arrays. overview overview of arrays creating arrays using arrays
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Arrays
Overview
Overview of ArraysCreating ArraysUsing Arrays
What Is an Array?
An array is a sequence of elements All elements in an array have the same type They remain the same size once they are created Individual elements are accessed using integer indexes The first element in every array is the zeroth element
Integer index 0(zero)
Integer index 4(four)
Array Declaration
You declare an array variable by specifying: The element type of the array The rank of the array The name of the variable
This specifies the rank of the array
This specifies the name of the array variable
This specifies the element type of the array
Dim name as Type()Dim name() as Type
Dim name as Type()Dim name() as Type
Array Rank
Rank is also known as the array dimension The number of indexes associated with each element
Rank 1: One-dimensionalSingle index associates with each Integer element
Rank 2: Two-dimensionalTwo indexes associate with each Integer element
Dim row as Integer()Dim row as Integer() Dim grid as Integer(,)Dim grid as Integer(,)
Accessing Array Elements
Supply an integer index for each rank Indexes are zero-based
33332222
1111
Dim row as Long()...row(3)
Dim row as Long()...row(3)
Dim grid as Integer(,)...grid(1,2)
Dim grid as Integer(,)...grid(1,2)
Checking Array Bounds
All array access attempts are bounds checked A bad index throws an IndexOutOfRangeException Use the Length property and the GetLength method
rowrow gridgrid
row.GetLength(0)=6row.GetLength(0)=6
row.Length=6row.Length=6
grid.GetLength(0)=2grid.GetLength(0)=2
grid.GetLength(1)=4grid.GetLength(1)=4
grid.Length=2*4grid.Length=2*4
Creating Array InstancesInitializing Array ElementsInitializing Multidimensional Array
ElementsCreating a Computed Size ArrayCopying Array Variables
Creating Arrays
1 ' Fig. 7.2: CreateArray.vb
2 ' Declaring and allocating an array.
3
4 Imports System.Windows.Forms
5
6 Module modCreateArray
7
8 Sub Main()
9 Dim output As String
10 Dim i As Integer
11
12 Dim array As Integer() ' declare array variable
13 array = New Integer(9) {} ' allocate memory for array
14
15 output &= "Subscript " & vbTab & "Value" & vbCrLf
16
17 ' display values in array
18 For i = 0 To array.GetUpperBound(0)
19 output &= i & vbTab & array(i) & vbCrLf
20 Next
21
22 output &= vbCrLf & "The array contains " & _
23 array.Length & " elements."
24
25 MessageBox.Show(output, "Array of Integer Values", _
26 MessageBoxButtons.OK, MessageBoxIcon.Information)
27 End Sub ' Main
28
29 End Module ' modCreateArray
A variable capable of storing a reference to an array of Integer elementsAllocate an array of 10 elements
using New and assigns it to arrayAppends to output the headings for the columns displayed by the program
For structure is used to append the index number and value of each array element to output
The length property returns the number of elements in the array
Creating Array Instances
Declaring an array variable does not create an array! You must use new to explicitly create the array instance Array elements have an implicit default value of zero
row
0 0 0 0
grid
0 0 00 0 0
Variable Instance
Dim row as Long()=new Long(3){}Or Dim row as Long()=new Long(0 to 3){}
Dim row as Long()=new Long(3){}Or Dim row as Long()=new Long(0 to 3){}
Dim grid as Integer(,)=new Integer(1,2){}Dim grid as Integer(,)=new Integer(1,2){}
Initializing Array Elements
The elements of an array can be explicitly initialized You can use a convenient shorthand
row
0 1 2 3
Equivalent
dim row as Long() = new long(3) {0, 1, 2, 3}dim row as Long() = new long(3) {0, 1, 2, 3}
dim row as Long() = {0, 1, 2, 3}dim row as Long() = {0, 1, 2, 3}
Initializing Multidimensional Array Elements
You can also initialize multidimensional array elements All elements must be specified
grid
5 4 32 1 0
Implicitly a new Integer(2,3) array
Dim grid as Integer(,) = {
{5, 4, 3},{2, 1, 0}
}
Dim grid as Integer(,) = {{5, 4, 3},{2, 1, 0}
}
Dim grid as Integer(,) = {{5, 4, 3},{2, 1 }
}
Dim grid as Integer(,) = {{5, 4, 3},{2, 1 }
}
Creating a Computed Size Array
The array size does not need to be a compile-time constant Any valid integer expression will work Accessing elements is equally fast in all cases
Array size specified by compile-time integer constant:
Array size specified by run-time integer value:
Dim row as Long() = new Long(3){}Dim row as Long() = new Long(3){}
dim s as String= Console.ReadLine()dim size as Integer = Int32.Parse(s)dim row as Long() = new Long(size-1){}
dim s as String= Console.ReadLine()dim size as Integer = Int32.Parse(s)dim row as Long() = new Long(size-1){}
Copying Array Variables
Copying an array variable copies the array variable only It does not copy the array instance Two array variables can refer to the same array
instance
copy
row
0 0 0 0
Variable Instance
dim row as Long() = new long(3){}dim copy as Long() = row...row(0)+=1dim value as Long = copy(0)Console.WriteLine(value)
dim row as Long() = new long(3){}dim copy as Long() = row...row(0)+=1dim value as Long = copy(0)Console.WriteLine(value)
7.5 Passing Arrays to Procedures
Passing the Array Specify the name of the array without using
parentheses
Method( arrayname)
Receiving the array The procedure’s parameter list must specify
that an array will be received
Sub Method( ByVal arrayname1() As Integer)
PassArray.vb
1 ' Fig. 7.8: PassArray.vb
2 ' Passing arrays and individual array elements to procedures.
3
4 Imports System.Windows.Forms
5
6 Module modPassArray
7 Dim output As String
8
9 Sub Main()
10 Dim array1 As Integer() = New Integer() {1, 2, 3, 4, 5}
11 Dim i As Integer
12
13 output = "EFFECTS OF PASSING ENTIRE ARRAY " & _
14 "BY REFERENCE:" & vbCrLf & vbCrLf & _
15 "The values of the original array are:" & vbCrLf
16
17 ' display original elements of array1
18 For i = 0 To array1.GetUpperBound(0)
19 output &= " " & array1(i)
20 Next
21
22 ModifyArray(array1) ' array is passed by reference
23
24 output &= vbCrLf & _
25 "The values of the modified array are:" & vbCrLf
26
27 ' display modified elements of array1
28 For i = 0 To array1.GetUpperBound(0)
29 output &= " " & array1(i)
30 Next
31
Appends the five elements of array1 to String output
Passes array1 to procedure ModifyArray
Appends the elements of array1 to output
PassArray.vb
32 output &= vbCrLf & vbCrLf & _
33 "EFFECTS OF PASSING ARRAY ELEMENT " & _
34 "BY VALUE:" & vbCrLf & vbCrLf & "array1(3) " & _
35 "before ModifyElementByVal: " & array1(3)
36
37 ' array element passed by value
38 ModifyElementByVal(array1(3))
39
40 output &= vbCrLf & "array1(3) after " & _
41 "ModifyElementByVal: " & array1(3)
42
43 output &= vbCrLf & vbCrLf & "EFFECTS OF PASSING " & _
44 "ARRAY ELEMENT BY REFERENCE: " & vbCrLf & vbCrLf & _
45 "array1(3) before ModifyElementByRef: " & array1(3)
46
47 ' array element passed by reference
48 ModifyElementByRef(array1(3))
49
50 output &= vbCrLf & "array1(3) after " & _
51 "ModifyElementByRef: " & array1(3)
52
53 MessageBox.Show(output, "Passing Arrays", _
54 MessageBoxButtons.OK, MessageBoxIcon.Information)
55 End Sub ' Main
57 ' procedure modifies array it receives (note ByVal)
58 Sub ModifyArray(ByVal arrayParameter As Integer())
59 Dim j As Integer
61 For j = 0 To arrayParameter.GetUpperBound(0)
62 arrayParameter(j) *= 2
63 Next
65 End Sub ' ModifyArray
Multiplies the elements of arrayParameter by 2
PassArray.vb
67 ' procedure modifies integer passed to it
68 ' original is not be modified (note ByVal)
69 Sub ModifyElementByVal(ByVal element As Integer)
70
71 output &= vbCrLf & "Value received in " & _
72 "ModifyElementByVal: " & element
73 element *= 2
74 output &= vbCrLf & "Value calculated in " & _
75 "ModifyElementByVal: " & element
76 End Sub ' ModifyElementByVal
77
78 ' procedure modifies integer passed to it
79 ' original is be modified (note ByRef)
80 Sub ModifyElementByRef(ByRef element As Integer)
81
82 output &= vbCrLf & "Value received in " & _
83 "ModifyElementByRef: " & element
84 element *= 2
85 output &= vbCrLf & "Value calculated in " & _
86 "ModifyElementByRef: " & element
87 End Sub ' ModifyElementByRef
88
89 End Module ' modPassArray
7.6 Passing Arrays: ByVal vs. ByRef
ByVal Causes the value of the argument to be copied to
a local variable in the procedure Changes to the local variable are reflected in the
local copy of that variable, not in the original variable in the calling program
But if the argument is of a reference type, like an array, passing it ByVal actually passes it by reference, so changes to the object affect the original objects in the callers
7.6 Passing Arrays: ByVal vs. ByRef
ByRef When an array is passed with ByRef the called
procedure gains control over the passed reference itself
This allows the called procedure to replace the original reference in the object with another object or even Nothing
ArrayReferenceTest.vb
1 ' Fig. 7.9: ArrayReferenceTest.vb
2 ' Testing the effects of passing array references using
3 ' ByVal and ByRef.
4
5 Module modArrayReferenceTest
6
7 Sub Main()
8 Dim i As Integer
9
10 ' declare array references
11 Dim firstArray As Integer()
12 Dim firstArrayCopy As Integer()
13
14 ' allocate firstArray and copy its reference
15 firstArray = New Integer() {1, 2, 3}
16 firstArrayCopy = firstArray
17
18 Console.WriteLine("Test passing array reference " & _
19 "using ByVal.")
20 Console.Write("Contents of firstArray before " & _
21 "calling FirstDouble: ")
22
23 ' print contents of firstArray
24 For i = 0 To firstArray.GetUpperBound(0)
25 Console.Write(firstArray(i) & " ")
26 Next
27
28 ' pass firstArray using ByVal
29 FirstDouble(firstArray)
31 Console.Write(vbCrLf & "Contents of firstArray after " & _
32 "calling FirstDouble: ")
firstArray is passed to FirstDouble
Prints contents first to verify that FirstDouble indeed changes the array’s contents
Copies reference firstArray to variable firstArrayCopy, now
they reference the same object
34 ' print contents of firstArray
35 For i = 0 To firstArray.GetUpperBound(0)
36 Console.Write(firstArray(i) & " ")
37 Next
38
39 ' test whether reference was changed by FirstDouble
40 If firstArray Is firstArrayCopy Then
41 Console.WriteLine(vbCrLf & "The references are " & _
42 "equal.")
43 Else
44 Console.WriteLine(vbCrLf & "The references are " & _
45 "not equal.")
46 End If
48 ' declare array references
49 Dim secondArray As Integer()
50 Dim secondArrayCopy As Integer()
52 ' allocate secondArray and copy its reference
53 secondArray = New Integer() {1, 2, 3}
54 secondArrayCopy = secondArray
56 Console.WriteLine(vbCrLf & "Test passing array " & _
57 "reference using ByRef.")
58 Console.Write("Contents of secondArray before " & _
59 "calling SecondDouble: ")
61 ' print contents of secondArray before procedure call
62 For i = 0 To secondArray.GetUpperBound(0)
63 Console.Write(secondArray(i) & " ")
64 Next
66 ' pass secondArray using ByRef
67 SecondDouble(secondArray)
68
Compares references firstArray and firstArrayCopy
69 Console.Write(vbCrLf & "Contents of secondArray " & _
70 "after calling SecondDouble: ")
72 ' print contents of secondArray after procedure call
73 For i = 0 To secondArray.GetUpperBound(0)
74 Console.Write(secondArray(i) & " ")
75 Next
77 ' test whether the reference was changed by SecondDouble
78 If secondArray Is secondArrayCopy Then
79 Console.WriteLine(vbCrLf & "The references are " & _
80 "equal.")
81 Else
82 Console.WriteLine(vbCrLf & "The references are " & _
83 "not equal.")
84 End If
86 End Sub ' Main
87
88 ' procedure modifies elements of array and assigns
89 ' new reference (note ByVal)
90 Sub FirstDouble(ByVal array As Integer())
91 Dim i As Integer
92
93 ' double each element value
94 For i = 0 To array.GetUpperBound(0)
95 array(i) *= 2
96 Next
97
98 ' create new reference and assign it to array
99 array = New Integer() {11, 12, 13}
100 End Sub ' FirstDouble
101
Allocates a new array, and attempts to assign it’s reference to parameter array, attempting to overwrite reference firstArray in memory, but
will fail because the reference was passed ByVal
Multiplies all the elements of the array by 2
Reference is passed ByVal
102 ' procedure modifies elements of array and assigns
103 ' new reference (note ByRef)
104 Sub SecondDouble(ByRef array As Integer())
105 Dim i As Integer
106
107 ' double contents of array
108 For i = 0 To array.GetUpperBound(0)
109 array(i) *= 2
110 Next
111
112 ' create new reference and assign it to array
113 array = New Integer() {11, 12, 13}
114 End Sub ' SecondDouble
115
116 End Module ' modPassArray
Test passing array reference using ByVal.Contents of firstArray before calling FirstDouble: 1 2 3Contents of firstArray after calling FirstDouble: 2 4 6The references are equal. Test passing array reference using ByRef.Contents of secondArray before calling SecondDouble: 1 2 3Contents of secondArray after calling SecondDouble: 11 12 13The references are not equal.
Because the reference was passed with ByRef, the called procedure has the ability to modify what the reference
actually points to
Reference is passed ByRef
7.10 Variable-Length Parameter Lists
Keyword ParamArray Makes it possible to create procedures that
receive a variable number of arguments You can not use ParamArray with a
multidimensional array You can not use ByRef with ParamArray All arguments passed to the ParamArray array
must be of the same type as the array
1 ' Fig. 7.18: ParamArrayTest.vb
2 ' Using ParamArray to create variable-length parameter lists.
4 Module modParamArrayTest
5
6 Sub Main()
7 AnyNumberArguments()
8 AnyNumberArguments(2, 3)
9 AnyNumberArguments(7, 8, 9, 10, 11, 12)
10
11 End Sub ' Main
12
13 ' receives any number of arguments in array
14 Sub AnyNumberArguments(ByVal ParamArray array1 _
15 As Integer())
16
17 Dim i, total As Integer
18 total = 0
19
20 If array1.Length = 0 Then
21 Console.WriteLine("Procedure AnyNumberArguments" & _
22 " received 0 arguments.")
23 Else
24 Console.Write("The total of ")
25
26 For i = 0 To array1.GetUpperBound(0)
27 Console.Write(array1(i) & " ")
28 total += array1(i)
29 Next
31 Console.WriteLine("is {0}.", total)
32 End If
33
Determines whether or not zero arguments where passed, if not displays array1’s elements and their sum
Applies keyword ParamArray to array1
Calls procedure AnyNumberArguments, passing a different number of arguments each time
34 End Sub ' AnyNumberArguments
35
36 End Module ' modParamArrayTest
Procedure AnyNumberArguments received 0 arguments.The total of 2 3 is 5.The total of 7 8 9 10 11 12 is 57.
7.11 For Each/Next Repetition Structure
For Each/Next Provided to iterate through the values in a data
structure, such as an array Instead of a counter, For Each/Next uses a
variable to represent the value of each element Useful when the indices of the elements are not
important Particularly useful for looping through arrays of
objects
ForEach.vb
1 ' Fig. 7.19: ForEach.vb
2 ' Program uses For Each/Next to find a minimum grade.
3
4 Module modForEach
5
6 Sub Main()
7 Dim gradeArray As Integer(,) = New Integer(,) _
8 {{77, 68, 86, 73}, {98, 87, 89, 81}, {70, 90, 86, 81}}
9
10 Dim grade As Integer
11 Dim lowGrade As Integer = 100
12
13 For Each grade In gradeArray
14
15 If grade < lowGrade Then
16 lowGrade = grade
17 End If
18
19 Next
20
21 Console.WriteLine("The minimum grade is: {0}", lowGrade)
22 End Sub ' Main
23
24 End Module ' modForEach
The minimum grade is: 68
Specifies a variable grade and an array gradeArray. The structure iterates through all the elements in gradeArray, sequentially assigning
each value to variable grade
The values are compared to variable lowGrade, which stores the lowest grade in the array
Dynamic Arrays
Use ReDim keyword If you want to change its size in order to add items Clean up space when you remove items
when an array is redimensioned, by default all of the values in the array are cleared.
You can solve it by using Preserve Keyword
Tutorial
In the above Form, the user is invited to enter values into three Textboxes. The first Textbox is for whatever times table he or she wants. The second Textbox asks for the starting value of the times table. The third Textbox is for the end number of the times table. In other words, 1 times 4, 2 times 4, 3 times 4, right up to 12 times 4.
Dim numbers() As Integer Dim startAt As Integer Dim endAt As Integer Dim times As Integer Dim StoreAnswer As Integer Dim i As Integer
times = Val(TextBox1.Text) startAt = Val(TextBox2.Text) endAt = Val(TextBox3.Text)
ReDim numbers(endAt)
For i = startAt To endAt
StoreAnswer = i * times numbers(i) = StoreAnswer ListBox1.Items.Add(times & " times " & i & " = " & numbers(i))
Next
Jagged Arrays
Jagged arrays are maintained as arrays of arrays. Unlike rectangular arrays, rows in jagged arrays can be of different lengths .
Dim array1 As Integer()() = New Integer(2)(){}array1(0) = New Integer() {1, 2}array1(1) = New Integer() {3} array1(2) = New Integer() {4, 5, 6}
Dim array1 As Integer()() = New Integer(2)(){}array1(0) = New Integer() {1, 2}array1(1) = New Integer() {3} array1(2) = New Integer() {4, 5, 6}
50 For i = 0 To array1.GetUpperBound(0) 51 52 For j = 0 To array1(i).GetYpperBound(0) 53 array(i)(j)= 9
58 Next 59 60 Next
Array Methods
Commonly used methods Sort – sorts the elements in an array of rank 1
Array.Sort(Array name) Array.Reverse(Array name)
Clear – sets a range of elements to zero or null Array.Clear(Array name, Start Index, length)
Clone – creates a copy of the array ArrayCopy = Array1.Clone
IndexOf – returns the index of the first occurrence of a value Array.IndexOf( Array name, Value)