241-423 Advanced Data Structures
and Algorithms Semester 2, 2013-2014
9. Maps
Objectives
– examples of maps, and introduce map
collection views
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Contents
1.
2.
3.
4.
5.
What is a Map?
Map Interfaces
The TreeMap Collection
Map Collection Views
Create a Concordance
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1. What is a Map?
• A map stores data in key-value pairs.
• A key acts like an index to locate the
corresponding value in the map
– a map is also called an associative array
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2. The Map Hierarchy
• The dashed lines indicate that the class implements
the interface.
• The Map interface is separate from the Collection
hierarchy, since if defines methods not relevant to
general collections.
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The Map Interface
interface MAP<K,V> (partial)
ds.util
void clear()
Removes all mappings from this map.
key)
K
boole containsKey(Object
an
Returns true if this map contains a mapping for the specified key.
boole isEmpty()
an
Returns true if this map contains no key-value mappings.
key)
V remove(Object
K
Removes the mapping for this key from this map if present. Returns the previous
value associated with specified key, or null if there was no mapping for key.
int size()
Returns the number of key-value mappings in this map.
Access/Update Methods
key)
K
VK get(Object
Returns the value to which this map maps the specified key or null if the map
contains no mapping for this key.
V put(K key, V value)
Associates the specified value with the specified key in this map. Returns the
previous value associated with key, or null if there was no mapping for key.
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• size(), isEmpty(), and clear() are like those in
the Collection interface.
• A map does not have an iterator to scan its
elements
– instead, keySet() and entrySet() return the keys and
the entries in a map as a set (see later)
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3. The TreeMap Collection
• TreeMap is an ordered collection that
accesses elements in ascending order of its
keys.
• The class implements the OrderedMap
interface (see slide 3)
– firstKey() and lastKey() return the values
corresponding to the minimum and maximum
keys
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class TreeMap<K,V> implements OrderedMap<K,V>
ds.util
Constructor
TreeMap()
Creates an empty ordered map. The key type K must implement
Comparable.
Methods
VT firstKey()
Returns the value associated with the entry that has the minimum key.
VT lastKey()
Returns the value associated with the entry that has the maximum key.
String toString()
Returns a comma-separated list of entries enclosed in braces ("{}"). Each
entry has the format "key = value".
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Brief TreeMap Example
// arrays for class names and their enrollment (student nos)
String[] className = {"ECON 101","CS 173","ENGL 25"};
int[] enrollment = {85, 14, 30};
// create a TreeMap object
TreeMap<String, Integer> tm = new TreeMap<String, Integer>();
for(int i = 0; i < 3; i++)
tm.put(className[i], enrollment[i]);
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3.1. Student Working Hours Map
import java.util.Scanner;
import java.io.FileReader;
import java.io.FileNotFoundException;
import ds.util.TreeMap;
import ds.time.Time24;
the key-value pair is
student name and
hours worked
public class CalcHours
{
public static void main(String[] args)
{
TreeMap<String, Time24> workMap =
new TreeMap<String,Time24>();
:
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/* access the student info file:
each line consists of a student name and
their work time in hours and mins
*/
Scanner fin = null;
try {
// load student info
fin = new Scanner(new FileReader("studwk.txt"));
}
catch (FileNotFoundException e) {
System.err.println("Cannot open "\"studwk.txt"");
System.exit(1);
}
:
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// input names and times
while (fin.hasNext()) {
String studName = fin.next();
// get hours and minutes from the input line
int hours = fin.nextInt();
int mins = fin.nextInt();
Time24 workTime = new Time24(hours,mins);
// access entry for the student name
Time24 timeValue = workMap.get(studName);
:
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if (timeValue == null)
// add new entry
workMap.put(studName, workTime);
else {
// update existing entry
timeValue.addTime(hours*60 + mins);
workMap.put(studName, timeValue);
}
}
}
}
// display the work Map
System.out.println("Student-Time: " + workMap);
// end of main()
// end of CalcHours class
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Execution
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3.2. A Software Map
import java.io.FileReader;
import java.io.FileNotFoundException;
import java.util.Scanner;
import ds.util.TreeMap;
import ds.util.TreeSet;
the key-value pair is
company name and
a set of software names
public class MakeSoftMap
{
public static void main(String[] args)
{
TreeMap<String, TreeSet<String>> softwareMap =
new TreeMap<String, TreeSet<String>>();
:
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/* access the product info file:
each line consists of a company name and
software product name, separated by a tab
*/
Scanner fin = null;
try {
// load product info
fin = new Scanner(new FileReader("product.txt"));
fin.useDelimiter("[\t\n\r]+");
}
catch (FileNotFoundException e){
System.err.println("Cannot open "\"product.txt\"");
System.exit(1);
}
:
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while(fin.hasNext()){
String company = fin.next();
String product = fin.next();
// look for software set for the company
TreeSet<String> prodSet = softwareMap.get(company);
// if no entry found, then create empty software set
if (prodSet == null)
prodSet = new TreeSet<String>();
prodSet.add(product);
// add product name to set
softwareMap.put(company, prodSet); // add entry
}
}
}
// display contents of software Map
System.out.println(softwareMap);
// end of main()
// end of MakeSoftMap class
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Execution
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4. Map Collection Views
• A map does not have an iterator for accessing
its elements.
• Instead we can use two collection views, which
are sets that act on the original map
– the keySet collection view
– the entrySet collection view
• The original map is sometimes called the
backing collection.
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4.1. The keySet Collection View
• In Map, keySet() returns a set of map keys.
This set is a collection view for the map.
Set<String> keys = peopleMap.keySet();
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Deletion
• Deleting a key from the set removes the
corresponding entry from the map.
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Insertion
• The Set interface defines an add() operation,
but this makes no sense for a keySet view:
– add() would have to add a key-value pair to the
backing collection, but what value should be
added?
– Instead add() throws an
UnsupportedOperationException in a keySet
view
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4.2. The entrySet Collection View
• In Map, entrySet() returns a view called an
entry set, is a set of key-value entries.
• The entries implement the Map.Entry interface
– an interface inside the Map interface
Set< Map.Entry<String, Integer> > entriesSet =
peopleMap.entrySet();
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• entriesSet is a set of Map.Entry objects from
the peopleMap.
• Operations on the set affect the map.
interface MAP.ENTRY<K,V>
ds.util.Map
K getKey()
Returns the key corresponding to this entry..
V getValue()
Returns the value corresponding to this entry..
V setValue(V value)
Replaces the value corresponding to this entry with the specified value. Returns
the old value corresponding to the entry
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Entry Set View Example
• confTimeMap holds conference activities and
their times.
TreeMap<String, Time24> confTimeMap =
new TreeMap<String, Time24>();
confTimeMap.put("Session 1", new Time24(9,30));
confTimeMap.put("Session 2", new Time24(14,00));
confTimeMap.put("Lunch", new Time24(12,0));
confTimeMap.put("Dinner", new Time24(17,30));
confTimeMap
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// create an entry set for confTimeMap
Set< Map.Entry<String,Time24> > entries =
confTimeMap.entrySet();
entries set
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4.3. Scanning a Map's Entries
• A Map.Entry set can be used to define an
iterator for scanning the map entries.
• The iterator can access an entry's key and
access/update an entry's value.
• The iterator remove() method removes an
entry from the map
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Entry Set Iterators
• An entry set iterator provides a way to scan the
entries in a map.
• The iterator references a Map.Entry element in
the map
– the programmer can use Map.Entry methods to
access the map components
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Examples
// create an entry set for confTimeMap
Set< Map.Entry<String,Time24> > entries =
confTimeMap.entrySet();
// create an iterator for the entry set
Iterator< Map.Entry<String, Time24> > iter =
entries.iterator();
entries set
iter
iterator
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map
entries
set
entries
iterator
changes will affect
original map
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• Delay all activities by 30 minutes.
// scan map entries
while (iter.hasNext()) {
Map.Entry<String, Time24> me = iter.next();
// get next Map.Entry object
Time24 t = me.getValue();
t.addTime(30);
// add 30 mins
me.setValue(t);
// update map entry
}
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• List session starting time.
for (Map.Entry<String,Time24> i : entries) {
String activity = (String)i.getKey();
if (activity.indexOf("Session") != -1) //is activity a session?
System.out.println("Activity " + activity +
"
Starting time " + i.getValue());
}
Activity
Activity
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Session 1
Session 2
Starting time 10:00
Starting time 14:30
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5. Create a Concordance
• A concordance is a list of all the unique
words from a file along with the line
numbers where the words appear.
• We implement the concordance as a
TreeMap.
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Input File
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Execution
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import
import
import
import
import
java.io.*;
java.util.regex.*;
java.util.StringTokenizer;
java.util.Scanner;
ds.util.*;
public class CreateConcordance
{
private static Pattern identifierPattern =
Pattern.compile("[a-zA-Z][a-zA-Z0-9]*");
public static void main(String[] args) throws IOException
{
System.out.print("Enter the file name: ");
Scanner keyIn = new Scanner(System.in);
String filename = keyIn.nextLine();
System.out.println();
concordance(filename);
// create concordance
} // end of main()
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public static void concordance(String filename)
throws IOException
{
the key-value pair is
identifier name and
a set of line numbers
// create the concordance treemap
TreeMap<String, TreeSet<Integer>> concordanceMap =
new TreeMap<String, TreeSet<Integer>>();
// create scanner to input from document file
Scanner fin = new Scanner(new FileReader(filename));
:
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// read the file a line at a time
int lineNumber = 0;
while(fin.hasNext()) {
String inputLine = fin.nextLine();
lineNumber++;
// read a line
// create matcher to find identifiers in line
Matcher matcher = identifierPattern.matcher(inputLine);
// extract identifiers until end of line
while (matcher.find()) {
String ident = inputLine.substring(
matcher.start(), matcher.end());
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// find line number set for the identifier
TreeSet<Integer> lineNums =
concordanceMap.get(ident);
if (lineNums == null)
// no set; make one
lineNums = new TreeSet<Integer>();
// add line number to set
lineNums.add(lineNumber);
concordanceMap.put(ident, lineNums);
}
}
// output the concordance
writeConcordance(concordanceMap);
} // end of concordance()
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public static void writeConcordance(
TreeMap<String,TreeSet<Integer>> map)
{
// create entry view for the map
Set<Map.Entry<String,TreeSet<Integer>>> entries =
map.entrySet();
// create iterator over the entry view
Iterator<Map.Entry<String,TreeSet<Integer>>> iter =
entries.iterator();
while (iter.hasNext()) {
Map.Entry<String,TreeSet<Integer>> e = iter.next();
System.out.print( e.getKey() );
// output key
// pad output to 12 characters using blanks
if (e.getKey().length() < 12)
for (int i=0;i < 12 - (e.getKey().length()); i++)
System.out.print(' ');
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// extract map value as a TreeSet
TreeSet<Integer> lineNumberSet = e.getValue();
// display identifier and line numbers
System.out.print(formatInt(4, lineNumberSet.size()) +
":
");
// iterate over TreeSet of line numbers
Iterator<Integer> setIter = lineNumberSet.iterator();
while (setIter.hasNext())
System.out.print( setIter.next() + "
");
System.out.println();
}
System.out.println();
}
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private static String formatInt(int w, int n)
/* returns a formatted string with integer n
right-justified in a field of w spaces;
used to line up output in concordance
*/
{ . . . }
}
// end of CreateConcordance class
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