How to find duplicates in arrayList?

Question

I have to check duplicate records in ArrayList. In ArrayList each item should have atleast 2 times before insert new Item. Per example

Example 1:

AL(0) = '1'
AL(1) = '1'
AL(2) = '2'
AL(3) = '2'
AL(4) = '2'
Method has to return = True, because each value has atleast 2 times in the list.

Example 2:

AL(0) = '1'
AL(1) = '1'
AL(2) = '2'
AL(3) = '3' //Trying to insert new Item, But It should not

Method has to return = 'false', because '2' has 1 time in the list. So I dont 
want insert '3' in ArrayList and return false. 

Answer 1

I do not know much VB.Net, but maybe the following C# code might help (using LINQ).

array.Distinct().All(item => array.Count(other => other == item) > 1)

I'm guessing the VB syntax (might be wrong)

Array.Distinct().All(Function(item) Array.Count(Function(other) other = item) > 1)

Here array holds the list item items you're interested about

Answer 2

Use the following check on arraylist to check if there are duplicate data or not in the arraylist

   return arrList.ToArray().Distinct().Count() == arrList.Count;

if you want to check before inserting that the data is already present in the arraylist or not use the following check

if (!arrList.Contains(data)) {
    arrList.Add(data);                
}

Answer 3

If your ArrayList is ordered, and contains strings of numbers (as illustrated in your post,) then the below function should work:

Private Function OKToInsertSorted(ByVal theArrayList As ArrayList, _
                                  ByVal stringToInsert As String) As Boolean

    With theArrayList
        If CInt(stringToInsert) < CInt(.Item(.Count - 1)) Then Return False
        If .Count <= 1 Then
            If stringToInsert = "1" Then Return True Else Return False
        End If
        If .Item(.Count - 1).ToString = .Item(.Count - 2).ToString Then
            Return True
        Else
            Return False
        End If
    End With

End Function

If your ArrayList is NOT ordered, but still contains strings of numbers (assuming you started with number "1",) then the following function should work:

Private Function OKToInsertUNSorted(ByVal theArrayList As ArrayList, _
                                    ByVal stringToInsert As String) As Boolean

    If stringToInsert = "1" Then Return True

    Dim stringToCheck As String = CStr(CInt(stringToInsert) - 1)

    Dim qry = From stringItem In theArrayList _
              Where stringItem.ToString = stringToCheck _
              Group By stringItem Into _
              stringCount = Count()

    For Each result In qry
        If result.stringCount >= 2 Then Return True
    Next

    Return False

End Function

I put quite a bit of validation code in the first function based on some assumptions about what you were looking for, so your mileage may vary.