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How to overwrte an arraylist location

Posted on 2009-04-02
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Last Modified: 2012-05-06
How can I overwrite an arraylist location?

Example
4000
0
4001

I need to overwrite the value 0 which in in my arraylist with the value 4000

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Question by:cmdolcet
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14 Comments
 
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Expert Comment

by:jpaulino
ID: 24053667
Can you simply remove it ?
        Dim arr As New ArrayList

        arr.Add(4000)

        arr.Add(0)

        arr.Add(4001)
 

        ' Removes the 0

        arr.Remove(0)

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Author Comment

by:cmdolcet
ID: 24053873
but I don't want to remove it i want to update it with the previous arraylist value
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Expert Comment

by:Mike Tomlinson
ID: 24053903
Just access it by Index:

    arr(1) = 4000 ' replace the second element with 4000
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by:jpaulino
ID: 24053978
Then follow the Idle_Mind suggestion ... I thought you wanted to delete the wrong entry.
If you don't know the index of the 0 (zero) you can use:

arr(arr.IndexOf(0)) = New Value
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Author Comment

by:cmdolcet
ID: 24054275
I keep getting a break error after the second loop through
I have _Arraylist_1Inch which has 24 spot in the arraylist. Every 2 index position I have written a 0 integer value what this loop should do is loop through the       _Arraylist_1Inch       and replace the 0 value with the goodpre_intarrayindex value

However its breaking


    For Each intarrayindex In _Arraylist_1Inch
                    If intarrayindex = 0 Then
                        _Arraylist_1Inch(intarrayindex) = pre_intarrayindex
                    Else
                        pre_intarrayindex = 0
                        pre_intarrayindex = intarrayindex
                    End If
                Next
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by:jpaulino
ID: 24054548

intarrayindex gives you the value in the array and you're trying to use as the index of the  array  _Arraylist_1In in here
  _Arraylist_1Inch(intarrayindex)
I don't know what you want to replace with but you can do something like this (and change the 123 with your value)

        Dim pos As Integer

        While True

            pos = _Arraylist_1Inch.IndexOf(0)

            If pos = -1 Then Exit While

            _Arraylist_1Inch(pos) = 123

        End While

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Expert Comment

by:Mike Tomlinson
ID: 24054582
I think you want?

        For intarrayindex As Integer = 0 To _Arraylist_1Inch.Count - 1
            If _Arraylist_1Inch(intarrayindex) = 0 Then
                _Arraylist_1Inch(intarrayindex) = pre_intarrayindex
            End If
        Next
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by:jpaulino
ID: 24054676
Nice solution Idle_Mind, the only problem is that loops in all the arraylist.
But, could be not a problem :)
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Accepted Solution

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Mike Tomlinson earned 500 total points
ID: 24054910
Yeah...it's not very clear how it's supposed to work...

If you LITERALLY want to visit "Every 2 index position" then use the Step keyword on the end of the For loop:

        For intarrayindex As Integer = 0 To _Arraylist_1Inch.Count - 1 Step 2
            If _Arraylist_1Inch(intarrayindex) = 0 Then
                _Arraylist_1Inch(intarrayindex) = pre_intarrayindex
            End If
        Next

You would just need to start at 0 or 1 depending on which slot was the first one you needed.
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Expert Comment

by:Mike Tomlinson
ID: 24055059
@jpaulino: With regard to this part of your comment:

    "...the only problem is that loops in all the arraylist."

I do hope you realize that your code is ALSO visiting all slots in the ArrayList.  Internally, the IndexOf() method is doing exactly what I have done: loop thru iteratively, inorder, until it finds a match.

In fact, your algorithm is potentially ALOT worse because EACH call to IndexOf() is starting from the BEGINNING of the ArrayList and looking for a match.  So you will be iterating over some nodes multiple times!  In fact, if the whole thing was full of zeroes then you would be doing multiple passes with a total of (1 + 2 + 3 + 4 + ... + n) checks instead of a just 1 pass with n checks.   =\

You would need to use the IndexOf() overload that accepts a "starting position" to overcome this:

        Dim pos As Integer, startIndex = 0
        While True
            pos = _Arraylist_1Inch.IndexOf(0, startIndex)
            If pos = -1 Then Exit While
            _Arraylist_1Inch(pos) = 123
            startIndex = pos + 1
        End While

~IM   =)
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Expert Comment

by:jpaulino
ID: 24055526
Strange I have made a few tests and the time from your method and my (including you change) is very close. Sometimes my runs fasts, sometimes your code runs faster.
But that the startIndex it's needed or else it will increase ALLOT the cycle time
Loved you sign ~IM :)
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Expert Comment

by:jpaulino
ID: 24055534
Typo ... Should be:

Loved your sign ~IM :)
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Expert Comment

by:Mike Tomlinson
ID: 24055604
Lol...and of course, if the size of the list is small, then the milliseconds saved doesn't really matter!   =O


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Expert Comment

by:jpaulino
ID: 24055717
I have tested with 20000 records!
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