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‎Algorithms 4/Part Ⅰ/week 2/Checklist_ Deques and Randomized Queues.html

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<!-- saved from url=(0061)http://coursera.cs.princeton.edu/algs4/checklists/queues.html -->
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<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<title>
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Programming Assignment 2 Checklist: Deques and Randomized Queues
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</title>
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</head>
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<body>
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<p><br>
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</p><h2>
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Programming Assignment 2 Checklist: Deques and Randomized Queues
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</h2>
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<p><br>
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<table border="0" cellpadding="2" cellspacing="0" width="100%">
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<tbody><tr align="left">
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<td bgcolor="000000">
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<font size="+0" face="helvetica" color="ffffff">
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<center><b>Frequently Asked Questions</b></center>
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</font></td></tr></tbody></table>
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</p><p><b>Should I use arrays or linked lists in my implementations?</b>
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In general we don't tell you <em>how</em> to implement your data structures—you
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can use arrays, linked lists, or maybe even invent your own new structure provide
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you abide by the specified time and space requirements.
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So, before you begin to write the code, make sure that your data structure
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will achieve the required resource bounds.
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</p><p><b>How serious are you about not calling any external library function other than those in <tt>stdlib.jar</tt>?</b>
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You will receive a substantial deduction.
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The goal of this assignment is to implement data types from first principles, using
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resizing arrays and linked lists—feel free to use
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<a href="http://docs.oracle.com/javase/7/docs/api/java/util/LinkedList.html">java.util.LinkedList</a>
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and
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<a href="http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html">java.util.ArrayList</a>
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on future programming assignments.
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We also require you to use <tt>StdIn</tt> (instead of
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<a href="http://docs.oracle.com/javase/7/docs/api/java/util/Scanner.html">java.util.Scanner</a>)
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because we will intercept the calls to <tt>StdIn</tt> in our testing.
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</p><p><b>
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Can I add extra public methods to the <tt>Deque</tt> or
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<tt>RandomizedQueue</tt> APIs?
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Can I use different names for the methods?</b>
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No, you must implement the API exactly as specified.
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The only exception is the <tt>main()</tt> method, which you should use for
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unit testing.
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<!--
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<p><b>If a client creates two randomized queue iterators, should
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they return the items in the same order?</b>
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No, each should return them in random order, independent
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of the other.
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-->
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</p><p><b>What is meant by uniformly at random?</b>
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If there are <em>n</em> items in the randomized queue, then you should choose each one
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with probability 1/<em>n</em>, up to the randomness of <tt>StdRandom.uniform()</tt>,
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independent of past decisions.
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You can generate a pseudo-random integer between 0 and <em>n</em>−1 using
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<tt>StdRandom.uniform(n)</tt> from
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<a href="http://algs4.cs.princeton.edu/code/javadoc/edu/princeton/cs/algs4/StdRandom.html"><tt>StdRandom</tt></a>.
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</p><p><b>Given an array, how can I rearrange the entries in random order?</b>
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Use <tt>StdRandom.shuffle()</tt>—it implements the Knuth shuffle
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discussed in lecture and runs in linear time. Note that depending on your implementation,
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you may not need to call this method.
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</p><p><b>What should my deque (or randomized queue) iterator do if the deque
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(or randomized queue) is structurally modified
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at any time after the iterator is created (but before it is done iterating)?</b>
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You don't need to worry about this in your solution.
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An industrial-strength solution (used in the Java libraries)
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is to make the iterator <em>fail-fast</em>: throw a
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<tt>java.lang.ConcurrentModificationException</tt> as soon as this is detected.
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</p><p><b>Why does the following code lead to a
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<tt>generic array creation</tt> compile-time error when <tt>Item</tt> is a generic
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type parameter?</b>
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</p><blockquote>
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<pre>Item[] a = new Item[1];
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</pre>
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</blockquote>
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Java prohibits the creation of arrays of generic types. See the
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<a href="http://algs4.cs.princeton.edu/13stacks/index.php#Q+A">Q+A in Section 1.3</a>
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for a brief discussion. Instead, use a cast.
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<blockquote>
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<pre>Item[] a = (Item[]) new Object[1];
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</pre>
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</blockquote>
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Unfortunately, this leads to a compiler warning.
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<!---
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<p><b>I'm using a linked list and don't like dealing with these special cases when the list is empty or almost
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empty. Is there some way to simplify them?</b> Yes. One common method is to include sentinel nodes. A sentinel node
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is just a special node created by the constructor which is never removed, and which contains dummy data which is
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never used. The trick is that your head (and tail, if applicable) pointers always point at these special sentinel
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nodes, even if the list is empty. In other words, the head (and tail) pointers are never null, thus avoiding the
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need to check to see if the head (or tail) is null.
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-->
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<p><b>The compiler says that my program uses unchecked or unsafe operations
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and to recompile with -Xlint:unchecked for details.</b>
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Usually this means you did a potentially unsafe cast.
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When implementing a generic stack with an array, this is unavoidable since
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Java does not allow generic array creation. For example, the compiler outputs
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the following warning with
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<a href="http://algs4.cs.princeton.edu/13stacks/ResizingArrayStack.java.html">ResizingArrayStack.java</a>:
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</p><blockquote>
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<pre>% javac ResizingArrayStack.java
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Note: ResizingArrayStack.java uses unchecked or unsafe operations.
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Note: Recompile with -Xlint:unchecked for details.
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% javac -Xlint:unchecked ResizingArrayStack.java
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ResizingArrayStack.java:25: warning: [unchecked] unchecked cast
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found : java.lang.Object[]
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required: Item[]
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a = (Item[]) new Object[2];
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^
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ResizingArrayStack.java:36: warning: [unchecked] unchecked cast
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found : java.lang.Object[]
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required: Item[]
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Item[] temp = (Item[]) new Object[capacity];
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^
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2 warnings
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</pre>
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</blockquote>
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<p>
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You should not make any other casts.
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</p><p><b>Checkstyle complains that my nested class' instance variables must be
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private and have accessor methods that are not private. Do I need to make them private? </b>
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No, but there's no harm in doing so.
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The access modifier of a nested class' instance variable
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is irrelevant—regardless of its access modifiers, it can be accessed
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anywhere in the file. (Of course, the enclosing class' instance
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variables should be private.)
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</p><p><b>Can a nested class have a constructor?</b>
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Yes.
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</p><p><b>What assumptions can I make about the input to <tt>Permutation</tt>?</b>
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Standard input can contain any sequence of strings.
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You may assume that there is one integer command-line argument <em>k</em>
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and it is between 0 and the number of strings on standard input.
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</p><p><b>Will I lose points for loitering?</b>
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Yes. <!-- See p. 137 of the textbook for a discussion of loitering. -->
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Loitering is maintaining a useless reference to an object that could otherwise be garbage collected.
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</p><p>
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<table border="0" cellpadding="2" cellspacing="0" width="100%">
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<tbody><tr align="left">
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<td bgcolor="000000">
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<font size="+0" face="helvetica" color="ffffff">
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<center><b>Possible Progress Steps</b></center>
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</font></td></tr></tbody></table>
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</p><p>
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These are purely suggestions for how you might make progress. You do
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not have to follow these steps. These same steps apply to each of the two data
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types that you will be implementing.
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</p><ol>
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<p></p><li><b>
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Make sure you understand the performance requirements for both <tt>Deque</tt> and <tt>RandomizedQueue</tt>.</b>
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<!--
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Your Deque should use space proportional to the number of items <i>currently</i> in the deque, and each
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operation must complete in <i>constant worst-case time</i>. Your Deque iterator must support each iteration
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operation (including construction) in <i>constant worst-case time</i>, and use a <i>constant</i> amount of
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extra space per iterator. Your RandomizeQueue should use space proportional to the number of items
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<i>currently</i> in the queue, and each operation should complete in <i>constant <b>amortized</b> time</i>.
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Your RandomizedQueue iterator implementation must support each iteration operation (<b>excluding</b>
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construction) in <i>constant worst-case time</i>, and use a <i>linear</i> amount of extra space per iterator.-->
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They are summarized in the table below.
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<em>Every detail in these performance requirements is important.
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Do not proceed until you understand them.</em>
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<center>
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<p><table border="1" cellpadding="4" cellspacing="0">
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<tbody>
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<!-- Results table headers -->
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<tr>
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<th></th>
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<th>Deque</th>
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<th>Randomized Queue</th>
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</tr>
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<tr>
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<td><b>Non-iterator operations</b></td>
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<td>Constant worst-case time</td>
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<td>Constant amortized time</td>
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</tr>
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<tr>
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<td><b>Iterator constructor</b></td>
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<td>Constant worst-case time</td>
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<td>linear in current # of items</td>
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</tr>
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<tr>
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<td><b>Other iterator operations</b></td>
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<td>Constant worst-case time</td>
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<td>Constant worst-case time</td>
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</tr>
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<tr>
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<td><b>Non-iterator memory use</b></td>
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<td>Linear in current # of items</td>
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<td>Linear in current # of items</td>
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</tr>
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<tr>
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<td><b>Memory per iterator</b></td>
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<td>Constant</td>
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<td>Linear in current # of items</td>
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</tr>
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</tbody>
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</table>
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</p></center>
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<p></p></li><li><b>Decide whether you want to use an array, linked list, or your own class.</b>
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This choice should be made based on the performance requirements discussed above.
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You may make different choices for
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<tt>Deque</tt> and <tt>RandomizedQueue</tt>.
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You might start by considering why a resizing array does not support
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<i>constant worst-case</i> time operations in a stack.
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<p></p></li><li><b>Use our example programs as a guide when implementing your methods.</b>
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There are many new ideas in this programming assignment, including resizing arrays, linked lists, iterators, the
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<i>foreach</i> keyword, and generics. If you are not familiar with these topics, our example code should make things
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much easier.
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<a href="http://algs4.cs.princeton.edu/13stacks/ResizingArrayStack.java.html">ResizingArrayStack.java</a>
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uses a resizing array;
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<a href="http://algs4.cs.princeton.edu/13stacks/LinkedStack.java.html">LinkedStack.java</a> uses a singly-linked list.
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Both examples use iterators, foreach, and generics.
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<!-- <a href="http://algs4.cs.princeton.edu/13stacks/DoublyLinkedList.java.html">DoublyLinkedList</a> provides an example of
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a linked list where each node has two links, which is useful for being able to go backwards in a linked list.
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-->
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<p></p></li><li>
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<b>We strongly recommend that you develop <a href="http://en.wikipedia.org/wiki/Unit_testing">unit tests</a>
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for your code as soon as you've written enough methods to allow for testing.</b>
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As an example for <tt>Deque</tt>, you
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know that if you call <tt>addFirst()</tt> with the numbers 1 through <em>n</em> in ascending order, then call
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<tt>removeLast()</tt> <em>n</em> times, you should see the numbers 1 through <em>n</em> in ascending order.
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As soon as you have
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those two methods written, you can write a unit test for these methods. Arguably even better are randomized
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unit tests (which we employ heavily in our correctness testing). We recommend that you create a client class
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with a name like <tt>TestDeque</tt>, where each unit test is a method in this class. Don't forget to test
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your iterator.
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</li></ol>
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<table border="0" cellpadding="2" cellspacing="0" width="100%">
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<tbody><tr align="left">
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<td bgcolor="000000">
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<font size="+0" face="helvetica" color="ffffff">
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<center><b>Programming Tricks and Common Pitfalls</b></center>
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</font></td></tr></tbody></table>
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<p>
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</p><ol>
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<p></p><li>
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<b>It is very important that you carefully plan your implementation before you begin.</b>
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In particular, for each data structure that you're implementing (<tt>RandomizedQueue</tt> and
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<tt>Deque</tt>), you must decide whether to use a linked list, an array, or something else.
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If you make the wrong choice, you will not achieve the performance requirements and
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you will have to abandon your code and start over.
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<p></p></li><li>
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<b>Make sure that your memory use is linear in the current number of items, as opposed to the
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greatest number of items that has ever been in the data structure since its instantiation.</b>
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If you're using a resizing array, you must resize the array when it becomes sufficiently empty.
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You must also take care to avoid loitering anytime you remove an item.
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<p></p></li><li>
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<b>Make sure to test what happens when your data structures are emptied.</b> One very common
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bug is for something to go wrong when your data structure goes from non-empty to empty and
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then back to non-empty. Make sure to include this in your tests.
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<p></p></li><li>
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<b>Make sure to test that multiple iterators can be used simultaneously.</b>
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You can test this with a nested <i>foreach</i> loop.
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The iterators should operate independently of one another.
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<p></p></li><li>
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<b>Don't rely on our automated tests for debugging.</b>
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You don't have access to the source code of our testing suite,
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so the <em>Assessment Details</em> may be hard to utilize for debugging.
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As suggested above, write your own unit tests; it's good practice.
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<p></p></li><li>
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<b>If you use a linked list, consider using a sentinel node (or nodes).</b> Sentinel
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nodes can simplify your code and prevent bugs. However, they are not
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required (and we have not provided examples that use sentinel nodes).
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</li></ol>
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<!--
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<p><b>When I am asked to give an answer using tilde notation or order-of-growth notation,
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should I include the leading coefficient and the lower order terms?</b>
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By definition, tilde notation includes the leading coefficient but discards the lower order terms;
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order of growth notation discards both the leading coefficient and the lower order terms.
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See pp. 178-179 of the textbook.
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-->
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<!--
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<p><br>
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<TABLE BORDER = 0 CELLPADDING = 2 CELLSPACING = 0 WIDTH = 100%>
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<tr align = left>
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<td bgcolor = "000000">
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<font size = +0 face = "helvetica" color = "ffffff">
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<center><b>Testing and Submitting</b></center>
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</table>
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<!--
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<p><li><b>Getting started.</b>
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Review the code from the textbook for generic stacks and queues
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with iterators.
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You can download the code from
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the
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<a href = "http://algs4.cs.princeton.edu/code">booksite</a>.
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If you adapt our code, you should include a citation to the original source
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from either the textbook or the booksite.
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-->
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</body><div></div></html>

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