Video: Strings and Text Processing in C# (Bulgarian)

Topics covered:

• What is String?
• Creating and Using Strings
  • Declaring, Creating, Reading and Printing
• Manipulating Strings
  • Comparing, Concatenating, Searching, Extracting Substrings, Splitting
• Other String Operations
  • Replacing Substrings, Deleting Substrings, Changing Character Casing, Trimming
• Building and Modifying Strings
  • Why the + Operator is Slow?
  • Using the StringBuilder Class
• Formatting Strings
  • Formatting Numbers, Dates and Currency
• Cultures and Culture-Sensitive Formatting
  • Accessing and Assigning the Current Culture
  • Parsing Numbers and Dates

Video (in Bulgarian)

Presentation Content

What Is String?

• Strings are sequences of characters
• Each character is a Unicode symbol
• Represented by the string data type in C# ( System.String )
• Example: string s = "Hello, C#";

H	e	l	l	o	,		C	#

The System.String Class

• Strings are represented by System.String objects in .NET Framework
  • String objects contain an immutable (read-only) sequence of characters
  • Strings use Unicode to support multiple languages and alphabets
• Strings are stored in the dynamic memory ( managed heap )
• System.String is reference type
• String objects are like arrays of characters ( char[] )
  • Have fixed length ( String.Length )
  • Elements can be accessed directly by index
    • The index is in the range [0...Length-1]

0	1	2	3	4	5
H	e	l	l	o	!

Declaring Strings

• Several ways of declaring string variables:
  • Using the C# keyword string
  • Using the .NET’s fully qualified class name System.String
  • The next three declarations are equivalent
    • string str1;
    • System.String str2;
    • String str3;

Creating Strings

• Before initializing a string variable has null value
• Strings can be initialized by:
  • Assigning a string literal to the string variable
  • Assigning the value of another string variable
  • Assigning the result of operation of type string
• Not initialized variables has value of null
• Assigning a string literal
  • string s; // s is equal to null
• Assigning from another string variable
  • string s = "I am a string literal!";
• Assigning from the result of string operation
  • string s = 42.ToString();

Comparing Strings

• Several ways to compare two strings:
  • Dictionary-based string comparison
    • Case-insensitive
    • Case-sensitive
• Equality checking by operator ==
  • Performs case-sensitive compare
• Using the case-sensitive Equals() method
  • The same effect like the operator ==

Concatenating Strings

• There are two ways to combine strings:
  • Using the Concat() method
  • Using the + or the += operators
• Any object can be appended to a string

Searching in Strings

• Finding a character or substring within given string
  • First occurrence
    • IndexOf(string str)
  • First occurrence starting at given position
    • IndexOf(string str, int startIndex)
  • Last occurrence
    • LastIndexOf(string)

0	1	2	3	4	5	6	7	8	9	10	11	12	13	…
C	#		P	r	o	g	r	a	m	m	i	n	g	…

Extracting Substrings

• Extracting substrings
  • str.Substring(int startIndex, int length)
  • str.Substring(int startIndex)

0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19
C	:	\	P	i	c	s	\	R	i	l	a	2	0	0	5	.	j	p	g

Replacing and Deleting Substrings

• Replace(string, string) – replaces all occurrences of given string with another
  • The result is new string (strings are immutable)
• Re move ( index , length ) – deletes part of a string and produces new string as result

Constructing Strings

• Strings are immutable !
  • C oncat() , R eplace() , T rim() , ... return new string, do not modify the old one
• Do not use " + " for strings in a loop!
  • It runs very, very inefficiently!
• StringBuilder keeps a buffer memory, allocated in advance
  • Most operations use the buffer memory and do not allocate new objects

How the + Operator Performs String Concatenations?

• Consider the following string concatenation:
• It is equivalent to this code:
• Several new objects are created and left to the garbage collector for deallocation
  • What happens when using + in a loop?

The StringBuilder Class

• StringBuilder(int _ _ capacity) constructor allocates in advance buffer of given size
  • By default 16 characters are allocated
• Capacity holds the currently allocated space (in characters)
• this[int _ _ index] (indexer in C#) gives access to the char value at given position
• Length holds the length of the string in the buffer
• Append(…) appends a string or another object after the last character in the buffer
• Remove (int start Index , int length ) removes the characters in given range
• Insert(int index , string str) inserts given string (or object) at given position
• Replace(string oldStr, string newStr) replaces all occurrences of a substring
• ToString() converts the StringBuilder to String

Method ToString()

• All classes in C# have public virtual method ToString()
  • Returns a human-readable, culture-sensitive string representing the object
  • Most .NET Framework types have own implementation of ToString()
    • int , float , bool , DateTime
• We can apply specific formatting when converting objects to string
  • ToString(formatString) method

Formatting Strings

• The formatting strings are different for the different types
• Some formatting strings for numbers:
  • D – number (for integer types)
  • C – currency (according to current culture)
  • E – number in exponential notation
  • P – percentage
  • X – hexadecimal number
  • F – fixed point (for real numbers)

Formatting Dates

• Dates have their own formatting strings
  • d , dd – day (with/without leading zero)
  • M , MM – month
  • yy , yyyy – year (2 or 4 digits)
  • h , HH , m , mm , s , ss – hour, minute, second
• Cultures in .NET specify formatting / parsing settings specific to country / region / language

Summary

• Strings are immutable sequences of characters (instances of System.String )
  • Declared by the keyword string in C#
  • Can be initialized by string literals
• Most important string processing members are:
  • Length, this[] , Compare(str1, str2) , IndexOf(str) , LastIndexOf(str) , Substring(startIndex, length) , Replace(oldStr, newStr) , Remove(startIndex, length) , ToLower() , ToUpper() , Trim()
• Objects can be converted to strings and can be formatted in different styles (using ToString () method)
• Strings can be constructed by using placeholders and formatting strings ( String.Format (…) )