Programming Assignment 2 Checklist: Deques and Randomized Queues

Frequently Asked Questions

Should I use arrays or linked lists in my implementations? In general we don't tell you how to implement your data structures—you can use arrays, linked lists, or maybe even invent your own new structure provide you abide by the specified time and space requirements. So, before you begin to write the code, make sure that your data structure will achieve the required resource bounds.

How serious are you about not calling any external library function other than those in stdlib.jar? You will receive a substantial deduction. The goal of this assignment is to implement data types from first principles, using resizing arrays and linked lists—feel free to use java.util.LinkedList and java.util.ArrayList on future programming assignments. We also require you to use StdIn (instead of java.util.Scanner) because we will intercept the calls to StdIn in our testing.

Can I add extra public methods to the Deque or RandomizedQueue APIs? Can I use different names for the methods? No, you must implement the API exactly as specified. The only exception is the main() method, which you should use for unit testing.

What is meant by uniformly at random? If there are n items in the randomized queue, then you should choose each one with probability 1/n, up to the randomness of StdRandom.uniform(), independent of past decisions. You can generate a pseudo-random integer between 0 and n−1 using StdRandom.uniform(n) from StdRandom.

Given an array, how can I rearrange the entries in random order? Use StdRandom.shuffle()—it implements the Knuth shuffle discussed in lecture and runs in linear time. Note that depending on your implementation, you may not need to call this method.

What should my deque (or randomized queue) iterator do if the deque (or randomized queue) is structurally modified at any time after the iterator is created (but before it is done iterating)? You don't need to worry about this in your solution. An industrial-strength solution (used in the Java libraries) is to make the iterator fail-fast: throw a java.lang.ConcurrentModificationException as soon as this is detected.

Why does the following code lead to a generic array creation compile-time error when Item is a generic type parameter?

Item[] a = new Item[1];
Java prohibits the creation of arrays of generic types. See the Q+A in Section 1.3 for a brief discussion. Instead, use a cast.
Item[] a = (Item[]) new Object[1];
Unfortunately, this leads to a compiler warning.

The compiler says that my program uses unchecked or unsafe operations and to recompile with -Xlint:unchecked for details. Usually this means you did a potentially unsafe cast. When implementing a generic stack with an array, this is unavoidable since Java does not allow generic array creation. For example, the compiler outputs the following warning with

% javac
Note: uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

% javac -Xlint:unchecked warning: [unchecked] unchecked cast
found   : java.lang.Object[]
required: Item[]
        a = (Item[]) new Object[2];
                     ^ warning: [unchecked] unchecked cast
found   : java.lang.Object[]
required: Item[]
        Item[] temp = (Item[]) new Object[capacity];
2 warnings

You should not make any other casts.

Checkstyle complains that my nested class' instance variables must be private and have accessor methods that are not private. Do I need to make them private? No, but there's no harm in doing so. The access modifier of a nested class' instance variable is irrelevant—regardless of its access modifiers, it can be accessed anywhere in the file. (Of course, the enclosing class' instance variables should be private.)

Can a nested class have a constructor? Yes.

What assumptions can I make about the input to Permutation? Standard input can contain any sequence of strings. You may assume that there is one integer command-line argument k and it is between 0 and the number of strings on standard input.

Will I lose points for loitering? Yes. Loitering is maintaining a useless reference to an object that could otherwise be garbage collected.

Possible Progress Steps

These are purely suggestions for how you might make progress. You do not have to follow these steps. These same steps apply to each of the two data types that you will be implementing.

  1. Make sure you understand the performance requirements for both Deque and RandomizedQueue. They are summarized in the table below. Every detail in these performance requirements is important. Do not proceed until you understand them.

    Deque Randomized Queue
    Non-iterator operations Constant worst-case time Constant amortized time
    Iterator constructor Constant worst-case time linear in current # of items
    Other iterator operations Constant worst-case time Constant worst-case time
    Non-iterator memory use Linear in current # of items Linear in current # of items
    Memory per iterator Constant Linear in current # of items

  2. Decide whether you want to use an array, linked list, or your own class. This choice should be made based on the performance requirements discussed above. You may make different choices for Deque and RandomizedQueue. You might start by considering why a resizing array does not support constant worst-case time operations in a stack.

  3. Use our example programs as a guide when implementing your methods. There are many new ideas in this programming assignment, including resizing arrays, linked lists, iterators, the foreach keyword, and generics. If you are not familiar with these topics, our example code should make things much easier. uses a resizing array; uses a singly-linked list. Both examples use iterators, foreach, and generics.

  4. We strongly recommend that you develop unit tests for your code as soon as you've written enough methods to allow for testing. As an example for Deque, you know that if you call addFirst() with the numbers 1 through n in ascending order, then call removeLast() n times, you should see the numbers 1 through n in ascending order. As soon as you have those two methods written, you can write a unit test for these methods. Arguably even better are randomized unit tests (which we employ heavily in our correctness testing). We recommend that you create a client class with a name like TestDeque, where each unit test is a method in this class. Don't forget to test your iterator.
Programming Tricks and Common Pitfalls

  1. It is very important that you carefully plan your implementation before you begin. In particular, for each data structure that you're implementing (RandomizedQueue and Deque), you must decide whether to use a linked list, an array, or something else. If you make the wrong choice, you will not achieve the performance requirements and you will have to abandon your code and start over.

  2. Make sure that your memory use is linear in the current number of items, as opposed to the greatest number of items that has ever been in the data structure since its instantiation. If you're using a resizing array, you must resize the array when it becomes sufficiently empty. You must also take care to avoid loitering anytime you remove an item.

  3. Make sure to test what happens when your data structures are emptied. One very common bug is for something to go wrong when your data structure goes from non-empty to empty and then back to non-empty. Make sure to include this in your tests.

  4. Make sure to test that multiple iterators can be used simultaneously. You can test this with a nested foreach loop. The iterators should operate independently of one another.

  5. Don't rely on our automated tests for debugging. You don't have access to the source code of our testing suite, so the Assessment Details may be hard to utilize for debugging. As suggested above, write your own unit tests; it's good practice.

  6. If you use a linked list, consider using a sentinel node (or nodes). Sentinel nodes can simplify your code and prevent bugs. However, they are not required (and we have not provided examples that use sentinel nodes).