Chapter12 - Working with String potx

Roadmap 12 .1 String Overview 12.2 String Literals 12.3 Format Specifiers and Globalization 12.4 Working String from Outsite Sources 12 5 StringBuilder 12.6 Searching Strings with Regula

Chapter 12 Working With String

Hoang Anh Viet

VietHA@it-hut.edu.vn

HaNoi University of Technology

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“Describes how strings are a first-class type in the CLR and how to use them effectively in C# A large portion of the chapter covers the string-formatting capabilities of various types in the NET Framework and how to make your defined types behave similarly by implementing IFormattable Additionally, I introduce you to the globalization capabilities of the framework and how to create custom CultureInfo for cultures and regions that the NET Framework doesn’t already know about .”

Roadmap

12 1 String Overview

12.2 String Literals

12.3 Format Specifiers and Globalization

12.4 Working String from Outsite Sources

12 5 StringBuilder

12.6 Searching Strings with Regular Expression

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12.1 String Overview

 In C#, String is a built-in type

 In the built-in type collection , String is a reference type and but most of the built-in types are value types

String Basics

 A string is an object of type String whose value is text

 The text is stored as a readonly collection of Char

objects

 Each of which represents one Unicode character

encoded in UTF-16

 There is no null-terminating character at the end of a

C# string (unlike C and C++) therefore a C# string can contain any number of embedded null characters ('\0')

 The length of a string represents the number of

characters regardless of whether the characters are

formed from Unicode surrogate pairs or not

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Alias and String Class

 Alias

• In C#, the string keyword is an alias for String -> string and

String are equivalent

Declaring and Initializing Strings

 We can declare and initialize strings in various ways, as shown in the following example:

// Declare without initializing

string message1;

// Initialize to null

string message2 = null;

// Initialize as an empty string

// Use the Empty constant instead of the literal ""

string message3 = System.String.Empty;

//Initialize with a regular string literal

string oldPath = "c:\\Program Files\\Microsoft Visual Studio 8.0";

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// Use System.String if we prefer

System.String greeting = "Hello World!";

// In local variables (i.e within a method body)

// you can use implicit typing

var temp = "I'm still a strongly-typed System.String!";

// Use a const string to prevent 'message4' from /

/ being used to store another string value

const string message4 = "You can't get rid of me!";

// Use the String constructor only when creating

// a string from a char*, char[], or sbyte* See

// System.String documentation for details

char[] letters = { 'A', 'B', 'C' };

string alphabet = new string(letters);

Declaring and Initializing Strings

Immutability of String Objects

 String objects are immutable: they cannot be changed after they have been created.

 All of the String methods and C# operators that appear to

modify a string actually return the results in a new string

object.

 For example:

string s1 = "A string is more ";

string s2 = "than the sum of its chars.";

// Concatenate s1 and s2 This actually creates a new

// string object and stores it in s1, releasing the

// reference to the original object

s1 += s2;

System.Console.WriteLine(s1);

// Output: A string is more than the sum of its chars.

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Immutability of String Objects

 Note:

• When create a reference to a string, and then "modify" the

original string, the reference will continue to point to the original object instead of the new object that was created when the string was modified.

Remark

 When we declare a string in your C# code, the compiler creates a System.String object for us

 And then it places into an internal table in the module

called the intern pool.

 The compiler first checks to see if we’ve declared the

same string elsewhere, and if we have, then the code simply references the one already interned

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12 1 String Overview

12.2 String Literals

12.3 Format Specifiers and Globalization

12.4 Working String from Outsite Sources

12 5 StringBuilder

12.6 Searching Strings with Regular Expression

string columns = "Column 1\tColumn 2\tColumn 3";

//Output: Column 1 Column 2 Column 3

string rows = "Row 1\r\nRow 2\r\nRow 3";

string title = "\"The \u00C6olean Harp\", by Samuel Taylor Coleridge";

//Output: "The olean Harp", by Samuel Taylor Coleridge

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Regular and Verbatim String Literals

 Use verbatim strings for convenience and better

readability when the string text contains backslash

characters, for example in file paths

 Verbatim strings use the delaration preceded with the @ character

 For example:

string filePath = @"C:\Users\scoleridge\Documents\";

//Output: C:\Users\scoleridge\Documents\

string text = @"My pensive SARA ! thy soft

cheek reclined

Thus on mine arm, most soothing sweet it is

To sit beside our Cot, ";

/* Output:

My pensive SARA ! thy soft cheek reclined

Thus on mine arm, most soothing sweet it is

To sit beside our Cot,

*/

string quote = @"Her name was ""Sara.""";

//Output: Her name was "Sara."

Example

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String Escape Sequences

\U Unicode escape sequence for surrogate pairs. \Unnnnnnnn

\u Unicode escape sequence \u0041 = "A"

Roadmap

12 1 String Overview

12.2 String Literals

12.3 Format Specifiers and Globalization

12.4 Working String from Outsite Sources

12 5 StringBuilder

12.6 Searching Strings with Regular Expression

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12.3 Format Specifiers and

Globalization

 Format the data to display to users in a specific way

• Dealing with these sorts of issues

• Handling formatting of values,so on.

 For example:

• Display a floating-point value representing some tangible

metric in exponential form or

• Display in fixed-point form.

Format Strings

 A format string is a string whose contents can be

determined dynamically at runtime

 We create a format string by using

• Using the static Format method

• Embedding placeholders in braces that will be replaced by other values at runtime

 The built-in numeric objects use the standard numeric format strings or the custom numeric format strings

defined by the NET Framework

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Format Strings

 The standard format strings are typically of the form Axx

• A is the desired format requested

• And xx is an optional precision specifier.

 Examples of format specifiers for numbers are

• "C" for currency

• "D" for decimal

• "E" for scientific notation

• "F" for fixed-point notation

• And "X" for hexadecimal notation

• "G" for general This is the default format specifier, and is also the format that we get when we call Object.ToString.

• Suports one of the custom format strings.

class FormatString

{

static void Main()

{

// Get user input.

System Console.WriteLine("Enter a number");

string input = System Console.ReadLine();

// Convert the input string to an int.

int j;

System Int32.TryParse(input, out j);

// Write a different string each iteration.

string s;

for (int i = 0; i < 10; i++)

{

// A simple format string with no alignment formatting.

s = System String.Format("{0} times {1} = {2}", i, j, (i * j));

Object.ToString, IFormattable

 All built-in numeric types as well as date-time types

implement this interface

 An object that implements the IFormatProvider interface is—surprise—a format provider

Roadmap

12 1 String Overview

12.2 String Literals

12.3 Format Specifiers and Globalization

12.4 Working String from Outsite Sources

12 5 StringBuilder

12.6 Searching Strings with Regular Expression

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12.4 Working String from Outside

Remark

 Encoding is the process of transforming a set of Unicode characters into a sequence of bytes

 Decoding is the reverse; it is the process of transforming

a sequence of encoded bytes into a set of Unicode

characters

 The Unicode Standard assigns a code point (a number)

to each character in every supported script

 A Unicode Transformation Format (UTF) is a way to

encode that code point

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The Standard Unicode

UTF-x Describle

UTF-8 Which represents each code point as a sequence of one

to four bytes.

UTF-16 Which represents each code point as a sequence of one

to two 16-bit integers

UTF-32 Which represents each code point as a 32-bit integer

are supported

UTF32Encoding Encodes Unicode characters using the

UTF-32 encoding

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GetEncoding and method

 Use the GetEncoding method to obtain other encodings

 Use the GetEncodings method to get a list of all

encodings

GetByteCount and GetByte method

 The GetByteCount method determines how many bytes result in encoding a set of Unicode characters

 The GetBytes method performs the actual encoding

 Likewise, the GetCharCount method determines how

many characters result in decoding a sequence of bytes

 The GetChars method performs the actual decoding

 For example:

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string unicodeString = "This string contains the

unicode character Pi(\u03a0)";

// Create two different encodings.

Encoding ascii = Encoding.ASCII;

Encoding unicode = Encoding.Unicode;

// Convert the string into a byte[].

byte[ ] unicodeBytes =

unicode.GetBytes(unicodeString);

Example

// Perform the conversion from one encoding to the other.

byte[] asciiBytes = Encoding.Convert(unicode, ascii,

// the use of GetCharCount/GetChars.

char[ ] asciiChars = new

char[ascii.GetCharCount(asciiBytes, 0,

asciiBytes.Length)];

ascii.GetChars(asciiBytes, 0, asciiBytes.Length,

asciiChars, 0);

string asciiString = new string(asciiChars);

// Display the strings created before and after the

conversion.

Console.WriteLine( "Original string: {0}", unicodeString);

Console.WriteLine("Ascii converted string: {0}",

12 1 String Overview

12.2 String Literals

12.3 Format Specifiers and Globalization

12.4 Working String from Outsite Sources

12 5 StringBuilder

12.6 Searching Strings with Regular Expression

12.5 StringBuilder

 The System.Text.StringBuilder class can be used to

modify a string without creating a new object

• To decrease to store strings in the memory after performing

repeated modifications to a string.

• To boost performance when concatenating many strings

together in a loop

 We can create a new instance of the StringBuilder

class by initializing a variable with one of the overloaded constructor methods

StringBuilder MyStringBuilder = new

StringBuilder("Hello World!");

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Setting the Capacity and Length

 The maximum number of characters that a StringBuilder can hold is called capacity of the object

 Distinguish between Capacity and Length that a

StringBuider hold

StringBuilder MyStringBuilder = new

StringBuilder("Hello World!", 25);

Capacity of a StringBuider

Setting the Capacity and Length

 If the Capacity property is geater than the Length

property The Capacity property does not change

 If otherwise, the Capacity property is automatically

changed to the same value as the Length property

 We can use the read/write Capacity property to set the maximum length of your object

 The EnsureCapacity method can be used to check the

capacity of the current StringBuilder

 The Length property can also be viewed or set

MyStringBuilder.Capacity = 25;

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Modifying the StringBuilder String

 The following table lists the methods to modify the contents of a

StringBuilder

StringBuilder.Append Appends information to the end of the current

StringBuilder

StringBuilder.Append

Format Replaces a format specifier passed in a string with formatted text StringBuilder.Insert Inserts a string or object into the specified

index of the current StringBuilder

StringBuilder.Remove Removes a specified number of characters

from the current StringBuilder.

StringBuilder.Replace Replaces a specified character at a specified

index

Append

 The Append method can be used to add text or a string

representation of an object to the end of a string represented by the

StringBuilder MyStringBuilder = new

StringBuilder("Your total is ");

MyStringBuilder.AppendFormat("{0:C} ", MyInt);

Console.WriteLine(MyStringBuilder);

Result: Your total is $25.00

Insert

 The Insert method adds a string or object to a specified position in the current StringBuilder.

StringBuilder MyStringBuilder = new

StringBuilder("Hello World!");

MyStringBuilder.Insert(6,"Beautiful ");

Console.WriteLine(MyStringBuilder);

Result: Hello Beautiful World!

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 The Remove method is used to remove a specified

number of characters from the current StringBuilder,

beginning at a specified zero-based index

StringBuilder MyStringBuilder = new

StringBuilder("Hello World!");

MyStringBuilder.Remove(5,7);

Console.WriteLine(MyStringBuilder);

Result: Hello

StringBuilder MyStringBuilder = new

StringBuilder("Hello World!");

MyStringBuilder.Replace('!', '?');

Console.WriteLine(MyStringBuilder);

Result: Hello World?

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Regular Expression

 Regular Expression enables to do the folowings:

• Creating, comparing, and modifying strings

• rapidly parsing large amounts of text and data to search for,

remove, and replace text patterns

Roadmap

12 1 String Overview

12.2 String Literals

12.3 Format Specifiers and Globalization

12.4 Working String from Outsite Sources

12 5 StringBuilder

12.6 Searching Strings with Regular Expression

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12.6 Searching Strings with Regular

Expression.

 The System.Text.RegularExpression.Regex class can

be used to search strings

 These searches can range in complexity from very

simple to making full use of regular expressions

 The static method Regex.IsMatch performs the search given the string to search and a string that contains the search pattern

 For example:

foreach (string s in numbers)

// Keep the console window open in debug mode.

System.Console.WriteLine( "Press any key to exit.");

System.Console.ReadKey();

} //end of main

}//end of TestRegularExpressionValidation

Replacing Text with Regex

 Using NET regular expressions via the Regex.Replace method overloads, we can replace text

 For example:

using System.Text.RegularExpressions;

public class EntryPoint {

static void Main(string[] args)

 In this chapter, we have known:

• The string-handling capabilities of the NET Framework and C#

• Why the string is included in the base class library and why the CLR designers chose to annex it into the set of built-in types.

• How common string usage is Furthermore, the library provides

a thorough implementation of cultural-specific patterns, via CultureInfo.

• Can create our own cultures easily using the

CultureAndRegionInfoBuilder class.

• Gaven a brief tour of the regular-expression capabilities of

the NET Framework, even though a full treatment of the regularexpression language is outside the scope of this book

• The string and text-handling facilities built into the CLR,

the NET Framework, and the C# language are welldesigned and easy to use.