the back index must be incremented first before adding the element into the array After Enque: 60 Before Enque: 60 0 1 2 3 4 0 1 2 3 4 50 50 40 40 60 front = 3 front = 3 back = 4 back = 0 CS1s - 07B-Queues (v1.00)

  38. Implementing a Queue - Deque • With deque, the element must be added into the array before incrementing the front index Before deque: 20 After deque: 20 0 1 2 3 4 0 1 2 3 4 20 30 30 front = 1 front = 2 back = 2 back = 2 CS1s - 07B-Queues (v1.00)

  39. Implementing a Queue - Preconditions • There’s two preconditions we must worry about before we can write enqueue and dequeue • What should happen if we try to enque an element and the array is already full? Write our solution here: • What should happen if we try to deque an element and the array is empty? Write our solution here: CS1s - 07B-Queues (v1.00)

  40. Implementing the Queue - Print • For debugging purposes, we need a way to print out the contents of the queue • Consider this example: • We need to create a loop variable which starts at the front index and moves until all the elements in the queue have been printed 0 1 2 3 4 50 40 60 front = 3 size = 5 # elements = 2 back = 0 CS1s - 07B-Queues (v1.00)

  41. Implementing a Queue - Print • Here we want to print out indexes 3, 4 and 5: myQueue[3] myQueue[4] myQueue[0] • Write down the algorithm here: 0 1 2 3 4 50 40 60 front = 3 size = 5 # elements = 2 back = 0 CS1s - 07B-Queues (v1.00)

  42. Implementing a Queue – Enqueue & Print • In Class Activity • Write the enqueue method • Be sure to include the precondition discussed earlier • Remember that the back index must be incremented before adding the element into the array • Don’t forget to change the number of elements in the queue if the element was added • Next write the printQue method so the elements that are enqueued can be printout out • Finally, in your TestQueue class, write a test program which adds elements to the queue and prints them out • Make sure you check that adding elements to a full queue doesn’t blow up CS1s - 07B-Queues (v1.00)

  43. Implementing a Queue - Deque • In Class Activity • The final activity is to write the deque method • Be sure to handle the precondition where the queue is empty (return 0) • Remember that deque uses the front index • The element must be accessed in the array using the front index first (before incrementing the front index) • Write some tests in your TestQueue class which tests that you can correctly deque elements • Make sure you can handle the case where the queue is empty and a deque is requested CS1s - 07B-Queues (v1.00)

  44. Revision History • Revision History • v1.00, 10/21/2004 11:42 AM, sps Initial revision. -- v2.00, 10/24/2005, chr Minor correction CS1s - 07B-Queues (v1.00)

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