In my Windows Forms application at runtime I will be resizing an array each time I add an element. So first I must resize to the size + 1, and then add a member to this index. How do I do this?
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9 Answers
You could use the ReDim statement, but this really isn't your best option. If your array will be changing sizes often, especially as it sounds like you're just appending, you should probably use a generic List(Of T) or similar collection type.
You can use it just like you use an array, with the addition that adding an item to the end is as easy as MyList.Add(item)
To use a generic list, add Imports System.Collections.Generics to the top of the file. Then, you would declare a new integer list like this:
Dim MyList As New List(Of Integer)()
or a string list like this:
Dim MyList As New List(Of String)()
You should get the idea.
2 Comments
Alex Gordon
hi joel! you're right this is the best solution. can you please show me how to declare a list?
Dillie-O
Oh yes, definitely +1 for List<T>. I should have asked if you had other object options available first. 8^D
The suggested ReDim's need the Preserve keyword for this scenario.
ReDim Preserve MyArray(n)
1 Comment
Joel Coehoorn
Since this question is over 30,000 views now, I thought it was worth commenting that the "
Preserve" keyword does NOT in fact preserve the original array, but rather allocates a whole new array and copies the elements from the original one by one... which is fine if that's what you want, but usually programmers are expecting something more efficient.Using a generic list is (as suggested) the best idea. If you however want to change the size of an Array, you can use Array.Resize(ByRef arr, newSize).
ReDim is not a good (pretty bad) idea (VB specific legacy, extremely slow).
3 Comments
Blackwood
This appears to be mainly commentary on two of the existing answers.
Corniel Nobel
Well, nobody mentioned Array.Resize() So, I don't agree.
Ruud Helderman
I agree
Array.Resize should be preferred over the archaic ReDim. The bad news is, the two are probably equally slow.I would prefer some type of collection class, but if you WANT to use an array do it like this:
Dim arr() As Integer
Dim cnt As Integer = 0
Dim ix As Integer
For ix = 1 To 1000
cnt = cnt + 1
ReDim arr(cnt)
arr(cnt - 1) = ix
Next
2 Comments
Peter Mortensen
This becomes N^2. Can you say Shlemiel the painter's algorithm?
Peter Mortensen
The argument in the ReDim line is not the length, it is the last index. Thus in that line the length of the array becomes cnt+1, leaving an undefined value at the end of the array (well, not actually undefined, but equal to Nothing). (It is not very clear from the documentation, but perhaps more clear by trying it using Visual Studio's debugger. However, somewhat down in the documentation it says "The upper bound is the highest possible index value for that dimension, not the length of the dimension")
You can also make your own collection class. A good programming exercise for new programmers.
Public Class MyList
Private Items() As String
Private No As Integer = 0
Public Sub Add(ByVal NewItem As String)
''Create a temporary new string array
Dim CopyString(No) As String
''Copy values from Global Variable Items() to new CopyString array
For i As Integer = 0 To No - 1
CopyString(i) = Items(i)
Next
''Add new value - NewItem - to CopyString
CopyString(No) = NewItem
''Increment No to No + 1
No += 1
''Copy CopyString to Items
Items = CopyString
'Discard CopyString
CopyString = Nothing
End Sub
Public Sub Show(ByVal index As Integer)
MsgBox(Items(index))
End Sub
End Class
''Now create a form with a TextBox name - txt, Button1 and Button2
Public Class Form1
''Declare txts as a new MyList Class
Private txts As New MyList
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
''Add text to txts which is a MyList Class
txts.Add(txt.Text)
txt.Text = ""
End Sub
Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
''Display value at a specific index
txts.Show(Convert.ToInt16(txt.Text))
txt.Text = ""
End Sub
End Class
Comments
Use the ReDim command to specify the new size.
ReDim MyArray(MyArray.Length + 1)
3 Comments
Alex Gordon
this one doesnt work, it gives me Error 1 'ReDim' statements require a parenthesized list of the new bounds of each dimension of the array.
Alex Gordon
thanks got it to work this way Redim MyArray, (MyArray.Length + 1)
Ruud Helderman
This increases the array size by 2 rather than 1, due to the fact that
ReDim expects as its parameter the new upper bound, not the length.As Joel says, use a list.
Dim MyList As New List(Of String)
Don't forget to change Of String to be Of whichever datatype you're using.
Comments
This work for me
Dim Table1 As New DataTable
' Define columns
Table1.Columns.Add("Column1", GetType(System.String))
Table1.Columns.Add("Column2", GetType(System.Int32))
Table1.Columns.Add("Column3", GetType(System.Int32))
' Add a row of data
Table1.Rows.Add("Item1", 44, 99)
Table1.Rows.Add("Item2", 42, 3)
Table1.Rows.Add("Item3", 42, 3)
Table1.Rows.Add("Item4", 42, 3)
Dim arr(-1) As String
For Each dr As DataRow In Table1.Rows
ReDim Preserve arr(arr.Length)
arr(arr.Length - 1) = dr("Column1")
Next
Comments
As stated by Joel Coehoorn (the accepted answer) you should just use a list, but you if you REALLY want to use an array you could do something like this:
Imports System.Runtime.CompilerServices
Module Foo
''' <summary>
''' Adds the specified object to the end of the collection.
''' </summary>
''' <param name="array">
''' The object collection to append.
''' </param>
''' <param name="value">
''' The object to add to the end of the collection.
''' </param>
<Extension> ' Attribute Info: https://learn.microsoft.com/en-us/dotnet/api/system.runtime.compilerservices.extensionattribute?view=net-8.0
Public Sub Add(ByRef array As Object(), value As Object)
' Optional:
' Dim aType As Type = array.GetType.GetElementType
' Dim vType As Type = value.GetType
' If aType <> vType Then Throw New ArrayTypeMismatchException()
Dim length As Integer = array.Count
ReDim Preserve array(length)
array(length) = value
End Sub
End Module
