Fastest way to add an Item to an Array

Question

What is the fastest way to add a new item to an existing array   Dim arr As Integer      1  2  3  Dim newItem As Integer   4    I already know that when working with dynamic list of items you should rather use a List  ArrayList or similar IEnumerables  But what to do if you re stuck to legacy code that uses arrays    What I ve tried so far     A  converting to List  add item and convert back Dim list As List Of Integer  arr  list Add newItem  arr   list ToArray     -- gt  duration for adding 100 000 items  33270 msec    B  redim array and add item ReDim Preserve arr arr Length  arr arr Length - 1    newItem   -- gt  duration for adding 100 000 items  9237 msec    C  using Array Resize Array Resize arr  arr Length   1  arr arr Length - 1    newItem   -- gt  duration for adding 100 000 items  1 msec   -- gt  duration for adding 100 000 000 items  1168 msec   A  seems very slow since every time an item is added two conversions of the whole array are done  B  seems faster but still the array is copied once during the ReDim Preserve  C  seems to be the fastest at this point  Is there anything better

User · Answer

It depends on how often you insert or read. You can increase the array by more than one if needed.

numberOfItems = ??

' ...

If numberOfItems+1 >= arr.Length Then
    Array.Resize(arr, arr.Length + 10)
End If

arr(numberOfItems) = newItem
numberOfItems += 1

Also for A, you only need to get the array if needed.

Dim list As List(Of Integer)(arr) ' Do this only once, keep a reference to the list
                                  ' If you create a new List everything you add an item then this will never be fast

'...

list.Add(newItem)
arrayWasModified = True

' ...

Function GetArray()

    If arrayWasModified Then
        arr = list.ToArray()
    End If

    Return Arr
End Function

If you have the time, I suggest you convert it all to List and remove arrays.

* My code might not compile

User · Answer

Case C  is the fastest  Having this as an extension   Public Module MyExtensions      lt Extension   gt        Public Sub Add Of T  ByRef arr As T    item As T          Array Resize arr  arr Length   1          arr arr Length - 1    item     End Sub End Module   Usage   Dim arr As Integer      1  2  3  Dim newItem As Integer   4 arr Add newItem     -- gt  duration for adding 100 000 items  1 msec   -- gt  duration for adding 100 000 000 items  1168 msec

User · Answer

For those who didn t know what next  just add new module file and put  jor code  with my little hacked  supporting  nothing  array  below   Module ArrayExtension      lt Extension   gt        Public Sub Add Of T  ByRef arr As T    item As T          If arr IsNot Nothing Then             Array Resize arr  arr Length   1              arr arr Length - 1    item         Else             ReDim arr 0              arr 0    item         End If      End Sub End Module

User · Answer

Not very clean but it works     Dim arr As Integer      1  2  3  Dim newItem As Integer   4  arr   arr Concat  newItem   ToArray

User · Answer

Dim arr As Integer      1  2  3  Dim newItem As Integer   4 ReDim Preserve arr  3  arr 3  newItem   for more info Redim

[arrays] Fastest way to add an Item to an Array

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