LEARN-VISUAL-BASIC 跟我一起来学VisualBasic源码一份

## Arrays and Collections --- ### **Arrays** An array is a container object that holds a fixed number of values of a single type. The length of an array is established when the array is created. After creation, its length is fixed. In a simple way an array is just variable that can contain multiple values unlike a simple variable. Syntax : ``` Dim name_of_variable As DataType = {value1, value2, ...} ``` Example : ```vbnet Dim x() As Integer = {4, 5 , 15, 95} Dim students() As String = {"Marie", "Peter", "Helene"} ' Declare an array but initialize it later Dim countries(2) As String countries(0) = "Italy" countries(1) = "Germany" countries(2) = "France" ``` We can not print out the array in *Console.WriteLine()* or *Console.Write()* as we do for simple variable. ```vbnet Console.WriteLine(students) ``` output: ``` System.String[] ``` It doesn't display elements stored in array but just display the data type of the array. So to access any element of the array we must specify the position (index) of the element. for example : ```vbnet students(2) ``` output : ``` Helene ``` We start counting the index from 0 ```vbnet x(0) ``` output : ``` 4 ``` But can show all the elements in array by using a loop __*Using For Loop :*__ ```vbnet For i As Integer = 0 To 2 Console.WriteLine(students(i)) Next ``` output : ``` Marie Peter Helene ``` Remember that *i* will be counting from 0 to 2 ``` students(0) = Marie students(1) = Peter students(2) = Helene ``` __*Using Do Loop :*__ ```vbnet Dim i As Integer = 0 Do While (i < 3) Console.WriteLine(students(i)) i += 1 Loop ``` output : ``` Marie Peter Helene ``` __*Using For each Loop :*__ In the precious section we didn't talk about **For each loop**, because this loop is only used on arrays and collection. Syntax : ``` For Each element [ As datatype ] In group [ statements ] [ Continue For ] [ statements ] [ Exit For ] [ statements ] Next [ element ] ``` ```vbnet For Each studentName As String In students Console.WriteLine(studentName) Next ``` output : ``` Marie Peter Helene ``` **Properties of The Array** Name | Description -----|------------ IsFixedSize |Gets a value indicating whether the Array has a fixed size. IsReadOnly |Gets a value indicating whether the Array is read-only Length | Gets the total number of elements in all the dimensions of the Array. Long Length | Gets a 64-bit integer that represents the total number of elements in all the dimensions of the Array. Rank | Gets the rank (number of dimensions) of the Array. For example, a one-dimensional array returns 1, a two-dimensional array returns 2, and so on. SyncRoot|Gets an object that can be used to synchronize access to the Array. IsSynchronized|Gets a value indicating whether access to the Array is synchronized (thread safe). Go [here](https://msdn.microsoft.com/en-us/library/system.array(v=vs.110).aspx) for more details. Let try some properties ```vbnet Console.WriteLine("IsFixedSize : " & students.IsFixedSize) Console.WriteLine("IsReadOnly : " & students.IsReadOnly) Console.WriteLine("Length : " & students.Length) Console.WriteLine("IsSynchronized : " & students.IsSynchronized) ``` output : ``` IsFixedSize : True IsReadOnly : False Length : 3 IsSynchronized : False ``` **Methods :** Name | Description ----- | --------- BinarySearch(Array, Object) | Searches an entire one-dimensional sorted array for a specific element, using the IComparable interface implemented by each element of the array and by the specified object. Clear(Array, Int32, Int32) | Sets a range of elements in an array to the default value of each element type. ConstrainedCopy(Array, Int32, Array, Int32, Int32) |Copies a range of elements from an Array starting at the specified source index and pastes them to another Array starting at the specified destination index. Guarantees that all changes are undone if the copy does not succeed completely. ConvertAll<TInput, TOutput>(TInput[], Converter<TInput, TOutput>) | Converts an array of one type to an array of another type. Copy(Array, Array, Int32)|Copies a range of elements from an Array starting at the first element and pastes them into another Array starting at the first element. The length is specified as a 32-bit integer. IndexOf(Array, Object) |Searches for the specified object and returns the index of its first occurrence in a one-dimensional array. SetValue(Object, Int32)|Sets a value to the element at the specified position in the one-dimensional Array. The index is specified as a 32-bit integer. Sort(Array) | Sorts the elements in an entire one-dimensional Array using the IComparable implementation of each element of the Array. Go [here](https://msdn.microsoft.com/en-us/library/system.array(v=vs.110).aspx) for more details. example : Sorting an array ```vbnet Dim x() As Integer = {8, 41, 0, 74, -1, 4, 25, 452} Array.Sort(x) For Each y As Integer In x Console.WriteLine(y) Next ``` output : ``` -1 0 4 8 25 41 74 452 ``` Get the sum of elements in an array ```vbnet Dim x() As Integer = {8, 40} Console.WriteLine(x.Sum()) ``` output : ``` 48 ``` Get the smallest and biggest elements in an array ```vbnet Dim x() As Integer = {8, 41, 0, 74, -1, 4, 452, 12} Console.WriteLine("Smallest : " & x.Min()) Console.WriteLine("Biggest : " & x.Max()) ``` output : ``` Smallest : -1 Biggest : 452 ``` __*Modify elements in an Array :*__ We gonna now try to add or modify elements in an array after it has been declare. Has we saw previously for accessing an element in array we must specify the index of the element, it's also the case here ```vbnet Dim x() As Integer = {8, 41, 0, 74, -1, 4, 25, 452} ' Modify the element at index 2 of x x(2) = 10 Console.WriteLine(x(2)) ``` output : ``` 10 ``` In the above example we modified the element at index 2 that was *0* we changed it to *10* __*Searching in an array :*__ Other things we can do is searching an element in array. *Using Contains method* ```vbnet Dim x() As Integer = {8, 41, 0, 74, -1, 4, 25, 452} ' Searching for 410 Console.WriteLine(x.Contains(410)) ' Searching for -1 Console.WriteLine(x.Contains(-1)) ``` output : ``` False True ``` The above method only search one elements in an array, let try to create something to will search for more then 1 element. ```vbnet ' This code, will search for element containing in both x and y Dim x() As Integer = {20, 85, 62, 41} Dim y() As Integer = {93, -5, 20, 85} For Each elementX As Integer In x ' Search elementX in y For Each elementY As Integer In y If elementX = elementY Then Console.WriteLine(elementX & " is found in x and y") End If Next Next ``` output : ``` 20 is found in x and y 85 is found in x and y ``` __*Multi dimensional Array*__: A multi dimensional array is a array that contain other array in it. ```vbnet ' This is a two dimensional array, all elements in the students are arrays. Dim students(,) = {{"Robert", 17}, {"Alan", 28}, {"Milan", 10}} ``` Accessing element in multi dimensional array ```vbnet Console.WriteLine(students(0, 0)) Console.WriteLine(students(0, 1)) ``` output : ``` Robert 17 ``` Or can also declare a multidimensional array as follow : ```vbnet ' this array is populated by 3 arrays Dim students()() = {New String() {"Robert", 17}, New String() {"Alan", 28}, New String() {"Milan", 10}} ``` ```vbnet 'This select the first array in students Console.WriteLine(students(0)) ``` output : ``` System.String[] ``` ```vbnet 'This select the second element in the first array of students Console.WriteLine(students(0)(1)) ``` output : ``` 17 ``` Iterating over the a multi dimensional array ```vbnet Dim students()() =