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Windows Programming Using C#. Arrays, Collections. Contents. Arrays Collections Generics Strings. Arrays. Arrays are similar to Java int[] intArray; Creates a reference to an array intArray = new int[5]; Creates an array object and assigns it

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windows programming using c

Windows Programming Using C#

Arrays, Collections

  • Arrays
  • Collections
  • Generics
  • Strings
  • Arrays are similar to Java
    • int[] intArray;
  • Creates a reference to an array
    • intArray = new int[5];
  • Creates an array object and assigns it
  • Array objects are reference types created on the heap
  • The integers in the array are value types
  • Array indexing is zero based
system array
  • All arrays are derived from this class
  • Properties
    • Length – number of elements in all dimensions
    • Rank – number of dimensions
  • Methods
    • BinarySearch – performs a binary search for a member
system array1
  • Clear – sets a range of elements to 0 or null
  • Clone – creates a shallow copy
  • Copy – copies elements from one array to another
  • getLength – returns the length of a single dimension
  • IndexOf – returns the first occurrence of a value
  • Sort – sorts the array
arrays of objects
Arrays of Objects
  • When you create an array of objects, it is simply an array of references to objects, not the actual objects
  • These references are initialized to null
  • You must then create objects and assign to each member of the array
the foreach statement
The foreach Statement
  • This iterates through all elements of an array or collection
    • foreach (type ident in expression) statement

int[] ar = new int[5] {1,2,3,4,5};

foreach(int i in ar) {



rectangular arrays
Rectangular Arrays
  • C# supports rectangular and jagged arrays
  • Arrays can be of 1 or more dimensions
  • To declare a rectangular array

int[,] int2d = new int[2,3];

for (int i = 0; I < 2; i++) {

for(int j = 0; j < 3; j++) {

int2d[i,j] = i+j;



jagged arrays
Jagged Arrays
  • These are really arrays of arrays
  • Each member must be created as a separate array
  • Note the difference in subscripting

int jag[][] = new int[2][];

jag[0] = new int[5];

jag[1] = new int[10];

jag[0][1] = 15;

  • Indexers allow any class to be indexed like an array
  • Indexers are declared in a way similar to properties
  • They always use the keyword this
  • The index can be an integer or even a string
  • * see IndexedArray
  • Create a simple array which can be indexed
    • By an integer
    • By a string using “first”, “middle” or “last”

class IndexedArray {

int[] data;

public IndexedArray(int sz) {

data = new int[sz];


  • An indexer with integer indices

public int this[int index]{

get { return data[index]; }

set { data[index] = value; }


  • An indexer with string indices

public int this[string index] {

get {

int idx = 0;

switch (index){

case "middle":

idx = data.Length / 2;


case "first":

idx = 0;


case "last":

idx = data.Length - 1;



return data[idx];



  • Arrays
  • Collections
  • Generics
  • Strings
  • Collections are generic data structures for containing objects or primitives
  • These are often in the form of common data structures like lists, and dictionaries
  • Many of the collections have interfaces representing their functionality
the ienumerable interface
The IEnumerable Interface
  • This states that the class supports an enumerator or iterator which steps through the elements one-by-one
  • Classes which implement this interface can be used with the foreach statement

public interface IEnumerable {

IEnumerator GetEnumerator();


the ienumerator interface
The IEnumerator Interface
  • This is the interface the enumerator must implement
  • It is usually implemented as a private class within the class being enumerated

public interface IEnumerator {

bool MoveNext();

bool Reset();

object Current { get; }



private class IndexedArrayEnumerator: IEnumerator {

int idx;

IndexedArray theArray;

public IndexedArrayEnumerator(IndexedArray ar) {

theArray = ar;

idx = -1;


public bool MoveNext() {


if(idx >=

return false;


return true;


public void Reset() {

idx = -1;


public object Current {

get{ return theArray[idx]; }



getting an enumerator
Getting an Enumerator

public IEnumerator GetEnumerator()


return new IndexedArrayEnumerator(this);


  • This creates and returns an enumerator
  • Using a separate enumerator class allows several enumerators to operate at the same time
the icollection interface
The ICollection Interface
  • Defines properties and methods for all collections
  • Implements: IEnumerable
    • Count
      • the number of elements in the collection
    • CopyTo(Array ar, int idx)
      • Copies all elements to an array starting at the index
  • The problem with arrays is their fixed size
  • ArrayLists fix this by resizing the array when the addition of new members exceeds the capacity
  • The class has an extensive set of methods
  • Sorting primitives is easy since comparing one to another is well-defined
  • Sorting user-defined types is more difficult since you do not know how to compare one to another
  • There are two solutions
    • IComparable
    • IComparer
the icomparable interface
The IComparable Interface
  • This require one method CompareTo which returns
    • -1 if the first value is less than the second
    • 0 if the values are equal
    • 1 if the first value is greater than the second
  • This is a member of the class and compares this to an instance passed as a parameter

public interface IComparable {

int CompareTo(object obj)


the icomparer interface
The IComparer Interface
  • This is similar to IComparable but is designed to be implemented in a class outside the class whose instances are being compared
  • Compare() works just like CompareTo()

public interface IComparer {

int Compare(object o1, object o2);


sorting an arraylist
Sorting an ArrayList
  • To use CompareTo() of IComparable
    • ArrayList.Sort()
  • To use a custom comparer object
    • ArrayList.Sort(IComparer cmp)
  • To sort a range
    • ArrayList.Sort(int start, int len, IComparer cmp)
implementing icomparer
Implementing IComparer
  • To sort people based on age

class PersonComparer: IComparer


public int Compare(object o1, object o2)


PersonBase p1 = (PersonBase)o1;

PersonBase p2 = (PersonBase)o2;

return p1.Age.CompareTo(p2.Age);



icloneable interface
ICloneable Interface
  • This guarantees that a class can be cloned
  • The Clone method can be implemented to make a shallow or deep clone

public interface ICloneable {

object Clone();


queue class
Queue Class
  • Implements: ICollection, IComparable, ICloneable
stack class
Stack Class
  • Implements: ICollection, IComparable, ICloneable
idictionary interface
IDictionary Interface
  • A dictionary is an associative array
  • It associates a key with a value and allows a value to be retrieved by providing the key
  • Implements: ICollection, IEnumerable
  • The hashtable is a common implementation of the IDictionary interface
  • If the key is not an integer then the hashcode for the key is used as an index into the hashtable
  • Keys used with hashtables must have unique hashcode for every value
idictionaryenumerator interface
IDictionaryEnumerator Interface
  • This is the type of enumerator used with dictionaries
  • It implements IEnumerator
  • Has properties
    • Key
      • Returns the key for the item
    • Value
      • Returns the value for the item
bitarray class
BitArray Class
  • Long bit strings can be difficult to store efficiently
  • Since data structures can be addressed on the byte level, we end up storing one bit per byte, wasting 7 bits
  • The BitArray class stores the bits efficiently while providing access
bitarray class1
BitArray Class
  • The constructor is overloaded
    • BitArray(Boolean[])
      • Makes a BitArray from an array of Booleans
    • BitArray(Byte[])
      • Makes an array from an array of Bytes where each byte represents 8 bits
    • BitArray(int len)
      • Creates a BitArray of len bytes
    • BitArray(int[])
      • Makes a BitArray from the 32 bits in each int in an array of ints
bitarray indexer
BitArray Indexer
  • The BitArray has an indexer providing both get and set

BitSet bs = new BitSet(8);

bs[0] = true;


  • There are also Get and Set methods

bool Get(int index)

void Set(int index, bool value)

bitarray operations
BitArray Operations
  • Various Boolean operations are provided
    • BitArray And(BitArray)
    • BitArray Or(BitArray)
    • BitArray Xor(BitArray)
    • BitArray Not()
  • You can set all of the bits at once
    • void SetAll(bool value)
  • Arrays
  • Collections
  • Generics
  • Strings
  • All of the containers so far have stored data as objects
  • This means
    • Containers hold any type
    • Operations must test the type before operating on the objects in the container
    • When objects are removed from the container they must be cast to their true type
    • The programmer must remember the type of the object placed in the container
    • CLR must test to see that the cast is legal
  • All of this
    • Places an extra burden on the programmer having to remember the types
    • Introduces new potential sources of error
    • Forces expensive run-time type checking
  • What is needed is a way to make a class work with many types but
    • Do so efficiently
    • Enforce compile type strong type checking
  • The solution is generics!
  • This is almost the same as the template facility of C++
  • Classes and methods are parameterized with a type
  • Every time the class or method is used with a new type, the compiler generates the code for that type and compiles it
  • This allows strong type checking at compile time
generic classes
Generic Classes
  • To create a generic class, we parameterize it with one or more type parameters
  • The parameter types can then be used as types within the class and will be replaced with the actual types used when the class instance is created
generic array
Generic Array
  • A growable array which can hold any type

class GenericArray<T>


T[] data;

public GenericArray(int sz)


if (sz < 1) sz = 1;

data = new T[sz];



  • * see GenericArray
creating generic class instances
Creating Generic Class Instances
  • To create an instance of our generic array for integers

GenericArray<int> iar = new GenericArray<int>(5);

  • This will cause the compiler to write a new class and replace every occurrence of the parameter T by int and compile this new class
generic methods
Generic Methods
  • A class can also have generic methods which can work with any type
  • We will demonstrate this by writing a method to test the Generic Array
  • This method will be placed in the class containing the Main method
generic methods1
Generic Methods

static void test<E>(string id, GenericArray<E> ar,

E[] data) {

for (int i = 0; i < data.Length; i++){

ar[i] = data[i];


for (int i = 0; i < data.Length; i++){

Console.WriteLine("{0}[{1}] = {2}",

id, i, ar[i]);


Console.WriteLine("final capacity={0}",



generic collections
Generic Collections
  • There are actually three different namespaces for collections
    • System.Collections
      • Non-generic collections with data stored as objects
    • System.Collections.Generic
      • Generic collections
    • System.Collections.Specialized
      • Specialized, strongly typed collections designed to work efficiently with specific types
  • Arrays
  • Collections
  • Generics
  • Strings
  • C# strings are really instances of System.String
  • The class implements
    • IComparable, ICloneable, IConvertable, IEnumerable, IEnumerable<string>, IComparable<string>, IEquatable
  • The class provides a large number of methods for manipulating strings
creating strings
Creating Strings
  • The easiest way is to assign a string in quotes

string s = “abc”;

  • All builtin types have a ToString() method which can be used to convert to a string

int n = 5;

string s1 = n.ToString();

  • Verbatim strings start with an @ symbol and do not have escape characters replaced
    • string exact = @”ab\nc”
manipulating string contents
Manipulating String Contents
  • Strings are immutable
  • This means that any attempt to change a string will create a new string
  • This has major performance implications
  • If you will be making major changes to a string, you should use a StringBuilder
accessing string contents
Accessing String Contents
  • Strings have an indexer which supports get only

String s = “abc”;


  • There is also a substring method

Substring(int startIdx);

Substring(int start, int len);

  • Find the string length with Length property


comparing strings
Comparing Strings
  • A static method to compare two strings
    • static int compare(string, string);
  • An instance method to compare this to another string
    • int CompareTo(string);
  • Static & instance versions of Equals
    • int Equals(string)
    • static bool Equals(string, string)
comparing strings1
Comparing Strings
  • Comparing the starts and ends of strings
    • bool startsWith(string)
    • bool endsWith(string)
searching strings
Searching Strings
  • To find the first index of a char
    • int IndexOf(char)
    • int IndexOf(char, int startIndex)
  • To find the first index of a string
    • int IndexOf(string)
    • int IndexOf(string, int startIndex)
  • To find the last index of a char
    • int LastIndexOf(char)
    • int LastIndexOf(char, int startIndex)
  • To find the last index of a string
    • int LastIndexOf(string)
    • int LastIndexOf(string, int startIndex)
searching strings1
Searching Strings
  • Find the first occurrence of any of a set of characters
    • int IndexOfAny(char[])
    • int IndexOfAny(char[], int startIndex)
  • Find the last occurrence of any of a set of characters
    • int LastIndexOfAny(char[])
    • int LastIndexOfAny(char[], int startIndex)
handling spaces
Handling Spaces
  • To remove a set of chars from start or end
    • TrimStart(chars[])
    • TrimEnd(chars[])
  • To remove spaces from both ends
    • Trim()
  • To remove a set of chars from both ends
    • Trim(char[])
  • To pad spaces on either side
    • PadRight(int totalLength)
    • PadLeft(int totalLength)
converting case
Converting Case
  • ToUpper()
  • ToLower()
splitting and joining
Splitting and Joining
  • To split a string into an array of strings at every occurrence of a separator char
    • string[] split(string, char[])
  • To join an array of strings into one string with a separator string between every pair of string
    • string join(string sep, string[])
  • To format an object into a string
    • string format(string fmt, object)
  • The format string is the same as used by WriteLine where an argument is referenced by an index
    • string s = s1.format(“{0:fmt}”, i);
    • Where fmt is one of the format codes on the next page
stringbuilder class
StringBuilder Class
  • The trouble with strings is that they are immutable
  • Every time you have to concatenate two strings, a new string is produced
  • This creates a huge amount of memory allocation and garbage collection
  • The StringBuilder is a class which can build a string much more efficiently
stringbuilder class1
StringBuilder Class
  • To create one


StringBuilder(int capacity)

  • If the capacity of the StringBuilder is exceeded, new space is automatically allocated
  • Ensuring sufficient capacity at the start is more efficient than forcing reallocation
appending to a stringbuilder
Appending to a StringBuilder
  • StringBuilder Append(anyPrimitive)
    • This produces a string representation of the primitive and appends it onto the end of the contents of the StringBuilder
    • This can be used to append strings too
  • You can also append a formatted string similar to the formatting done by WriteLine

sb.AppendFormat("2) {0}, {1}", var1, var2);

appending to a stringbuilder1
Appending to a StringBuilder
  • You can also append a line using the default line terminator



  • Without a parameter, it just inserts a line terminator
  • With a parameter, the string is appended followed by the line terminator
getting the content
Getting the Content
  • Building a string is useless unless you can retrieve it
  • To get the whole StringBuffer

string ToString()

  • To get just part of the StringBuffer

string ToString(int start, int len)