VB.NET Array.Clear quick question.

2011-12-02

Question

_{Friday, December 2, 2011 9:25 PM}

I've got a 2-dimensional array that I need to re-initialize (clear) and re-use many times.

Does this clear all the contents of both dimensions?

Dim S_Array(99, 40) As String Array.Clear(S_Array, 0, S_Array.Length)

All replies (20)

_{Friday, December 2, 2011 10:11 PM ✅Answered | 2 votes}

I've got a 2-dimensional array that I need to re-initialize (clear) and re-use many times.

Does this clear all the contents of both dimensions?

Dim S_Array(99, 40) As String

Array.Clear(S_Array, 0, S_Array.Length)

Yes it will set all items in the array to Nothing, same as Redim, just a lot faster.

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_{Friday, December 2, 2011 9:44 PM}

I'd bet that it didn't, but then I've never really used System.Array.Clear myself. If you call Clear then it has to iterate through the whole array and set it to NULL or NOTHING or 0 or whatever, and then when you go to re-use it you iterate through the whole array again to set it to the new values.

What I'd do is just ReDim the array every time - same effect as System.Array.Clear without the iteration. Either that or (if the size of the array never changes when you re-use it) just change the values in it without clearing it first.

It never hurts to try. In a worst case scenario, you'll learn from it.

Thanks, I'll try Redim instead. I'm not using preserve, so, I guess that should do the trick...

_{Friday, December 2, 2011 9:48 PM | 1 vote}

Hi,

Yes you are correct. :-D

Use this line.>>

Array.Clear(S_Array, 0, S_Array.Length)

For proof this works try this code.

Add one Button and one RichTextBox to a Form then run this please.

Option Strict On
Option Explicit On
Option Infer On

Public Class Form1

    Dim S_Array(99, 40) As String
    Dim myRand As New Random

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

        For x As Integer = 0 To 99
            For y As Integer = 0 To 40
                S_Array(x, y) = myRand.Next(0, 100).ToString
            Next
        Next

    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

        Dim sb As New System.Text.StringBuilder

        For x As Integer = 0 To 99
            For y As Integer = 0 To 40
                sb.Append(S_Array(x, y) & ",")
            Next
        Next

        RichTextBox1.Text = sb.ToString
        MessageBox.Show("Click OK when you are ready please.")

        Array.Clear(S_Array, 0, S_Array.Length)

        sb.Remove(0, sb.Length)
        For x As Integer = 0 To 99
            For y As Integer = 0 To 40
                sb.Append(S_Array(x, y))
            Next
        Next

        RichTextBox1.Text = sb.ToString
        MessageBox.Show("StringBuilder length = " & sb.ToString.Length.ToString)

    End Sub

End Class

Regards,

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_{Friday, December 2, 2011 9:52 PM | 1 vote}

Hi,

I would not use ReDim

Microsoft did not create the Array.Clear method for no reason whatsoever.

ReDim creates a new array in memory, whether you use Preserve or not.

Please read the remarks under ReDim here.>>

http://msdn.microsoft.com/en-us/library/w8k3cys2(VS.71).aspx

Regards,

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_{Friday, December 2, 2011 10:07 PM | 1 vote}

I agree with John. This test shows the speed difference between the legacy method and the .Net version.

Public Class Form1

    Const sizeA As Integer = 99
    Const tries As Integer = 1000
    Dim testA(sizeA, sizeA) As String
    Dim stpw As New Stopwatch

Private Sub Button1_Click(sender As System.Object, _
e As System.EventArgs) Handles Button1.Click

        initA()
        stpw.Restart()
        For x As Integer = 1 To tries
            Array.Clear(testA, 0, testA.Length)
        Next
        stpw.Stop()
        Debug.WriteLine(stpw.Elapsed)

        initA()
        stpw.Restart()
        For x As Integer = 1 To tries
            ReDim testA(sizeA, sizeA)
        Next
        stpw.Stop()
        Debug.WriteLine(stpw.Elapsed)

End Sub

    Private Sub initA()
        For x As Integer = 0 To testA.GetUpperBound(0)
            For y As Integer = 0 To testA.GetUpperBound(1)
                testA(x, y) = x.ToString
            Next
        Next
    End Sub
End Class

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_{Friday, December 2, 2011 10:16 PM}

I've got a 2-dimensional array that I need to re-initialize (clear) and re-use many times.

Does this clear all the contents of both dimensions?

Dim S_Array(99, 40) As String

Array.Clear(S_Array, 0, S_Array.Length)

Yes it will set all items in the array to Nothing, same as Redim, just a lot faster.

Serial Port Random Check Internet Connection Microsoft® Community Contributor 2011

Great. I'll change it back then. I'm all for speed! Thanks a lot guy's :)

- Red

_{Friday, December 2, 2011 10:18 PM | 1 vote}

I've got a 2-dimensional array that I need to re-initialize (clear) and re-use many times.

Does this clear all the contents of both dimensions?

Dim S_Array(99, 40) As String

Array.Clear(S_Array, 0, S_Array.Length)

Yes it will set all items in the array to Nothing, same as Redim, just a lot faster.

Serial Port Random Check Internet Connection Microsoft® Community Contributor 2011

Great. I'll change it back then. I'm all for speed! Thanks a lot guy's :)

- Red

You should credit John with the answer so that future readers see hist post first.
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_{Friday, December 2, 2011 10:23 PM}

Great. I'll change it back then. I'm all for speed! Thanks a lot guy's :)

- Red

Hi again,

Then please mark the replies as answer which better answer your question as well.

The suggestion from Andrew B. Painter is okay but not as fast so you can leave those marked as answer if you wish.

Regards,

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_{Friday, December 2, 2011 10:28 PM}

Okay, you guy's, now don't let's be fighting over who all gets credit! :)

_{Friday, December 2, 2011 10:50 PM}

You should credit John with the answer so that future readers see hist post first.

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Hi dbasnett,

Thank you for your suggestion to the OP

REDTAIL_CODEGUY

:-D

I will leave it to the OP which post(s) to leave marked as answer.

Regards,

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_{Friday, December 2, 2011 11:32 PM}

For Benchmark testing, you need to clear main-memory on your machine before each test, otherwise you won't get accurate results...

_{Friday, December 2, 2011 11:39 PM}

Changing the data type(string to int) and the dimensions(2 to 1) I found that it depended on the size of the array. When I used 99,999 as the size Clear was faster. When the array was 999,999 redim was faster, in both test methods. edit: Where did Andrews post go?

Public Class Form1

    Const sizeA As Integer = 99999 '999999
    Const tries As Integer = 1000
    Dim testA(sizeA) As Integer
    Dim stpw As New Stopwatch

Private Sub Button1_Click(sender As System.Object, _
e As System.EventArgs) Handles Button1.Click

Array.Clear(testA, 0, testA.Length)
Debug.WriteLine(testA.Length.ToString)

initA()

        stpw.Restart()
        For x As Integer = 1 To tries
            Array.Clear(testA, 0, testA.Length)
        Next
        stpw.Stop()
        Debug.WriteLine(stpw.Elapsed)

Array.Clear(testA, 0, testA.Length)
initA()

        stpw.Restart()
        For x As Integer = 1 To tries
            ReDim testA(sizeA)
        Next
        stpw.Stop()
        Debug.WriteLine(stpw.Elapsed)

End Sub

    Private Sub initA()
        For x As Integer = 0 To testA.GetUpperBound(0)
            testA(x) = x
        Next
    End Sub

Private Sub Button2_Click(sender As System.Object, e As System.EventArgs) Handles Button2.Click
Dim HugeCount(sizeA) As System.Int32

Dim Timer As New System.Diagnostics.Stopwatch

        'For a proper test, We'll iterate through the whole
        'array and set each indice's value to the index it
        'occupies.
        For hc_idx As System.Int32 = 0 To (HugeCount.Length - 1)
            HugeCount(hc_idx) = hc_idx
        Next hc_idx

        'Now we'll test the time to clear.
        Timer.Reset()
        Timer.Start()
        System.Array.Clear(HugeCount, 0, HugeCount.Length)
        Timer.Stop()
        Dim ClearTime As System.Int64 = Timer.ElapsedTicks

        'We don't actually have to reset every value to look into ReDim
        For hc_idx As System.Int32 = 0 To (HugeCount.Length - 1)
            HugeCount(hc_idx) = hc_idx
        Next hc_idx

        Timer.Reset()
        Timer.Start()
        ReDim HugeCount(sizeA)
        Timer.Stop()
        Dim RedimTime As System.Int64 = Timer.ElapsedTicks

        Debug.WriteLine("Time to clear: " & ClearTime.ToString & System.Environment.NewLine & "Time to ReDim: " & RedimTime)
        'Turns out ReDim is more than twice as fast as Array.Clear.
    End Sub
End Class

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_{Friday, December 2, 2011 11:50 PM}

Hi Andrew,

Very good point I feel. :-D

Sorry, I did not do a StopWatch test like the one you have just shown on the code from dbasnett.

However if you do not want VB.Net to create a duplicate array and

dispose of the old one then you could use the "sometimes" faster Array.Clear method.

<edit> I have just tried the code from dbasnett on a two dimensional array of STRING

ReDim on that type of array is 8 times slower for the number of tries.

So I guess it depends on the TYPE of the array elements or / and the number of array dimensions.

:-S

</edit>

To dbasnett,

I am just going to check if the .Restart method is new to FrameWork 4.0 or not.

The Public Shared method

Stopwatch.StartNew()

does not appear to be working for me. :-(

Regards,

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_{Friday, December 2, 2011 11:52 PM}

@JAO/red - very curious. A quick search of the web shed no light. Where did Andrews post go?

.Restart is new in 4.0 (yea!).

I see Andrew has found another of my post abusive. You have to stop Andrew.

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_{Saturday, December 3, 2011 12:11 AM}

@JAO/red - very curious. A quick search of the web shed no light. Where did Andrews post go?

.Restart is new in 4.0 (yea!).

I see Andrew has found another of my post abusive. You have to stop Andrew.

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Hi dbasnett,

I've just looked up .Restart

:-D

http://msdn.microsoft.com/en-us/library/system.diagnostics.stopwatch.restart.aspx

I guess that Andrew no longer wishes to participate in this thread and has deleted His posts.

Regards,

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_{Saturday, December 3, 2011 1:11 PM}

This morning I took another look at this using a one dimensional array. Sometimes Array.Clear is faster and sometimes ReDim is faster. For clearing I think I will stay with .Clear. Serial Port Random Check Internet Connection Microsoft® Community Contributor 2011

_{Saturday, December 3, 2011 1:48 PM}

This morning I took another look at this using a one dimensional array. Sometimes Array.Clear is faster and sometimes ReDim is faster. For clearing I think I will stay with .Clear. Serial Port Random Check Internet Connection Microsoft® Community Contributor 2011

Hi dbasnett,

I think that I will as well. :)

Regards,

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_{Saturday, December 3, 2011 2:55 PM}

Has anyone played around with "Erase"? Speed? Effectiveness?

_{Saturday, December 3, 2011 4:33 PM}

Has anyone played around with "Erase"? Speed? Effectiveness?

Hi again,

According to Intellisense ERASE deallocates any memory used as well. You might not want that to happen?

'Try this code snippet within the event for a Button Click or similar.
'It will throw an EXCEPTION error on the MessageBox line.>>

        Dim numbers(100) As Integer
        For index As Integer = 0 To 100
            numbers(index) = index * 2
        Next

        Erase numbers
        MessageBox.Show(numbers(77).ToString)

That is because the entire numbers array is ERASEd from memory.

Regards,

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_{Saturday, December 3, 2011 4:42 PM}

Yes. Looks like you have to Redim after the erase in order to use the array again, which would probably be slower than just a Redim or .Clear...

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VB.NET Array.Clear quick question.

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