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having a problem. Efficiency. I know very little about efficiency in coding, but I'm learning. Searching through these arraylists of strings for item1 and when found adding item2 to a new arrayList. The logic is great. Works fine. But takes about 30-40 seconds as there are 700 elements in arrLoList * 10 000 elements in arrCoList.

Now contains drops out as soon as it finds the item1, so I can't really improve on that. Would it help at all to cut some characters off the end of the arrCoList's strings?

How else could I improve this? I think changing from arrayLists?

For Each item1 In arrLoList             
        For Each item2 In arrCoList

            If arrCoList.contains(item1) Then

                arrNewList.Add(item2)


            Else
                intCouldntfind += 1     'not finding 7 million 

            End If
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2 Answers

up vote 2 down vote accepted

First, i would not use an old ArrayList which always requires to box/unbox the objects to their actual type(string here). Instead i would use a strongly typed List(Of String).

Second, you can use Enumerable.Except to get the set difference which is quite efficient since it uses a HashSet internally.

Dim sw = New System.Diagnostics.Stopwatch ' measure time 
Dim rnd As New Random ' create random strings
Dim loList = New List(Of String)(700)
Dim coList = New List(Of String)(10000)

' initialize with sample data
For i = 1 To 700
    loList.Add(String.Format("Item: {0} Random-Value: {1}", i, rnd.Next(1, 1000)))
Next
For i = 1 To 10000
    coList.Add(String.Format("Item: {0} Random-Value: {1}", i, rnd.Next(1, 1000)))
Next

sw.Start()

' *** Here is all you need: '
Dim newList = loList.Except(coList).ToList()

sw.Stop()

Result: 3 milliseconds at a max

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up vote 1 down vote

You are doing an innecesary nesting:

This should be quite a bit faster:

For Each item1 In arrLoList   
    If arrCoList.contains(item1) Then          
        For Each item2 In arrCoList               
            arrNewList.Add(item2)
        Next  
     End If
Next
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Thanks for this, very helpful, just not sure it works for me, as the .contains method doesn't find anything (what i'm searching for is part of a longer string eg 10815fmelmflw lwmflemkwl. So using the first 5 letters. Contains therefore doesn't find anything for me like this). – Jonny Apr 22 at 8:35

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