1. Collections 24: Collections Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

2. Objectives Explore collections in System.Collections namespace
memory management
containment testing
sorting
traversal
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

3. Collection A collection stores a group of objects
Provides operations to manipulate contents
add
remove
count?
contains?
traverse
sort
collection of strings
"blue"
"green"
"red"
"yellow"
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

4. Motivation Basic array is too simple for many applications
24: Collections
Motivation
Basic array is too simple for many applications
efficient, easy to use, and typesafe
but fixed size
array size fixed
at creation
int[] a = new int[10];
a[0] = 17;
a[1] = 99;
...
int x = a[1];
foreach (int i in a)
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

5. Collection classes Collection library offers many collections
in namespace System.Collections
store object reference to be generic
perform memory management internally
Core classes:
ArrayList
Stack
Queue
Hashtable
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

6. ArrayList ArrayList provides storage for sequence of elements
24: Collections
ArrayList
ArrayList provides storage for sequence of elements
duplicate values ok
data stored in array
array resized automatically as needed
using System.Collections;
class Department {
ArrayList employees = new ArrayList();
... }
create ArrayList
to store Employees
ArrayList
object
employees
array of object references
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

7. 24: Collections
ArrayList services
ArrayList is primary class to support concept of list
public class ArrayList : IList, ICloneable {
int Add (object value) ...
void Insert(int index, object value) ...
void Remove (object value) ...
void RemoveAt(int index) ...
void Clear () ...
bool Contains(object value) ...
int IndexOf (object value) ...
object this[int index] { get... set.. }
int Capacity { get... set... }
void TrimToSize() ...
... }
add new elements
remove
containment testing
read/write existing element
control of memory
in underlying array
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

8. ArrayList Add Use Add method to add new element to ArrayList
24: Collections
ArrayList Add
Use Add method to add new element to ArrayList
added at end
ArrayList a = new ArrayList();
a.Add("bbb");
a.Add("aaa");
a.Add("ddd");
...
element 0
element 1
element 2
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

9. ArrayList Count Count property records number of elements in ArrayList
24: Collections
ArrayList Count
Count property records number of elements in ArrayList
read only
ArrayList a = new ArrayList();
a.Add("bbb");
a.Add("aaa");
a.Add("ddd");
int n = a.Count;
Count is 3
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

10. 24: Collections
ArrayList Insert
Use Insert method to insert new element into ArrayList
specify data and index
index must be in range 0 through Count
index equal to Count means add to end
other elements shifted as needed to accommodate new one
ArrayList a = new ArrayList();
a.Add("bbb");
a.Add("aaa");
a.Add("ddd");
a.Insert(2, "ccc");
insert at index 2
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

11. 24: Collections
ArrayList RemoveAt
Use RemoveAt method to remove element from ArrayList
specify index in range 0 through Count-1
other elements shifted to fill empty position
ArrayList a = new ArrayList();
a.Add("bbb");
a.Add("aaa");
a.Add("ddd");
a.RemoveAt(1);
remove element
at index 1
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

12. ArrayList Remove Use Remove method to remove element from ArrayList
24: Collections
ArrayList Remove
Use Remove method to remove element from ArrayList
pass in element to remove
searches linearly through list
uses Equals method for comparison
removes first occurrence
other elements shifted to fill empty position
ArrayList a = new ArrayList();
a.Add("bbb");
a.Add("aaa");
a.Add("ddd");
a.Remove("aaa");
remove element
equal to "aaa"
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

13. 24: Collections
ArrayList Contains
Use Contains method to do containment test in ArrayList
performs linear search through underlying array
uses Equals method to determine equality
ArrayList a = new ArrayList();
a.Add("bbb");
a.Add("aaa");
a.Add("ddd");
if (a.Contains("aaa"))
Console.WriteLine("found");
else
Console.WriteLine("not found");
contains?
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

14. 24: Collections
ArrayList IndexOf
Use IndexOf method search for element index in ArrayList
performs linear search through underlying array
uses Equals method to determine equality
returns index of first occurrence or -1 if not found
ArrayList a = new ArrayList();
a.Add("bbb");
a.Add("aaa");
a.Add("ddd");
int i = a.IndexOf("aaa");
...
returns 1
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

15. ArrayList indexer Can use indexer to get/set elements
24: Collections
ArrayList indexer
Can use indexer to get/set elements
cannot use to add new elements
index must be in range 0 through Count-1
ArrayList a = new ArrayList();
a.Add("bbb");
a.Add("aaa");
a.Add("ddd");
a[2] = "ccc";
string s = (string)a[2];
use Add to add
new elements
change existing element
read existing element
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

16. 24: Collections
ArrayList Capacity
Capacity property used to control size of underlying array
can specify initial value during creation
can get or set
ArrayList will adjust capacity automatically
defaults to 16 and doubles whenever more space needed
should let list control capacity when size of dataset unknown
default capacity is 16
ArrayList a = new ArrayList();
ArrayList b = new ArrayList(50);
b.Capacity = 100;
int c = b.Capacity;
specify capacity of 50
set capacity to 100
get capacity
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

17. Box/unbox Collections store object reference
value types boxed when added
unboxed using cast when retrieved
ArrayList a = new ArrayList();
int x = 7;
a.Add(x);
int y = (int)a[0];
boxed
unboxed
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

18. Enumeration Collection contents traversed using enumerator
24: Collections
Enumeration
Collection contents traversed using enumerator
IEnumerable defines how to get an enumerator
IEnumerator used to perform traversal and access data
gives read-only access
available on all collection types
public interface IEnumerable {
IEnumerator GetEnumerator(); }
get enumerator
from collection
public interface IEnumerator {
object Current { get; }
bool MoveNext();
void Reset(); }
access data
traverse
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

19. Traversal with enumerator
24: Collections
Traversal with enumerator
Can traverse collection contents with enumerator
get IEnumerator from collection with GetEnumerator()
call MoveNext() on enumerator to advance to first element
use read-only Current property to access element data
call MoveNext() to advance to next element
repeat while MoveNext() returns true
ArrayList a = new ArrayList();
a.Add("bbb"); a.Add("aaa"); a.Add("ddd");
IEnumerator e = a.GetEnumerator();
while (e.MoveNext()) {
string t = (string)e.Current;
... }
get enumerator
from collection
advance and test
access and
cast return
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

20. Traversal with foreach
24: Collections
Traversal with foreach
Can traverse enumerable collection with foreach loop
less code than using enumerator
convenient since cast performed implicitly
better than indexer since no chance of index error
gives read-only access
ArrayList a = new ArrayList();
a.Add("bbb");
a.Add("aaa");
a.Add("ddd");
foreach (string s in a) {
... }
handles loop
and cast
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

21. Sort Can sort the contents of Array and ArrayList
24: Collections
Sort
Can sort the contents of Array and ArrayList
use variations on Sort method
Must provide comparison method
can build in by implementing IComparable
can supply separate IComparer object
simple types have comparison built in
public class ArrayList : IList, ICloneable {
void Sort() ...
void Sort(IComparer comparer) ...
void Sort(int index, int count, IComparer comparer) ...
... }
built-in
compare
separate
compare
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

22. IComparable Build in comparison code with IComparable.CompareTo
24: Collections
IComparable
Build in comparison code with IComparable.CompareTo
return negative, zero, or positive to indicate relative order
class Student : IComparable {
public string name;
public int id;
public int CompareTo(object o)
Student s = o as Student;
if (this.id < s.id) return -1;
if (this.id > s.id) return 1;
return 0; }
...
cast
this goes first
parameter goes first
equal
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

23. Using IComparable IComparable objects have CompareTo built in
24: Collections
Using IComparable
IComparable objects have CompareTo built in
will be called by no argument Sort method
ArrayList students = new ArrayList();
Student ann = new Student("Ann", 8000);
Student bob = new Student("Bob", 2000);
students.Add(ann);
students.Add(bob);
students.Sort();
sort according
to CompareTo
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

24. IComparer Separate comparison code with IComparer.Compare
24: Collections
IComparer
Separate comparison code with IComparer.Compare
return negative, zero, or positive to indicate relative order
typically implemented in different class
useful if original class does not build in desired comparison
class StudentNameComparer : IComparer {
public int Compare(object o1, object o2)
Student s1 = o1 as Student;
Student s2 = o2 as Student;
return s1.name.CompareTo(s2.name); }
cast
compare
names
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

25. Using IComparer IComparer object created and passed to Sort method
24: Collections
Using IComparer
IComparer object created and passed to Sort method
comparison object used in preference to built in comparison
ArrayList students = new ArrayList();
Student ann = new Student("Ann", 8000);
Student bob = new Student("Bob", 2000);
students.Add(ann);
students.Add(bob);
StudentNameComparer cmp = new StudentNameComparer();
students.Sort(cmp);
comparer
sort
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

26. Stack Stack provides last-in-first-out storage data stored in array
24: Collections
Stack
Stack provides last-in-first-out storage
data stored in array
array resized automatically as needed
using System.Collections;
class Trace {
Stack callChain = new Stack();
... }
create Stack
to store sequence
of method calls
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

27. Stack services Stack offers standard last-in-first-out services Push
24: Collections
Stack services
Stack offers standard last-in-first-out services
Push
Peek
Pop
Stack s = new Stack();
s.Push("aaa");
s.Push("bbb");
string t = (string)s.Peek();
string u = (string)s.Pop();
...
add
examine
remove
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

28. Queue Queue provides first-in-first-out storage data stored in array
24: Collections
Queue
Queue provides first-in-first-out storage
data stored in array
array resized automatically as needed
using System.Collections;
class Watcher {
Queue events = new Queue();
... }
create Queue
to store events
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

29. Queue services Queue offers standard first-in-first-out services
24: Collections
Queue services
Queue offers standard first-in-first-out services
Enqueue
Dequeue
Peek
Queue q = new Queue();
q.Enqueue("aaa");
q.Enqueue("bbb");
q.Enqueue("ccc");
string s = (string)q.Peek();
string t = (string)q.Dequeue();
add
examine
remove
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

30. Hashtable Hashtable provides collection of key/value pairs
24: Collections
Hashtable
Hashtable provides collection of key/value pairs
often called a map
data stored in hash table
stores object reference for both key and value
GetHashCode method of key used to determine placement
indexer provides convenient access
create
Hashtable ages = new Hashtable();
ages["Ann"] = 27;
ages["Bob"] = 32;
ages.Add("Tom", 15);
ages["Ann"] = 28;
int a = (int)ages["Ann"];
add
update
retrieve
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

31. Hashtable traversal Can traverse Hashtable contents
24: Collections
Hashtable traversal
Can traverse Hashtable contents
each element is DictionaryEntry struct
data exposed in Key and Value properties
Hashtable ages = new Hashtable();
ages["Ann"] = 27;
ages["Bob"] = 32;
ages["Tom"] = 15;
foreach (DictionaryEntry entry in ages) {
string name = (string)entry.Key;
int age = (int) entry.Value;
... }
enumerate entries
get key and value
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

32. Summary Collections provided in System.Collections namespace
primary collections: Array ArrayList Hashtable
specialized collections: Stack Queue
Collections store object reference
reference types are compatible
value types are automatically boxed
Traversal supported
enumerators
foreach
ArrayList can be sorted
must supply comparison method
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003