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CSC 427: Data Structures and Algorithm Analysis
Fall 2008
Inheritance and efficiency
 ArrayList  SortedArrayList
 tradeoffs with adding/searching
 timing code
 divide-and-conquer algorithms
1
Dictionary revisited
recall the Dictionary
class earlier
import
import
import
import
java.util.List;
java.util.ArrayList;
java.util.Scanner;
java.io.File;
public class Dictionary {
private List<String> words;
public Dictionary() {
this.words = new ArrayList<String>();
}
 the ArrayList add
method simply
appends the item at
the end  O(1)
public Dictionary(String filename) {
this();
try {
Scanner infile = new Scanner(new File(filename));
while (infile.hasNext()) {
String nextWord = infile.next();
this.words.add(nextWord.toLowerCase());
}
}
catch (java.io.FileNotFoundException e) {
System.out.println("FILE NOT FOUND");
}
 the ArrayList contains
method performs
sequential search
 O(N)
}
public void add(String newWord) {
this.words.add(newWord.toLowerCase());
}
this is OK if we are
doing lots of adds
and few searches
public void remove(String oldWord) {
this.words.remove(oldWord.toLowerCase());
}
public boolean contains(String testWord) {
return this.words.contains(testWord.toLowerCase());
}
}
2
Timing dictionary searches
we can use our
StopWatch class to
verify the O(N)
efficiency
dict. size
38,621
77,242
144,484
insert time
401 msec
612 msec
1123 msec
dict. size
38,621
77,242
144,484
search time
1.10 msec
2.61 msec
5.01 msec
execution time
roughly doubles as
dictionary size
doubles
import java.util.Scanner;
import java.io.File;
public class DictionaryTimer {
public static void main(String[] args) {
System.out.println("Enter name of dictionary file:");
Scanner input = new Scanner(System.in);
String dictFile = input.next();
StopWatch timer = new StopWatch();
timer.start();
Dictionary dict = new Dictionary(dictFile);
timer.stop();
System.out.println(timer.getElapsedTime());
timer.start();
for (int i = 0; i < 100; i++) {
dict.contains("zzyzyba");
}
timer.stop();
System.out.println(timer.getElapsedTime()/100.0);
}
}
3
Sorting the list
if searches were common, then we might want to make use of binary search
 this requires sorting the words first, however
we could change the Dictionary class to do the sorting and searching
 a more general solution would be to extend the ArrayList class to SortedArrayList
 could then be used in any application that called for a sorted list
recall:
public class java.util.ArrayList<E> implements List<E> {
public ArrayList() { … }
public boolean add(E item) { … }
public void add(int index, E item) { … }
public E get(int index) { … }
public E set(int index, E item) { … }
public int indexOf(Object item) { … }
public boolean contains(Object item) { … }
public boolean remove(Object item) { … }
public E remove(int index) { … }
…
}
4
SortedArrayList (v.1)
using inheritance, we only need to redefine what is new
 add method sorts after adding; indexOf uses binary search
 no additional fields required
 big-Oh for add? big-Oh for indexOf?
import java.util.ArrayList;
import java.util.Collections;
public class SortedArrayList<E extends Comparable<? super E>> extends ArrayList<E> {
public SortedArrayList() {
super();
}
public boolean add(E item) {
super.add(item);
Collections.sort(this);
return true;
}
public int indexOf(Object item) {
return Collections.binarySearch(this, (E)item);
}
}
5
SortedArrayList (v.2)
is this version any better? when?
 big-Oh for add?
 big-Oh for indexOf?
import java.util.ArrayList;
import java.util.Collections;
public class SortedArrayList<E extends Comparable<? super E>> extends ArrayList<E> {
public SortedArrayList() {
super();
}
public boolean add(E item) {
super.add(item);
return true;
}
// NOTE: COULD REMOVE THIS METHOD AND
// JUST INHERIT THE ADD METHOD FROM
// ARRAYLIST AS IS
public int indexOf(Object item) {
Collections.sort(this);
return Collections.binarySearch(this, (E)item);
}
}
6
SortedArrayList (v.3)
if insertions and searches are mixed, sorting for each insertion/search
is extremely inefficient
 instead, could take the time to insert each item into its correct position
 big-Oh for add? big-Oh for indexOf?
import java.util.ArrayList;
import java.util.Collections;
public class SortedArrayList<E extends Comparable<? super E>> extends ArrayList<E> {
public SortedArrayList() {
super();
}
public boolean add(E item) {
int i;
for (i = 0; i < this.size(); i++) {
if (item.compareTo(this.get(i)) < 0) {
break;
}
}
super.add(i, item);
return true;
}
public int indexOf(Object item) {
return Collections.binarySearch(this, (E)item);
}
}
search from the start vs.
from the end?
7
Dictionary using SortedArrayList
note that repeated
calls to add serve as
insertion sort
dict. size
38,621
77,242
144,484
import java.util.Scanner;
import java.io.File;
import java.util.Date;
public class DictionaryTimer {
insert time
29.2 sec
127.9 sec
526.2 sec
public static void main(String[] args) {
System.out.println("Enter name of dictionary file:");
Scanner input = new Scanner(System.in);
String dictFile = input.next();
StopWatch timer = new StopWatch();
dict. size
38,621
77,242
144,484
search time
0.0 msec
0.0 msec
0.1 msec
insertion time roughly
quadruples as
dictionary size
doubles; search time
is trivial
timer.start();
Dictionary dict = new Dictionary(dictFile);
timer.stop();
System.out.println(timer.getElapsedTime());
timer.start();
for (int i = 0; i < 100; i++) {
dict.contains("zzyzyba");
}
timer.stop();
System.out.println(timer.getElapsedTime()/100.0);
}
}
8
SortedArrayList (v.4)
if adds tend to be done in groups (as in loading the dictionary)
 it might pay to perform lazy insertions & keep track of whether sorted
 big-Oh for add? big-Oh for indexOf?
 if desired, could still provide addInOrder method (as before)
import java.util.ArrayList;
import java.util.Collections;
public class SortedArrayList<E extends Comparable<? super E>> extends ArrayList<E> {
private boolean isSorted;
public SortedArrayList() {
super();
this.isSorted = true;
}
public boolean add(E item) {
this.isSorted = false;
return super.add(item);
}
public int indexOf(Object item) {
if (!this.isSorted) {
Collections.sort(this);
this.isSorted = true;
}
return Collections.binarySearch(this, (E)item);
}
}
9
Timing the lazy dictionary on searches
modify the Dictionary
class to use the lazy
SortedArrayList
dict. size
38,621
77,242
144,484
insert time
340 msec
661 msec
1113 msec
dict. size
38,621
77,242
144,484
1st search
10 msec
61 msec
140 msec
dict. size
38,621
77,242
144,484
search time
0.0 msec
0.0 msec
0.1 msec
import java.util.Scanner;
import java.io.File;
import java.util.Date;
public class DictionaryTimer {
public static void main(String[] args) {
System.out.println("Enter name of dictionary file:");
Scanner input = new Scanner(System.in);
String dictFile = input.next();
StopWatch timer = new StopWatch()
timer.start();
Dictionary dict = new Dictionary(dictFile);
timer.stop();
System.out.println(timer.getElapsedTime());
timer.start();
dict.contains("zzyzyba");
timer.stop();
System.out.println(timer.getElapsedTime());
timer.start();
for (int i = 0; i < 100; i++) {
dict.contains("zzyzyba");
}
timer.stop();
System.out.println(timer.getElapsedTime()/100.0);
}
}
10
Divide & Conquer algorithms
recursive algorithms such as binary search and merge sort are known as
divide & conquer algorithms
the divide & conquer approach tackles a complex problem by breaking
into smaller pieces, solving each piece, and combining into an overall
solution
 e.g., to binary search a list, check the midpoint then binary search the
appropriate half of the list
divide & conquer is applicable when a problem can naturally be divided
into independent pieces
 e.g., merge sort divided the list into halves, conquered (sorted) each half, then
merged the results
11
Iterative vs. divide & conquer
many iterative algorithms can naturally be characterized as divide-andconquer
 sequential search for X in list[0..N-1] =
false
true
sequential search for X in list[1..N-1]
if N == 0
if X == list[0]
otherwise
 sum of list[0..N-1] =
0
list[0] + sum of list[1..N-1]
if N == 0
otherwise
 number of occurrences of X in a list[0..N-1] =
number of occurrences of X in list[1..N-1]
1 + number of occurrences of X in list[1..N-1]
if X != list[0]
if X == list[0]
interesting, but not very useful from a practical side (iteration is faster)
12
Euclid's algorithm
one of the oldest known algorithms is Euclid's algorithm for calculating the
greatest common divisor (gcd) of two integers
 appeared in Euclid's Elements around 300 B.C., but may be even 200 years older
 defines the gcd of two numbers recursively, in terms of the gcd of smaller numbers
/** Calculates greatest common divisor of a and b
*
@param a a positive integer
*
@param b a positive integer (a >= b)
*
@return the GCD of a and b
*/
public int gcd(int a, int b) {
if (b == 0) {
return a;
}
else {
return gcd(b, a % b);
}
}
e.g.., gcd(32, 12)
e.g.., gcd(1024, 96) = gcd(96, 64)
= gcd(64, 32)
= gcd(32, 0)
= 32
e.g.., gcd(17, 5)
if the larger number has N digits,
• Euclid's algorithm requires at most O(N) recursive calls
• however, each (a % b) requires O(N) steps
O(N2)
= gcd(12, 8)
= gcd(8, 4)
= gcd(4, 0)
=4
= gcd(5, 2)
= gcd(2, 1)
= gcd(1, 0)
=1
there is no known algorithm with better big-Oh (but is possible to reduce constants)
13
Multidimensional divide & conquer
we will see later that divide & conquer is especially useful when
manipulating multidimensional structures
 e.g., print values in a binary tree
public void traverse(TreeNode<String> root) {
if (root != null) {
traverse(root.getLeft());
System.out.println(root.getValue());
traverse(root.getRight());
}
}
“phillies”
“cubs”
“braves”
“reds”
“expos”
“pirates”
“rockies”
 e.g., find the distance of the closest pair of points in a space
1.
2.
3.
4.
5.
LDist = distance of closest pair in left half
RDist = distance of closest pair in right half
LClose = set of points whose x-coord are within min(LDist,RDist)
to the left of center
RClose = set of points whose x-coord are within min(LDist,RDist)
to the right of center
answer = min(LDist, RDist, distance(LClosei,RClosej))
14

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