CSC300: Linked List Accessors

Contents [0/11]

Video [1/11]
Starter code [2/11]
Loop [3/11]
Forward recursion, nullable argument [4/11]
Forward recursion, non-nullable argument [5/11]
Backward recursion, non-nullable argument [6/11]
Backward recursion, nullable argument [7/11]
Starter code with both array and linked list [8/11]
Another Problem [9/11]
Loop using current [10/11]
Loop using previous [11/11]

(Click here for one slide per page)


Video [1/11]

In three parts.

Only the first and third videos will be covered on exams. The second video is a preview of material that is covered in CSC 301/403.

Open Playlist

00:00 A Playground for Linked Lists
02:20 Why is Node a static class
03:57 How a list is created
08:52 How to access a specific item
10:08 Visualizing a pointer to a specific item
11:59 Writing a loop: The middle
14:27 Writing a loop: The end
15:07 Writing a loop: The beginning
15:57 Visualizing the execution of the loop

Open Playlist

00:00 Forward recursion with nullable parameter
03:51 Thinking about loops and recursion
06:41 Forward recursion with non-nullable parameter
10:04 Backward recursion with non-nullable parameter
13:33 Backward recursion with nullable parameter

Open Playlist

00:00 Using a current pointer
05:30 Using a previous pointer

Starter code [2/11]

file:Playground.java [source] [doc-public] [doc-private]
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package algs13;
import java.text.DecimalFormat;
import stdlib.*;
public class Playground {
  private Node first;
  static class Node { 
    public double item; 
    public Node next; 
    public Node (double item, Node next) { 
      this.item = item; 
      this.next = next; 
    }
  }

  public int numFives () {
    return StdRandom.uniform (100); //TODO: fix this
  }

  public static void main1 (String[] args) {
    Trace.showBuiltInObjects(true);
    Trace.drawStepsOfMethod ("numFives");
    Trace.drawStepsOfMethod ("numFivesH");
    Trace.run ();
    Playground list1 = Playground.of ("5 11 5 5");
    Playground list2 = Playground.of ("24 35 67");
    int result1 = list1.numFives ();
    Trace.draw();
    StdOut.println ("result: " + result1);
  }

  public static void main2 (String[] args) {
    Trace.showBuiltInObjects(true);
    Trace.drawStepsOfMethod ("main");
    Trace.drawStepsOfMethod ("example");
    //Trace.drawSteps();
    Trace.run ();
    Playground example1 = Playground.example (1);
    Playground example2 = Playground.example (5);
    Playground example3 = Playground.example (7);
    Node x = example1.first;
    StdOut.println (x.item);
  }

  /* A silly method to show list creation */
  public static Playground example(int i) {
    Node x1 = new Node (i+10, null);
    Node x2 = new Node (i+20, null);
    Node x3 = new Node (i+30, null);
    Node x4 = new Node (i+40, null);
    Playground result = new Playground ();
    result.first = x1;
    x1.next = x2;
    x2.next = x3;
    x3.next = x4;
    return result;
  }


  public static void main (String[] args) {
    testNumFives (2, "11 5 5 21");
    testNumFives (0, "11 21 31 41");
    testNumFives (1, "11 21 5 31 41");
    testNumFives (1, "5 11 21 31 41");
    testNumFives (1, "11 21 31 41 5");
    testNumFives (5, "5 11 21 5 5 31 5 41 5");
    testNumFives (3, "5 5 5");
    testNumFives (1, "5");
    testNumFives (0, "11");
    testNumFives (0, "");
    StdOut.println ("Finished tests");
  }

  private static void testNumFives (int expected, String sList) {
    Playground list = Playground.of (sList); 
    String sStart = list.toString ();
    int actual = list.numFives ();
    if (expected != actual) {
      StdOut.format ("Failed: Expecting [%d] Actual [%d] with argument %s\n", expected, actual, list);
    }
    String sEnd = list.toString ();
    if (! sStart.equals (sEnd)) {
      StdOut.format ("Failed %s.numFives(): List changed to %s\n", sStart, sEnd);
    }
  }

  /* ToString method to print */
  public String toString () { 
    // Use DecimalFormat #.### rather than String.format 0.3f to leave off trailing zeroes
    DecimalFormat format = new DecimalFormat ("#.###");
    StringBuilder result = new StringBuilder ("[ ");
    for (Node x = first; x != null; x = x.next) {
      result.append (format.format (x.item));
      result.append (" ");
    }
    result.append ("]");
    return result.toString ();
  }

  /* Method to create lists */
  public static Playground of(String s) {
    Playground result = new Playground ();
    if ("".equals (s)) return result;

    Node first = null;
    String[] nums = s.split (" ");
    for (int i = nums.length-1; i >= 0; i--) {
      try { 
        double num = Double.parseDouble (nums[i]); 
        first = new Node (num, first);      
      } catch (NumberFormatException e) {
        throw new IllegalArgumentException (String.format ("Bad argument \"%s\": could not parse \"%s\" as a double", s, nums[i]));
      }
    }
    result.first = first;
    return result;
  }

}

Loop [3/11]

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  public int numFives () {
    int result = 0;
    Node x = first;
    while (x != null) {
      if (x.item == 5) result = result + 1;
      x = x.next;
    }
    return result;
  }

For [5,11,5,5], the loop values (line 4) are 

x==[5,11,5,5], result==0
  x==[11,5,5], result==1
     x==[5,5], result==1
       x==[5], result==2
        x==[], result==3 (x==null)

There is an image to the right of the code.
Click to animate in single-slide mode.
To enter single-slide mode, click the slide number in the title bar.

numFivesLIO-

Forward recursion, nullable argument [4/11]

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  public int numFives () {
    return numFivesH (first, 0);
  }
  private static int numFivesH (Node x, int result) {
    if (x == null) return result;
    if (x.item == 5) result = result + 1;
    result = numFivesH (x.next, result);
    return result;
  }

For [5,11,5,5], the call tree is 

call@3 ([5,11,5,5], 0)
  call@4 ([11,5,5], 1)
    call@5  ([5,5], 1)
      call@6  ([5], 2)
        call@7 ([], 3)
        retn@7 ([], 3) : 3
      retn@6  ([5], 2) : 3
    retn@5  ([5,5], 1) : 3
  retn@4 ([11,5,5], 1) : 3
retn@3 ([5,11,5,5], 0) : 3
 numFivesLROF-

Forward recursion, non-nullable argument [5/11]

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  public int numFives () {
    if (first == null) return 0;
    return numFivesH (first, 0);
  }
  private static int numFivesH (Node x, int result) {
    if (x.item == 5) result = result + 1;
    if (x.next != null) result = numFivesH (x.next, result);
    return result;
  }

For [5,11,5,5], the call tree is 

call@3 ([5,11,5,5], 0)
  call@4 ([11,5,5], 1)
    call@5  ([5,5], 1)
      call@6  ([5], 2)
      retn@6  ([5], 2) : 3
    retn@5  ([5,5], 1) : 3
  retn@4 ([11,5,5], 1) : 3
retn@3 ([5,11,5,5], 0) : 3
 numFivesLRCF-

Backward recursion, non-nullable argument [6/11]

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  public int numFives () {
    if (first == null) return 0;
    return numFivesH (first);
  }
  private static int numFivesH (Node x) {
    int result = (x.next == null) ? 0 : numFivesH (x.next);
    if (x.item == 5) result = result + 1;
    return result;
  }
call@3 ([5,11,5,5])
  call@4 ([11,5,5])
    call@5  ([5,5])
      call@6  ([5])
      retn@6  ([5]) : 1
    retn@5  ([5,5]) : 2
  retn@4 ([11,5,5]) : 2
retn@3 ([5,11,5,5]) : 3

A more verbose version:

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  public int numFives () {
    if (first == null) return 0;
    return numFivesH (first);
  }
  private static int numFivesH (Node x) {
    int result = 0;
    if (x.next != null) {
      result = numFivesH (x.next);
    }
    if (x.item == 5) result = result + 1;
    return result;
  }
 numFivesLRCB-

Backward recursion, nullable argument [7/11]

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  public int numFives () {
    return numFivesH (first);
  }
  private static int numFivesH (Node x) {
    if (x == null) return 0;
    int result = numFivesH (x.next);
    if (x.item == 5) result = result + 1;
    return result;
  }

For [5,11,5,5], the call tree is 

call@3 ([5,11,5,5])
  call@4 ([11,5,5])
    call@5  ([5,5])
      call@6  ([5])
        call@7 ([])
        retn@7 ([]) : 0
      retn@6  ([5]) : 1
    retn@5  ([5,5]) : 2
  retn@4 ([11,5,5]) : 2
retn@3 ([5,11,5,5]) : 3
 numFivesLROB-

Starter code with both array and linked list [8/11]

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package algs13;
import java.text.DecimalFormat;
import stdlib.*;
public class Playground {
  private double a[]; 
  private Node first;
  static class Node { 
    public double item; 
    public Node next; 
    public Node (double item, Node next) { 
      this.item = item; 
      this.next = next; 
    }
  }

  public int numFives () {
    return StdRandom.uniform (100); //TODO: fix this
  }

  /* ToString method to print */
  public String toString () { 
    // Use DecimalFormat #.### rather than String.format 0.3f to leave off trailing zeroes
    DecimalFormat format = new DecimalFormat ("#.###");
    StringBuilder result = new StringBuilder ("[ ");
    for (Node x = first; x != null; x = x.next) {
      result.append (format.format (x.item));
      result.append (" ");
    }
    result.append ("]");
    return result.toString ();
  }

  /* Method to create lists */
  public static Playground of(String s) {
    Playground result = new Playground ();
    if ("".equals (s)) {
      // special case for the empty array
      result.first = null;
      result.a = new double [0];
    } else {
      String[] nums = s.split (" ");
      Node first = null;
      double[] a = new double[nums.length];
      for (int i = nums.length-1; i >= 0; i--) {
        double num = Double.NaN;
        try { 
          num = Double.parseDouble (nums[i]); 
        } catch (NumberFormatException e) { 
          throw new IllegalArgumentException (String.format ("Bad argument \"%s\": could not parse \"nums[i]\" as a double", s, nums[i]));
        }
        a[i] = num;
        first = new Node (num, first);  
      }
      result.a = a;
      result.first = first;
    } 
    return result;
  }

  public static void testNumFives (int expected, String sList) {
    Playground list = Playground.of (sList); 
    int actual = list.numFives ();
    if (expected != actual) {
      StdOut.format ("Failed: Expecting [%d] Actual [%d] with argument %s\n", expected, actual, list);
    }
  }
  public static void main (String[] args) {
    testNumFives (0, "");
    testNumFives (1, "5");
    testNumFives (0, "11");
    testNumFives (3, "5 5 5");
    testNumFives (0, "11 21 31 41");
    testNumFives (1, "5 11 21 31 41");
    testNumFives (1, "11 21 31 41 5");
    testNumFives (1, "11 21 5 31 41");
    testNumFives (5, "5 11 21 5 5 31 5 41 5");
    StdOut.println ("Finished tests");
  }
  public static void main1 (String[] args) {
    Trace.drawStepsOfMethod ("numFives");
    Trace.drawStepsOfMethod ("numFivesH");
    Trace.run ();
    Playground list0 = Playground.of ("5");
    Playground list1 = Playground.of ("5 11 5 5");
    int result = list1.numFives ();
    StdOut.println ("result: " + result);
  }
}

Another Problem [9/11]

file:PlaygroundNumUnique.java [source] [doc-public] [doc-private]
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package algs13;
import java.text.DecimalFormat;
import stdlib.*;
public class PlaygroundNumUnique {
  private Node first;
  static class Node { 
    public double item; 
    public Node next; 
    public Node (double item, Node next) { 
      this.item = item; 
      this.next = next; 
    }
  }
  
  /** Number of unique items, assuming the list is sorted */
  public int numUnique() {
    return StdRandom.uniform (100); //TODO: fix this
  }

  public static void main1 (String[] args) {
    Trace.showBuiltInObjects(true);
    Trace.drawStepsOfMethod ("numUnique");
    Trace.run ();
    PlaygroundNumUnique list1 = PlaygroundNumUnique.of ("11 21 21 21 31");
    int result1 = list1.numUnique ();
    Trace.draw();
    StdOut.println ("result: " + result1);
  }

  public static void main (String[] args) {
    testNumUnique(4, "11 21 21 21 31 41 41 41 41");
    testNumUnique(1, "11 11 11 11");
    testNumUnique(4, "11 21 31 41");
    testNumUnique(4, "11 11 11 21 31 31 31 31 41");
    testNumUnique(4, "11 11 21 21 21 31 31 41 41 41 41");
    testNumUnique(8, "11 11 11 11 21 31 41 41 41 41 41 51 51 61 71 81 81");
    testNumUnique(8, "11 21 31 41 41 41 41 41 51 51 61 71 81");
    testNumUnique(7, "11 11 11 11 21 31 41 41 41 41 41 51 51 61 71");
    testNumUnique(7, "11 21 31 41 41 41 41 41 51 51 61 71");
    testNumUnique(8, "-81 -81 -81 -81 -71 -61 -51 -51 -51 -51 -41 -41 -31 -21 -11 -11 -11");
    testNumUnique(3, "-11 -11 -11 0 0 11 11 11");
    testNumUnique(2, "0 11 11 11");
    testNumUnique(2, "-Infinity 11 11 11");
    testNumUnique(2, "11 11 11 Infinity");
    testNumUnique(1, "11 11");
    testNumUnique(1, "11");
    testNumUnique(0, "");
    
    StdOut.println ("Finished tests");
  }

  private static void testNumUnique (int expected, String sList) {
    PlaygroundNumUnique list = PlaygroundNumUnique.of (sList); 
    String sStart = list.toString ();
    int actual = list.numUnique ();
    if (expected != actual) {
      StdOut.format ("Failed: Expecting [%d] Actual [%d] with argument %s\n", expected, actual, list);
    }
    String sEnd = list.toString ();
    if (! sStart.equals (sEnd)) {
      StdOut.format ("Failed %s.numUnique(): List changed to %s\n", sStart, sEnd);
    }
  }

  /* ToString method to print */
  public String toString () { 
    // Use DecimalFormat #.### rather than String.format 0.3f to leave off trailing zeroes
    DecimalFormat format = new DecimalFormat ("#.###");
    StringBuilder result = new StringBuilder ("[ ");
    for (Node x = first; x != null; x = x.next) {
      result.append (format.format (x.item));
      result.append (" ");
    }
    result.append ("]");
    return result.toString ();
  }

  /* Method to create lists */
  public static PlaygroundNumUnique of(String s) {
    PlaygroundNumUnique result = new PlaygroundNumUnique ();
    if ("".equals (s)) return result;

    Node first = null;
    String[] nums = s.split (" ");
    for (int i = nums.length-1; i >= 0; i--) {
      try { 
        double num = Double.parseDouble (nums[i]); 
        first = new Node (num, first);      
      } catch (NumberFormatException e) {
        throw new IllegalArgumentException (String.format ("Bad argument \"%s\": could not parse \"%s\" as a double", s, nums[i]));
      }
    }
    result.first = first;
    return result;
  }
}

Loop using current [10/11]

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  public int numUnique() {
    if (first == null) return 0;
    int result = 1;
    Node curr = first.next;
    double prevVal = first.item;
    while (curr != null) {

      double currVal = curr.item;
      if (currVal != prevVal) {
        result += 1;
        prevVal = currVal;
      }
      curr = curr.next;
    }
    return result;
  }
 NumUniqueCurr-

Loop using previous [11/11]

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  public int numUnique() {
    if (first == null) return 0;
    int result = 1;
    
    Node prev = first;
    while (prev.next != null) {     
      double prevVal = prev.item;
      double currVal = prev.next.item;
      if (currVal != prevVal) {
        result += 1;

      }
      prev = prev.next;
    }
    return result;
  }
 NumUniquePrev-

Revised: 2008/03/17 13:01