Tài liệu Module 7: Strings, Arrays, and Collections pdf

.NET Framework Arrays Introduce the System.Array class as the base class of all array types that contains methods for creating, manipulating, searching, and sorting arrays!. The followi

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Contents

Overview 1

Strings 2

Terminology – Collections 20

.NET Framework Collections 39

Lab 7: Working with Strings, Enumerators,

Review 63

Module 7: Strings, Arrays, and Collections

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Instructor Notes

After completing this module, students will be able to:

! Parse, format, manipulate, and compare strings

! Use the classes in the System.Array and System.Collections namespaces

! Improve the type safety and performance of collections by using specialized collections and class-specific code

Materials and Preparation

This section provides the materials and preparation tasks that you need to teach this module

Required Materials

To teach this module, you need the Microsoft® PowerPoint® file 2349B_07.ppt

Preparation Tasks

To prepare for this module, you should:

! Read all of the materials for this module

! Practice the demonstrations

! Complete the lab

Presentation:

120 Minutes

Lab:

60 Minutes

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Demonstrations

This section provides demonstration procedures that will not fit in the margin notes or are not appropriate for the student notes

Sorting and Enumerating an Array

In this demonstration, you will show students how to sort and enumerate an array.

The code for this demonstration is contained in one project and is located in

<install folder>\Democode\Mod07\Demo07.1 In addition, the code for the

individual demonstration is provided in the student notes

ArrayList

In this demonstration, you will show students how ArrayList implements the

IList interface by using an array whose size is dynamically increased as

required

Hashtable

In this demonstration, you will show students how to create a hash table that is used for searches

In all of the preceding demonstrations, use the debugger to step through the code while you point out features

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Module Strategy

Use the following strategy to present this module:

! Strings Discuss how to work with strings in the Microsoft NET Framework, including common operations, such as parsing, formatting, manipulating, and comparing strings

! Terminology – Collections Define the term collection as it is used in this module and identify where collections are found in the NET Framework Be sure that students understand that the term collection is used in its broader sense: to describe a group of items

! NET Framework Arrays

Introduce the System.Array class as the base class of all array types that

contains methods for creating, manipulating, searching, and sorting arrays Discuss features of arrays that are specific to C# Explain the role of the

IEnumerable and IEnumerator interfaces in System.Array and System.Collections classes

Use the Sorting and Enumerating an Array demonstration to show how to sort and enumerate an array

! NET Framework Collections

Briefly introduce some commonly used classes in the System.Collections

namespace

Discuss the IList interface with regards to classes that represent an ordered

collection of objects that can be individually indexed Use the ArrayList demonstration to reinforce this concept

Discuss the IDictionary interface and the classes that it implements Use the Hashtable demonstration to show how to use the IDictionary interface

Provide guidelines to help students distinguish between collections and arrays, and explain when collections are used

Discuss runtime casting for type safety and the effects of runtime casting, and boxing and unboxing on performance Discuss techniques for handling boxing and unboxing to optimize performance

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Overview

! Strings

! Terminology – Collections

! .NET Framework Arrays

! .NET Framework Collections

***************************** ILLEGAL FOR NON - TRAINER USE ******************************

In this module, you will learn about some of the key classes in the Microsoft®

.NET Framework class library Specifically, you will learn how to work with strings, arrays, collections, and enumerators

After completing this module, you will be able to:

! Parse, format, manipulate, and compare strings

! Use the classes in the System.Array and System.Collections namespaces

! Improve the type safety and performance of collections by using specialized collections and class-specific code

In this module, you will learn

about some of the key

classes in the NET

Framework class library

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! Trim and Pad

! Split and Join

! StringBuilder

! C# Specifics

! Regular Expressions

In the C# language, string is an alias for System.String in the NET Framework The System.String type represents a string of Unicode characters

Working with strings is an everyday task in software development, and includes operations, such as parsing, formatting, manipulating, and comparing strings

The String object is immutable Therefore, every time you use one of the methods in the System.String class, you create a new string object When you

want to perform repeated modifications to a string, the overhead that is

associated with creating a new String object can be costly As an alternative, you can use the System.Text.StringBuilder class to modify a string without

creating a new object

In this section, you will learn how to work with strings in the NET Framework

Topic Objective

To introduce the topics in

the section

Lead-in

In this section, you will learn

how to work with strings in

the NET Framework

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Parse

! Parse Method Converts a Numeric String to a Numeric

! To Ignore Commas, Use the NumberStyles.AllowThousands Flag

// The output to the console is "123456"

string MyString = "123,456";

int MyInt = int.Parse(MyString, System.Globalization.NumberStyles.AllowThousands);Console.WriteLine(MyInt);

// The output to the console is "123456"

The Parse method converts a string that represents a NET Framework numeric

base type to an actual NET Framework numeric base type

The Parse method takes a combination of up to three parameters, as follows:

! The string to be converted

! One or more values from the System.Globalization.NumberStyles

enumeration

! A NumberFormatInfo class Because the Parse method assumes that all string input represents a base-10 value, non-base-10 values are not parsable The Parse method also does not parse strings that represent the values NaN (Not A Number), PositiveInfinity,

or NegativeInfinity of the Single and Double classes because they are not real

numbers

The following code example converts a string to an int value, increments that

value, and displays the result:

To explain how the Parse

method is used to convert

numeric strings to a NET

Framework numeric base

type

Lead-in

The Parse method converts

a string that represents a

.NET Framework numeric

base type to an actual NET

Framework numeric base

type

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Handling Nonnumeric Characters

The NumberStyles enumeration is useful if you have a string that contains

nonnumeric characters that you want to convert into a NET Framework numeric base type You must use this enumeration to parse a string with a currency symbol, decimal point, exponent, parentheses, and so on

For example, a string that contains a comma cannot be converted to an int value by using the Parse method unless you pass the

System.Globalization.NumberStyles enumeration

Incorrect Way to Parse a String with Nonnumeric Characters

The following code example is invalid and raises an exception It illustrates the incorrect way to parse a string that contains nonnumeric characters

exception in the preceding example

The following code example uses the same string as the preceding example but does not raise an exception

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The NET Framework provides several format strings, or specifiers, that you can use to format the appearance of strings that derive from other objects There are several advantages to converting base data types to strings before displaying them to users Strings are easily displayed and can be appended to the messages and dialog boxes of your application You can also use format specifiers to display the same numeric value in scientific format, monetary format, hexadecimal format, and so on

When to Use Format Strings

You can use format specifiers in situations where your application stores information in a format that is designed for use by the application, and not by the user For example, a business application may keep track of the current date

and time in a DateTime object to log when transactions are completed The

DateTime object stores information in which the user is not necessarily

interested, such as the number of milliseconds that have elapsed since the creation of the object

You can also use format specifiers to display only information that is of interest

to the user, such as the date and hour of the transaction Additionally, you can dynamically modify strings that are created by using format specifiers to represent monetary, date, and time conventions for the current culture For example, your application can display the date and time in the notation that is specific to the user’s current culture

Topic Objective

To explain how to use

format strings, or specifiers,

to format the appearance of

your application

Lead-in

The NET Framework

provides several format

strings that you can use to

format the appearance of

strings that derive from

other objects

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Methods Used with Format Strings

Format strings are used with any method that creates a string representation of a

.NET Framework data type, such as Int32, Int64, Single, Double,

Enumeration, DateTime, and so on Format strings are also used with Console.Writeline, String.Format, and several methods in the System.IO

namespace

Additionally, every base data type contains a ToString method that returns a

string representation of the data type’s value and accepts a string format specifier You can control the layout and design of strings that are created by any of these methods by using one of several format strings defined by the NET Framework

Using the ToString Method The ToString method is useful if you want to convert one of the standard NET

Framework data types to a string that represents that type in some other format

For example, if you have an integer value of 100 that you want to represent to the user as a currency value, you can easily use the ToString method and the

currency format string ("C") to produce a string of "$100.00" The original value that is contained in the data type is not converted, but a new string is returned that represents the resulting value This new string cannot be used for calculation until it is converted back to a NET base data type The original value, however, can be calculated at any time

Computers that do not have U.S English specified as the current culture will display whatever currency notation is used by the current culture

In the following code example, the ToString method displays the value of 100

as a currency-formatted string in a console window:

argument and can produce the same value as the preceding example

Console.Writeline accepts string format specifiers in the form, where the

characters inside the curly brackets specify the formatting to apply to the variable

The following code example uses the Console.WriteLine method to format the

value of MyInt to a currency value

int MyInt = 100;

Console.WriteLine("{0:C}", MyInt);

In the preceding example, the 0 character specifies the variable or value on which to apply formatting In this example, it is the first and only variable The characters that follow the colon are interpreted as string format specifiers

Note

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string MyString = MyInt.ToString("C" );

// In the U.S English culture: "$12,345.00"

int MyInt = 12345;

string MyString = MyInt.ToString("C" );

// In the U.S English culture: "$12,345.00"

DateTime MyDate = new DateTime(2000, 1, 10, 0, 0, 0);

string MyString = MyDate.ToString( "d" );

// In the U.S English culture: "1/10/2000"

The standard numeric, picture numeric, date and time, and enumeration format strings are described in detail in the NET Framework Software Development Kit (SDK) This topic provides examples of these format strings

The following examples show the use of the format string that returns the value

of MyInt in the currency format:

int MyInt = 12345;

string MyString = MyInt.ToString( "C" );

// In the U.S English culture, MyString has the value:

Example Format Pattern (en-US)

and short time)

Dd MMMM yyyy HH:mm

date and long time)

Dd MMMM yyyy HH:mm:ss

Topic Objective

To provide examples of

format strings that return

common numeric string

types

Lead-in

Let’s look at some examples

of format strings that return

the value of MyString in the

currency and date time

formats

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The following example shows the use of the format string that returns the value

of MyDate in the short date pattern format:

// In the U.S English culture, MyString has the value: // "1/10/2000"

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Changing Case

! You Can Easily Change the Case of a String

# String.ToUpper – converts to upper case

# String.ToLower – converts to lower case

string MyString = "hello world!";

// outputs: HELLO WORLD!

Console.WriteLine(MyString.ToUpper());

string MyString = "hello world!";

// outputs: HELLO WORLD!

Console.WriteLine(MyString.ToUpper());

string MyString = "HELLO WORLD!";

// outputs: hello world!

Console.WriteLine(MyString.ToLower());

string MyString = "HELLO WORLD!";

// outputs: hello world!

Console.WriteLine(MyString.ToLower());

You can easily change the case of a string by using the two methods that are described in the following table

String.ToUpper Converts all characters in a string to uppercase

String.ToLower Converts all characters in a string to lowercase

String.ToUpper and String.ToLower provide an override that accepts a

The preceding example displays HELLO WORLD! to the console

The String.ToLower method is similar to the String.ToUpper method, but it

converts all of the characters in a string to lowercase

The following code example converts the string "HELLO WORLD!" to lowercase string MyString = "HELLO WORLD!";

You can easily change the

case of a string by using the

String.ToUpper and

String.ToLower methods

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Compare

! The NET Framework Has Methods to Compare Strings

# For example, the Compare method compares the

current string object to another string or object, returning:

- Negative if first string is less than second string

- 0 if the two strings are equal

- Positive if first string is greater than second string

string MyString = "Hello World!";

The NET Framework provides several methods to compare the values of strings The following table describes some of the value-comparison methods

Method Name Use String.Compare Compares the values of two strings Returns an integer value

String.StartsWith Determines if a string begins with the string passed Returns a

boolean value

String.IndexOf Returns the index of the first occurrence of a String, or one or

more characters, within this instance

For more information about value comparison methods and for a complete list

of these methods, see “Comparing Strings” in the NET Framework SDK documentation

For example, the String.Compare method provides a thorough way to compare

the current string object to another string or object You can use this function to compare two strings or substrings of two strings

Additionally, overloads are provided that regard or disregard case and cultural variance

Topic Objective

To introduce some of the

value-comparison methods

that are used to compare

the values of strings

Lead-in

The NET Framework

provides several

value-comparison methods to

compare the values of

strings

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The following table shows the three integer values that are returned by the

Compare(string strA, string strB) method

The following code example uses the Compare method to determine whether

two strings are the same

Console.WriteLine(String.Compare(MyString, "Hello World!")); The preceding example displays 0 to the console

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Trim and Pad

! Trim Methods Remove Spaces

! Pad Methods Expand a Specific Number of Characters

string MyString = " Big ";

Console.WriteLine("Hello{0}World!", MyString ); string TrimString = MyString.Trim();

Console.WriteLine("Hello{0}World!", TrimString );

// outputs the following lines to the console:

//Hello Big World!

//HelloBigWorld!

Console.WriteLine("Hello{0}World!", MyString ); string TrimString = MyString.Trim();

// outputs the following lines to the console:

//Hello Big World!

//HelloBigWorld!

Console.WriteLine(MyString.PadLeft(20, '-'));

// outputs the following line to the console:

// -Hello World! to the console

// outputs the following line to the console:

// -Hello World! to the console

When you want to remove or extend the spaces around strings, the

System.String class provides methods for trimming and padding strings

The following table describes two of the available trim methods

Method Name Use String.Trim Removes white spaces from the beginning and end of a string

String.Remove Removes a specified number of characters from a specified index

position in a string

For example, you can easily remove white spaces from both ends of a string by

using the String.Trim method, as shown in the following code example

Console.WriteLine("Hello{0}World!", MyString );

string TrimString = MyString.Trim();

This code outputs the following lines to the console:

Hello Big World!

HelloBigWorld!

For a complete list of trim methods in the System.String class, see “Trimming

and Removing Characters” in the NET Framework SDK documentation

Topic Objective

To explain how to use

methods of the

System.String class to trim

and pad strings

Lead-in

You can remove or extend

the spaces around strings

by using methods of the

System.String class

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Padding

System.String also provides methods that you can use to create a new version

of an existing string that is expanded by a specific number of characters

The following table describes the available pad methods

Method Name Use String.PadLeft Right aligns and pads a string, so its rightmost character is a

specified distance from the beginning of the string

String.PadRight Left aligns and pads a string, so its rightmost character is a

specified distance from the beginning of the string

For example, the String.PadLeft method creates a new string that moves an

existing string to the right, so its last character is a specified number of spaces from the first index of the string White spaces are inserted if you do not use an override that allows you to specify your own custom padding character

The following code example uses the PadLeft method to create a new string

with a total length of 20 spaces

This example displays the following text to the console

-Hello World!

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Split and Join

! Split Method Is Used to Break Up a String Into an Array of Substrings

characters parameter

assumed to be the separator

! Join Method Is Used to Concatenate Strings

string array

string Line = "Hello World";

string[] Words = Line.Split(null);

// Words[0] = "Hello" and Words[1] = "World"

The System.String class provides the Split method to break up strings and the

Join method to concatenate strings

The Split Method

You use the Split method to break up a string instance into an array of

substrings at the positions that are specified by separator characters If separator

characters are omitted, that is to say, if the parameter is null, the whitespace

characters are assumed to be the separator If the separator is a zero-length string, a single-element array that contains the entire expression string is returned

The following example shows how to break up a string into a string array of words:

The Join Method

You use the Join method to concatenate a specified separator string between each element of a specified String array, which yields a single concatenated string If the separator is omitted, that is to say null, the space character (" ")

is used If the separator is a zero-length string (""), all of the items in the list are concatenated with no delimiters

Topic Objective

To explain how to use the

Split and Join methods to

break up and concatenate

strings

Lead-in

The System.String class

provides the Split method to

break up strings and the

Join method to concatenate

strings

For Your Information

You should cover the

String.Split method

because it is used in the lab

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StringBuilder

! The String Object is Immutable

! System.Text.StringBuilder – Allows You to Modify a String Without Creating a New Object

! You Can Specify the Maximum Number of Characters

! Methods Include:

# Append, AppendFormat, Insert, Remove, and Replace

StringBuilder MyStringBuilder = new StringBuilder("Hello");

// MyStringBuilder can hold a maximum of 25 characters StringBuilder MyStringBuilder =

new StringBuilder("Hello World!", 25);

// MyStringBuilder can hold a maximum of 25 characters StringBuilder MyStringBuilder =

new StringBuilder("Hello World!", 25);

The String object is immutable Therefore, every time you use one of the methods in the System.String class, you create a new string object When you

want to perform repeated modifications to a string, the overhead that is

associated with creating a new String object can be costly As an alternative, you can use the System.Text.StringBuilder class to modify a string without

Creating a StringBuilder Object

You can create a new instance of the StringBuilder object by initializing your

variable with one of the overloaded constructor methods, as shown in the following code example:

StringBuilder MyStringBuilder = new StringBuilder("Hello");

Although the StringBuilder object is a dynamic object that allows you to

expand the number of characters in the string that it encapsulates, you can specify a value for the maximum number of characters that it can hold This

value is called the capacity of the object and must not be confused with the

length of the string that the current StringBuilder object holds Any attempt to expand the StringBuilder class beyond the maximum range causes an

ArgumentOutOfRangeException to be thrown

The following code example specifies that the MyStringBuilder object can be

expanded to a maximum of 25 spaces

StringBuilder MyStringBuilder = new StringBuilder("Hello World!", 25);

Topic Objective

To explain how to use the

StringBuilder method to

modify a string without

class to modify a string

without creating a new

object

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StringBuilder Methods

The following table describes the methods that you can use to modify the

contents of the StringBuilder object

StringBuilder.Append Appends information to the end of the current

StringBuilder object

StringBuilder.AppendFormat Replaces zero or more format specifications with

the appropriately formatted value of an object

StringBuilder.Insert Inserts a string or object into the specified index of

the current StringBuilder object

StringBuilder.Remove Removes a specified number of indexes from the

current StringBuilder object

StringBuilder.Replace Replaces a specified index or character with the

passed character

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C# Specifics

! C# string Type Is a String of Unicode Characters

not references

// rather than "c:\\Docs\\Source\\a.txt"

The string type represents a string of Unicode characters; string is an alias for

System.String in the NET Framework

Although string is a reference type, the equality (==) operator and the inequality (!=) operator are defined to compare the values of string objects, not the references Comparing the values of string objects makes testing for string

equality more intuitive, as in the following example:

string a = "hello";

string b = "hello";

Console.WriteLine( a == b ); // output: True same value

The + operator concatenates strings, as in the following example:

string a = "good " + "morning";

The [ ] operator accesses individual characters of a string, as in the following

example:

char x = "test"[2]; // x = 's';

String literals are of type string and can be written in two forms: quoted and

@-quoted Quoted string literals are enclosed in quotation marks ("), as in the following example:

"good morning" // a string literal Quoted string literals can also contain any character literal, including escape sequences, as in the following example:

string a = "\\\u0066\n"; // backslash, letter f, new line

Topic Objective

To explain what the string

type is in the NET

Framework and to describe

the functions of the +, [ ],

and != operators, and

alias for System.String in

the NET Framework

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@-quoted string literals start with @ and are enclosed in quotation marks, as in the following example:

@"good morning" // a string literal The advantage of using @-quoted string literals is that escape sequences are not processed This makes it easy to write a fully qualified file name, as in the following example:

@"c:\Docs\Source\a.txt"

// rather than "c:\\Docs\\Source\\a.txt"

To include a quoted phrase in an @-quoted string, use two pairs of double quotation marks, as in the following example:

@"""Ahoy!"" cried the captain." // "Ahoy!" cried the captain The following code example uses the C# features that are discussed in this topic:

using System;

class test {

public static void Main( String[] args ) {

True

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Regular Expressions

! Regular Expressions – Powerful Text Processing

! Pattern-Matching Notation Allows You to:

# Find specific character patterns

# Extract, edit, replace, or delete text substrings

# Add the extracted strings to a collection to generate

a report

! Designed to be Compatible With Perl 5

Regular expressions provide a powerful, flexible, and efficient method to process text The extensive pattern-matching notation of regular expressions allows you to quickly parse large amounts of text to find specific character patterns; to extract, edit, replace, or delete text substrings; and to add the extracted strings to a collection in order to generate a report

For many applications that deal with strings, such as HTML processing, log file parsing, and HTTP header parsing, regular expressions are an essential tool The NET Framework regular expressions incorporate the most popular features

of other regular expression implementations, such as those used in Perl and awk Designed to be compatible with Perl 5 regular expressions, NET Framework regular expressions include features that are not yet available in other implementations, such as right-to-left matching and dynamic compilation The NET Framework regular expression classes are part of the NET

Framework class library and can be used with any language or tool that targets the common language runtime, including ASP.NET and Microsoft

Visual Studio® NET

A detailed explanation of how to use the regular expression classes is beyond the scope of this course For more information about using regular expression classes, see the NET Framework SDK documentation

Topic Objective

To briefly describe how

regular expressions can be

used in the NET

Framework

Lead-in

Regular expressions allow

you to quickly parse large

amounts of text in order to

find specific character

patterns; to extract, edit,

replace, or delete text

substrings; and to add the

expressions Do not spend

much time on this slide, but

encourage students to refer

to the NET Framework

SDK, especially for details

about using the regular

expression classes

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In this module, the term collection is used in its broader sense to refer to a group of items In the NET Framework, examples of collections are found in

the namespaces System.Array, and System.Collections

Topic Objective

To define the term collection

as it is used in this module

and to identify where

collections are found in the

.NET Framework

Lead-in

In this module, the term

collection is used in its

broader sense: to describe a

group of items

Do not spend much time on

this slide The objective of

this slide is to define the

term collection clearly,

before discussing the

collections that are found in

the NET Framework

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" NET Framework Arrays

The System.Array class is the base class of all array types and contains methods for creating, manipulating, searching, and sorting arrays The Array class is not part of the Collections namespace However, it is still a collection because it is based on the IList interface

In an array style of collection, an item in the collection is referred to by the term

element Specific elements are identified by their array index The low bound or

lower bound of an Array is the index of its first element

To understand the functionality of the various collection classes, you must understand their key interfaces In this section, you will learn about the

interfaces that are used by the methods of System.Array

Topic Objective

To introduce the topics in

the section

Lead-in

The System.Array class is

the base class of all array

types and contains methods

for creating, manipulating,

searching, and sorting

arrays

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System.Array

! System.Array Is the Base Class of All Array Types

! Arrays Implement the Following Interfaces

# ICloneable, IList, ICollection, and IEnumerable

! System.Array Has Methods For

# Creating, manipulating, searching, and sorting

! Null, Empty String, and Empty (0 item) Arrays Should Be Treated the Same

# Therefore, return an Empty array, instead of a null reference

The System.Array class is the base class of all array types and contains

methods for creating, manipulating, searching, and sorting arrays Arrays are always allocated on the garbage-collected heap Arrays can be single dimensional or multidimensional You can also create arrays of arrays, called

jagged arrays

For optimum performance, it is highly recommended that each rank of an array

be zero-based Additionally, each rank of an array must be zero-based when passing arrays between programming languages

Defining an Array Type

You define an array type by specifying the element type of the array, the rank,

or number of dimensions, of the array, and the upper and lower bounds of each dimension of the array All of these details are included in any signature of an array type

The runtime automatically creates exact array types as they are required No separate definition of the array type is needed Arrays of a particular type can only hold elements of that type If you need to manipulate a set of unlike objects or value types, consider using one of the collection types that are

defined in the System.Collections namespace

Topic Objective

To explain how array types

are used and defined

Lead-in

The System.Array class is

the base class of all array

types and contains methods

for creating, manipulating,

searching, and sorting

arrays

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Methods and Properties of System.Array

Some of the methods and properties for the System.Array class are described

in this topic Many of these methods are used to implement the class’s interfaces

System.Array implements the interfaces that are described in the following

table

Interface Use ICloneable Supports cloning, which creates a new instance of a class with the

same value as an existing instance

IList Represents a collection of objects that can be individually

The interfaces that are listed in the preceding table are supported not only by

arrays but also by many of the System.Collections classes Subsequent topics

in this module discuss specific interfaces and the classes that implement them The following tables describe some of the public members that are available

through the System.Array class

Static Method Use BinarySearch Overloaded Searches a one-dimensional sorted Array for a

value, using a binary search algorithm

CreateInstance Overloaded Initializes a new instance of the Array class

Sort Overloaded Sorts the elements in one-dimensional Array

objects

Property Use IsFixedSize Gets a value that indicates whether the Array has a fixed size

IsReadOnly Gets a value that indicates whether the Array is read-only

Length Gets the total number of elements in all of the dimensions of the

Array

Rank Gets the rank (number of dimensions) of the Array

The IsFixedSize and IsReadOnly properties are always false unless they

are overridden by a derived class

Note

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The following table describes some of the public instance methods that are

available through the System.Array class

Instance Method Use Clone Creates a shallow copy of the Array

GetEnumerator Returns an IEnumerator for the Array

GetLength Gets the number of elements in the specified dimension of the

Array

GetLowerBound Gets the lower bound of the specified dimension in the Array

GetUpperBound Gets the upper bound of the specified dimension in the Array

GetValue Overloaded Gets the values of the Array elements at the

specified indexes

SetValue Overloaded Sets the specified Array elements to the specified

value

For complete lists of public members of the System.Array class, see “Array

Members” in the NET Framework SDK documentation

Empty Arrays

Nulls should only be returned by reference properties that refer to another

object or component String and Array properties should never return null,

because a programmer typically does not expect null in this context For example, a programmer typically would assume that the following code works: public void DoSomething(…) {

string[] sa = SomeOtherFunc();

// The following line assumes sa is never null

if (sa.Length > 0) { // do something else }

} Generally, null, empty string, and empty (0 item) arrays should be treated in the

same way Therefore, return an Empty array, instead of a null reference

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C# Specifics

! C# Array Indexes Start at Zero

! Declaring an Array – Size Is Not Part of Its Type

! Creating an Array

! Initializing an Array

! Using System.Array Members

int[] numbers; // declare numbers as

// an int array of any size

int[] numbers; // declare numbers as

// an int array of any sizeint[] numbers = new int[5]; // declare and create

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

int LengthOfNumbers = numbers.Length;

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

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

C# arrays are zero-indexed That means the array indexes start at zero Arrays

in C# and arrays in most other popular languages work similarly, but there are a few differences that you should be aware of

When declaring an array, the brackets ([]) must come after the type, and not after the identifier Placing the brackets after the identifier is not legal syntax in C#, as shown in the following example:

int[] table; // not int table[];

In addition, and unlike the C language, the size of the array is not part of its

type This allows you to declare and assign to an array any array of int objects,

regardless of the array’s length, as in the following examples

int[] numbers; // declare numbers as an int array of any size numbers = new int[10]; // numbers is a 10-element array numbers = new int[20]; // now it's a 20-element array

Topic Objective

To explain features that are

unique to C# arrays

Lead-in

C# arrays are similar to

arrays in most other popular

languages, but you should

be aware of the differences

that are presented in this

topic

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The following code examples show the syntax to create an array for each of the arrays already discussed:

int[] numbers = new int[5]; // Single-dimensional array string[,] names = new string[5,4]; // Multi-dimensional array byte[][] scores = new byte[5][]; // Array-of-arrays

for (int x = 0; x < scores.Length; x++) {

scores[x] = new byte[4];

}

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The following example shows a complete C# program that declares and instantiates arrays as discussed in this topic

using System;

class DeclareArraysSample {

public static void Main() {

// Single-dimensional array int[] numbers = new int[5];

// Multidimensional array string[,] names = new string[5,4];

// Array-of-arrays (jagged array) byte[][] scores = new byte[5][];

// Create the jagged array for (int i = 0; i < scores.Length; i++) {

scores[i] = new byte[i+3];

} // Print length of each row for (int i = 0; i < scores.Length; i++) {

Console.WriteLine("Length of row {0} is {1}",

} } } The preceding program displays the following output:

Length of row 0 is 3 Length of row 1 is 4 Length of row 2 is 5 Length of row 3 is 6 Length of row 4 is 7

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Single-Dimensional Array

The following examples show different ways to initialize single-dimensional arrays:

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

string[] names = new string[3] {"Matt", "Joanne", "Robert"}; You can omit the size of the array, as in the following examples:

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

string[] names = new string[] {"Matt", "Joanne", "Robert"};

If an initializer is provided, you can also omit the new statement, as in the

You can omit the size of the array, as in the following examples:

int[,] numbers = new int[,] { {1, 2}, {3, 4}, {5, 6} }; string[,] siblings = new string[,] { {"Mike","Amy"}, {"Mary","Ray"} };

If an initializer is provided, you can also omit the new statement, as in the

following examples:

int[,] numbers = { {1, 2}, {3, 4}, {5, 6} };

string[,] siblings = { {"Mike", "Amy"}, {"Mary", "Albert"} };

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Jagged Array (Array-of-Arrays)

The following examples show different ways to initialize jagged arrays You can initialize jagged arrays by using the following style:

int[][] numbers = new int[2][] { new int[] {2,3,4}, new int[] {5,6,7,8,9} };

You can omit the size of the first array, as in the following example:

int[][] numbers = new int[][] { new int[] {2,3,4}, new int[] {5,6,7,8,9} };

Accessing Array Members

Accessing array members in C# is straightforward and similar to how you access array members in C and C++ For example, the following code creates

an array called numbers and then assigns 5 to the fifth element of the array: int[] numbers = {10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0};

numbers[4] = 5;

The following code example declares a multidimensional array and assigns 5 to the member located at [1, 1]:

int[,] numbers = { {1, 2}, {3, 4}, {5, 6}, {7, 8}, {9, 10} }; numbers[1, 1] = 5;

The following code example shows how to access a member of a jagged array: int[][] numbers = new int[][]

{ new int[] {1, 2}, new int[] {3, 4}

The System.Array class provides many other useful methods and properties,

such as methods for sorting, searching, and copying arrays

Tiêu đề	Module 7: Strings, Arrays, and Collections
Chuyên ngành	Computer Science
Thể loại	Tài liệu học phần
Năm xuất bản	2001-2002

Định dạng
Số trang	70
Dung lượng	0,93 MB