DSA - Lecture 04

C# String Theory

Dr.Haitham A. El-Ghareeb

Information Systems Department
Faculty of Computers and Information Sciences
Mansoura University
helghareeb@gmail.com

October 7, 2012

Dr.Haitham A. El-Ghareeb (CIS) Data Structures and Algorithms - 2012 October 7, 2012 1 / 47

Special Thanks GoTo
en.csharp-online.net
msdn.microsoft.com
www.codeproject.com
blogs.msdn.com


Objectives
By the end of this lecture, you shall be able to:
1 Tell the diﬀerence between string and String and String Builder
2 Capable of using String as the most famous Collection
3 Aware of the diﬀerent String Methods
4 Be familiar with the importance of Pattern Matching and Regular
Expression


C# Strings


C# Strings
In C#, a string is a sequential collection of Unicode characters that
represents text


C# Strings
represents text
a String object is a sequential collection of System.Char objects that
represents a string.


C# Strings
represents text
a String object is a sequential collection of System.Char objects that
represents a string.
A C# string has several properties which are critical to understanding
how to use them.


string is a Reference Type
A common misconception is that a C# string is a value type.
In many situations it does act a bit like a value type.
It is-in fact-a normal reference type to an object of type
System.String.


string is immutable
Once created, the data value in a string object can never be changed:
it is immutable (read-only)
Methods that appear to modify a String object-in fact-return a new
String object containing the modiﬁcation.
To modify the actual contents of a string-like object, the
System.Text.StringBuilder class can be used.


string may contain nulls
The String methods will handle null characters in string values
however, many classes-e.g. Windows Forms classes-may consider the
string terminated at the ﬁrst null.


string versus String
In the .NET framework, string is simply an alias for the Common
Type System (CTS) System.String class
String class represents a sequence of characters.
string is one of two predeﬁned C# reference types: The other is
object.
Use them interchangeably in your code.

String x = string . Copy ( ” x ” ) ;
string y = String . Copy ( ” y ” ) ;


C# String Literals
Contiguous sequence of zero or more characters enclosed in double
quotation marks (”)
representing string data, rather than a reference to string data.
A string literal is a literal of type string hard-coded in your program.
There are two methods of representing string literals-literal (quoted)
and verbatim ( -quoted).
The literal method requires certain characters such as quotation
marks (”) and whitespace to be escaped-that is, preceded by the
backslash escape character.
On the other hand, the verbatim method accepts any
characters-except quotation marks-including whitespace without
escaping.


String Class vs. String Object
The String class represents textual data, i.e., a series of Unicode
characters;
String object is actually a sequential collection of System.Char
objects.
The value of the char is the content of the collection
The value is immutable.


String Fields and Properties
Empty field: String.Empty is a read-only field which represents the
empty string.
Chars property: returns the character at a specified character position
in the instance.
Length property: returns the number of characters in this instance.


String Operators
String assignment: Strings can be assigned new values using either of
these assignment operators: =, +=
String index: The [ ] operator can be used to access individual
characters in a string like this: char letter = string1[5];
String concatenation: strings can be concatenated using the plus (+)
operator, or String.Concat


String Operators
String overloads the equality operators: Normally, the == operator
calls the Equals method which compares the pointer values found in
the reference variables for equality. This tests whether they point to
the same object.

string string1 = ” v a l u e ” ;
Object string2 = ” v a l u e ” ;
i f ( string1 == string2 ) // compares references
{
Console . WriteLine ( ”==” ) ; // comparison is true
}


String Manipulation
The String class contains many public methods which can be used to
manipulate strings.
The most frequently useful of them will be discussed here.
There are two types of string manipulation methods:
String instance methods are called via a string object, e.g.
string1.Equals(string2).
String static methods-or class methods-are called via the String class,
e.g. String.ReferenceEquals (string1, string2).


String instance methods
CompareTo
CopyTo
EndsWith
Equals
GetEnumerator
IndexOf
IndexOfAny
Insert
LastIndexOf
LastIndexOfAny
PadLeft
PadRight


String instance methods (cont.)
Remove
Replace
Split
StartsWith
Substring
ToCharArray
ToLower
ToUpper
Trim
TrimEnd
TrimStart


String static methods
Compare
CompareOrdinal
Concat
Copy
Format
Join
ReferenceEquals


StringBuilder Class
Represents a mutable string of characters.
This class cannot be inherited.
Use the StringBuilder class whenever you will be doing a great deal of
string manipulation. This will be speedier and save memory in most
cases.


StringBuilder Remarks
This class represents a string-like object whose value is a mutable
sequence of characters.
The value is said to be mutable because it can be modiﬁed after
creation by appending, removing, replacing, or inserting characters.
Most of the methods that modify an instance of this class return a
reference to that same instance, and you can call a method or
property on the reference.
This can be convenient if you want to write a single statement that
chains successive operations.


StringBuilder Remarks (cont.)
The capacity of a StringBuilder instance is the maximum number of
characters the instance can store at any given time.
The capacity is greater than, or equal to, the length of the string
representation of the value of the instance.
The capacity can be increased or decreased with the Capacity
property or EnsureCapacity method, but it cannot be less than the
value of the Length property.
If you don’t specify capacity or maximum capacity when you initialize
an instance of StringBuilder, implementation-speciﬁc default values
are used.


StringBuilder Functionality
The current size of a StringBuilder object is deﬁned by its Length
property. You can access the characters in the value of a StringBuilder
object by using the Chars property. Index positions start from zero.
The StringBuilder class includes methods that can reduce the size of
the current instance. The Clear method removes all characters and
sets the Length property to zero. The Remove method deletes a
range of characters.
The StringBuilder class also includes methods that can expand the
current instance. The Append and AppendLine methods add data to
the end of the StringBuilder object, and the Insert method inserts
data at a speciﬁed character position in the current StringBuilder
object. The AppendFormat method uses the composite formatting
feature to add formatted text to the end of a StringBuilder object.


StringBuilder Class Functionality (cont.)
The Replace method replaces all occurrences of a character or a string
in the entire StringBuilder object or in a particular character range.
You must convert the StringBuilder object to a String object before
you can pass the string represented by the StringBuilder object to a
method that has a String parameter or display it in the user interface.
You perform this conversion by calling the ToString method.


StringBuilder Performance Considerations
A String object concatenation operation always creates a new object
from the existing string and the new data.
A StringBuilder object maintains a buffer to accommodate the
concatenation of new data.
New data is appended to the buffer if room is available; otherwise, a
new, larger buffer is allocated, data from the original buffer is copied
to the new buffer, and the new data is then appended to the new
buffer.
The performance of a concatenation operation for a String or
StringBuilder object depends on the frequency of memory allocations.


StringBuilder Performance Considerations
A String concatenation operation always allocates memory, whereas a
StringBuilder concatenation operation allocates memory only if the
StringBuilder object buﬀer is too small to accommodate the new data.
Use the String class if you are concatenating a ﬁxed number of String
objects. In that case, the compiler may even combine individual
concatenation operations into a single operation.
Use a StringBuilder object if you are concatenating an arbitrary
number of strings; for example, if you’re using a loop to concatenate
a random number of strings of user input.


StringBuilder Memory Allocation
The default capacity for this implementation is 16, and the default
maximum capacity is Int32.MaxValue.
A StringBuilder object can allocate more memory to store characters
when the value of an instance is enlarged, and the capacity is
adjusted accordingly.
Append, AppendFormat, EnsureCapacity, Insert, and Replace
methods can enlarge the value of an instance.
The amount of memory allocated is implementation-speciﬁc, and an
exception (either ArgumentOutOfRangeException or
OutOfMemoryException) is thrown if the amount of memory required
is greater than the maximum capacity.


StringBuilder Class Constructors
StringBuilder(): Initializes a new instance of the StringBuilder class.
StringBuilder(Int32): Initializes a new instance of the StringBuilder
class using the specified capacity.
StringBuilder(String): Initializes a new instance of the StringBuilder
class using the specified string.
StringBuilder(Int32, Int32): Initializes a new instance of the
StringBuilder class that starts with a specified capacity and can grow
to a specified maximum.
StringBuilder(String, Int32): Initializes a new instance of the
StringBuilder class using the specified string and capacity.
StringBuilder(String, Int32, Int32, Int32): Initializes a new instance of
the StringBuilder class from the specified substring and capacity.


StringBuilder Class Properties
Capacity: Gets or sets the maximum number of characters that can
be contained in the memory allocated by the current instance.
Chars: Gets or sets the character at the speciﬁed character position in
this instance.
Length: Gets or sets the length of the current StringBuilder object.
MaxCapacity: Gets the maximum capacity of this instance.


StringBuilder Methods
Append(Boolean): Appends the string representation of a specified
Boolean value to this instance.
Append(Byte): Appends the string representation of a specified 8-bit
unsigned integer to this instance.
Append(Char): Appends the string representation of a specified
Unicode character to this instance.
Append(Char[]): Appends the string representation of the Unicode
characters in a specified array to this instance.
Append(Decimal): Appends the string representation of a specified
decimal number to this instance.


StringBuilder Methods (cont.)
Append(Double): Appends the string representation of a specified
double-precision floating-point number to this instance.
Append(Object): Appends the string representation of a specified
object to this instance.
Append(SByte): Appends the string representation of a specified
8-bit signed integer to this instance.
Append(Single): Appends the string representation of a specified
single-precision floating-point number to this instance.
Append(String): Appends a copy of the specified string to this
instance.


Append(UInt16): Appends the string representation of a specified
16-bit unsigned integer to this instance.
Append(Char, Int32): Appends a specified number of copies of the
string representation of a Unicode character to this instance.
Append(Char[], Int32, Int32):Appends the string representation of a
specified subarray of Unicode characters to this instance.


Append(String, Int32, Int32): Appends a copy of a speciﬁed substring
to this instance.
AppendFormat(String, Object): Appends the string returned by
processing a composite format string, which contains zero or more
format items, to this instance. Each format item is replaced by the
string representation of a single argument.
AppendFormat(String, Object[]): Appends the string returned by
processing a composite format string, which contains zero or more
string representation of a corresponding argument in a parameter
array.
AppendFormat(IFormatProvider, String, Object[]): Appends the
string returned by processing a composite format string, which
contains zero or more format items, to this instance. Each format
item is replaced by the string representation of a corresponding
argument in a parameter array using a speciﬁed format provider.

AppendFormat(String, Object, Object): Appends the string returned
by processing a composite format string, which contains zero or more
string representation of either of two arguments.
AppendFormat(String, Object, Object, Object): Appends the string
returned by processing a composite format string, which contains zero
or more format items, to this instance. Each format item is replaced
by the string representation of either of three arguments.
AppendLine(): Appends the default line terminator to the end of the
current StringBuilder object.
AppendLine(String): Appends a copy of the speciﬁed string followed
by the default line terminator to the end of the current StringBuilder
object.


Clear: Removes all characters from the current StringBuilder instance.
CopyTo: Copies the characters from a specified segment of this
instance to a specified segment of a destination Char array.
EnsureCapacity: Ensures that the capacity of this instance of
StringBuilder is at least the specified value.
Equals(Object): Determines whether the specified object is equal to
the current object. (Inherited from Object.)
Equals(StringBuilder): Returns a value indicating whether this
instance is equal to a specified object.
Finalize: Allows an object to try to free resources and perform other
cleanup operations before it is reclaimed by garbage collection.
(Inherited from Object.)


GetHashCode: Serves as a hash function for a particular type.
GetType: Gets the Type of the current instance. (Inherited from
Object.)
Insert(Int32, Boolean): Inserts the string representation of a Boolean
value into this instance at the specified character position.
Insert(Int32, Byte): Inserts the string representation of a specified
8-bit unsigned integer into this instance at the specified character
position.


Insert(Int32, Char): Inserts the string representation of a specified
Unicode character into this instance at the specified character
position.
Insert(Int32, Char[]): Inserts the string representation of a specified
array of Unicode characters into this instance at the specified
character position.
Insert(Int32, Decimal): Inserts the string representation of a decimal
number into this instance at the specified character position.
Insert(Int32, Double): Inserts the string representation of a
double-precision floating-point number into this instance at the
specified character position.


Insert(Int32, Int16): Inserts the string representation of a specified
16-bit signed integer into this instance at the specified character
position.
Insert(Int32, Int32): Inserts the string representation of a specified
position.
Insert(Int32, Int64): Inserts the string representation of a 64-bit
signed integer into this instance at the specified character position.
Insert(Int32, Object): Inserts the string representation of an object
into this instance at the specified character position.


Insert(Int32, SByte): Inserts the string representation of a specified
position.
Insert(Int32, Single): Inserts the string representation of a
single-precision floating point number into this instance at the
Insert(Int32, String): Inserts a string into this instance at the
Insert(Int32, UInt16): Inserts the string representation of a 16-bit
unsigned integer into this instance at the specified character position.


Insert(Int32, String, Int32): Inserts one or more copies of a speciﬁed
string into this instance at the speciﬁed character position.


Insert(Int32, Char[], Int32, Int32): Inserts the string representation of
a specified subarray of Unicode characters into this instance at the
MemberwiseClone: Creates a shallow copy of the current Object.
Remove: Removes the specified range of characters from this
instance.
Replace(Char, Char): Replaces all occurrences of a specified character
in this instance with another specified character.
Replace(String, String): Replaces all occurrences of a specified string
in this instance with another specified string.


Replace(Char, Char, Int32, Int32): Replaces, within a substring of this
instance, all occurrences of a specified character with another
specified character.
Replace(String, String, Int32, Int32): Replaces, within a substring of
this instance, all occurrences of a specified string with another
specified string.
ToString(): Converts the value of this instance to a String.
(Overrides Object.ToString().)
ToString(Int32, Int32): Converts the value of a substring of this
instance to a String.


C# String Optimization
The C# String class is designed to minimize unnecessary memory
allocations.
In the process, String operations can provide some unexpected results.
In the first case, two string variablessiteA, siteBare declared. The first
variable is assigned to a string literal (”C# Online.NET”).
The second variable is assigned to the first string variable.


Intern Pool
The intern pool is a list of strings which are currently referenced in your
C# application. When a new string is created, then the intern table is
checked ﬁrst to see if that exact string literal already exists in the pool. If
it does already exist, then both string variables will reference the same
string literal at the same memory location in the intern table. Therefore,
only a single copy of a unique string literal is ever created.


C# Best Practices
Use ”String” to refer specifically to the String class.
Use ”string” when referring to an object of the String class.
Avoid using the @ symbol in order to use C# keywords as identifiers.
It can obfuscate the code making it difficult to read and is just plain,
poor practice.
Use the StringBuilder class whenever you will be doing a great deal of
string manipulation. This will be speedier and save memory in most
cases.


DSA - Lecture 04

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