How do I slice an array in Excel VBA?

Question

What function can I use in Excel VBA to slice an array?

Answer 1

There is no direct slice function for arrays, different from many other recent languages.

However, there is a short code snippet very handy for this. Below, a complete solution for 1D arrays:

'*************************************************************
'*                      Fill(N1,N2)
'* Create 1 dimension array with values from N1 to N2 step 1
'*************************************************************
Function Fill(N1 As Long, N2 As Long) As Variant
 Dim Arr As Variant
 If N2 < N1 Then
   Fill = False
   Exit Function
 End If
 Fill = WorksheetFunction.Transpose(Evaluate("Row(" & N1 & ":" & N2 & ")"))
End Function

'**********************************************************************
'*                        Slice(AArray, [N1,N2])
'* Slice an array between indices N1 to N2
'***********************************************************************
Function Slice(VArray As Variant, Optional N1 As Long = 1, Optional N2 As Long = 0) As Variant
 Dim Indices As Variant
 If N2 = 0 Then N2 = UBound(VArray)
 If N1 = LBound(VArray) And N2 = UBound(VArray) Then
   Slice = VArray
 Else
   Indices = Fill(N1, N2)
   Slice = WorksheetFunction.Index(VArray, 1, Indices)
 End If
End Function

For testing

Var V As Variant
V = Fill(100,109)
PrintArr(Slice(V,3,5))

'************************************************
'*                 PrintArr(VArr)
'* Print the array VARR
'**************************************************
Function PrintArr(VArray As Variant)
 Dim S As String
 S = Join(VArray, ", ")
 MsgBox (S)
End Function

The results

102, 103, 104 

Answer 2

Here's a nifty function I wrote to subset a 2d array

Function Subset2D(arr As Variant, Optional rowStart As Long = 1, Optional rowStop As Long = -1, Optional colIndices As Variant) As Variant
    'Subset a 2d array (arr)
    'If rowStop = -1, all rows are returned
    'colIndices can be provided as a variant array like Array(1,3)
    'if colIndices is not provided, all columns are returned

    Dim newarr() As Variant, newRows As Long, newCols As Long, i As Long, k As Long, refCol As Long

    'Set the correct rowStop
    If rowStop = -1 Then rowStop = UBound(arr, 1)

    'Set the colIndices if they were not provided
    If IsMissing(colIndices) Then
        ReDim colIndices(1 To UBound(arr, 2))
        For k = 1 To UBound(arr, 2)
            colIndices(k) = k
        Next k
    End If

    'Get the dimensions of newarr
    newRows = rowStop - rowStart + 1
    newCols = UBound(colIndices) + 1
    ReDim newarr(1 To newRows, 1 To newCols)

    'Loop through each empty element of newarr and set its value
    For k = 1 To UBound(newarr, 2) 'Loop through each column
        refCol = colIndices(k - 1) 'Get the corresponding reference column
        For i = 1 To UBound(newarr, 1) 'Loop through each row
            newarr(i, k) = arr(i + rowStart - 1, refCol) 'Set the value
        Next i
    Next k

    Subset2D = newarr
End Function

Answer 3

You can use a combination of the Rows, Columns, Offset and Resize properties to get a subset of a range.

For example if you have a range that is 5 columns by 3 rows:

Set rng = Range("A1:E3")

You can get any subset by appropriately combining the above properties. For example, if you want to get the rightmost 3 cells on the second row (i.e. "C2:E2" in the above example), you could do something like:

   Set rngSubset = rng.Rows(2).Offset(0, rng.Columns.Count - 3).Resize(1, 3)

You could then wrap this up in a VBA function.

Answer 4

Two things, VBA doesn't support array slicing so whatever you use, you'll have to roll your own. But since this is just for Excel, you can use the build in worksheet function index for array slicing.

Sub Test()
    'All example return a 1 based 2D array.
    Dim myArr As Variant 'This var must be generic to work.
    'Get whole range:
    myArr = ActiveSheet.UsedRange
    'Get just column 1:
    myArr = WorksheetFunction.Index(ActiveSheet.UsedRange, 0, 1)
    'Get just row 5
    myArr = WorksheetFunction.Index(ActiveSheet.UsedRange, 5, 0)
End Sub

Answer 5

Below is a fast method to slice Excel variant arrays. Most of this was put together using the info from this excellent site http://bytecomb.com/vba-reference/

Essentially the destination array is pre-built as an empty 1d or 2d variant and passed to the sub with the source array and element index to be sliced. Due to the way arrays are stored in memory it's much faster to slice a column than a row as the memory layout allows a single block to be copied.

The good thing about this is it scales well beyond the Excel row limit.

Option Explicit

#If Win64 Then
    Public Const PTR_LENGTH As Long = 8
    Public Declare PtrSafe Function GetTickCount Lib "kernel32" () As Long
    Public Declare PtrSafe Sub Mem_Copy Lib "kernel32" Alias "RtlMoveMemory" (ByRef Destination As Any, ByRef Source As Any, ByVal Length As Long)
    Private Declare PtrSafe Function VarPtrArray Lib "VBE7" Alias "VarPtr" (ByRef Var() As Any) As LongPtr
    Private Declare PtrSafe Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (Destination As Any, Source As Any, ByVal Length As Long)
    Private Declare PtrSafe Sub FillMemory Lib "kernel32" Alias "RtlFillMemory" (Destination As Any, ByVal Length As Long, ByVal Fill As Byte)
#Else
    Public Const PTR_LENGTH As Long = 4
    Public Declare Function GetTickCount Lib "kernel32" () As Long
    Public Declare Sub Mem_Copy Lib "kernel32" Alias "RtlMoveMemory" (ByRef Destination As Any, ByRef Source As Any, ByVal Length As Long)
    Private Declare Function VarPtrArray Lib "VBE7" Alias "VarPtr" (ByRef Var() As Any) As LongPtr
    Private Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (Destination As Any, Source As Any, ByVal Length As Long)
    Private Declare Sub FillMemory Lib "kernel32" Alias "RtlFillMemory" (Destination As Any, ByVal Length As Long, ByVal Fill As Byte)
#End If

Private Type SAFEARRAYBOUND
    cElements    As Long
    lLbound      As Long
End Type

Private Type SAFEARRAY_VECTOR
    cDims        As Integer
    fFeatures    As Integer
    cbElements   As Long
    cLocks       As Long
    pvData       As LongPtr
    rgsabound(0) As SAFEARRAYBOUND
End Type

Sub SliceColumn(ByVal idx As Long, ByRef arrayToSlice() As Variant, ByRef slicedArray As Variant)
'slicedArray can be passed as a 1d or 2d array
'sliceArray can also be part bound, eg  slicedArray(1 to 100) or slicedArray(10 to 100)
Dim ptrToArrayVar As LongPtr
Dim ptrToSafeArray As LongPtr
Dim ptrToArrayData As LongPtr
Dim ptrToArrayData2 As LongPtr
Dim uSAFEARRAY As SAFEARRAY_VECTOR
Dim ptrCursor As LongPtr
Dim cbElements As Long
Dim atsBound1 As Long
Dim elSize As Long

    'determine bound1 of source array (ie row Count)
    atsBound1 = UBound(arrayToSlice, 1)
    'get pointer to source array Safearray
    ptrToArrayVar = VarPtrArray(arrayToSlice)
    CopyMemory ptrToSafeArray, ByVal ptrToArrayVar, PTR_LENGTH
    CopyMemory uSAFEARRAY, ByVal ptrToSafeArray, LenB(uSAFEARRAY)
    ptrToArrayData = uSAFEARRAY.pvData
    'determine byte size of source elements
    cbElements = uSAFEARRAY.cbElements

    'get pointer to destination array Safearray
    ptrToArrayVar = VarPtr(slicedArray) + 8 'Variant reserves first 8bytes
    CopyMemory ptrToSafeArray, ByVal ptrToArrayVar, PTR_LENGTH
    CopyMemory uSAFEARRAY, ByVal ptrToSafeArray, LenB(uSAFEARRAY)
    ptrToArrayData2 = uSAFEARRAY.pvData

    'determine elements size
    elSize = UBound(slicedArray, 1) - LBound(slicedArray, 1) + 1
    'determine start position of data in source array
    ptrCursor = ptrToArrayData + (((idx - 1) * atsBound1 + LBound(slicedArray, 1) - 1) * cbElements)
    'Copy source array to destination array
    CopyMemory ByVal ptrToArrayData2, ByVal ptrCursor, cbElements * elSize

End Sub

Sub SliceRow(ByVal idx As Long, ByRef arrayToSlice() As Variant, ByRef slicedArray As Variant)
'slicedArray can be passed as a 1d or 2d array
'sliceArray can also be part bound, eg  slicedArray(1 to 100) or slicedArray(10 to 100)
Dim ptrToArrayVar As LongPtr
Dim ptrToSafeArray As LongPtr
Dim ptrToArrayData As LongPtr
Dim ptrToArrayData2 As LongPtr
Dim uSAFEARRAY As SAFEARRAY_VECTOR
Dim ptrCursor As LongPtr
Dim cbElements As Long
Dim atsBound1 As Long
Dim i As Long

    'determine bound1 of source array (ie row Count)
    atsBound1 = UBound(arrayToSlice, 1)
    'get pointer to source array Safearray
    ptrToArrayVar = VarPtrArray(arrayToSlice)
    CopyMemory ptrToSafeArray, ByVal ptrToArrayVar, PTR_LENGTH
    CopyMemory uSAFEARRAY, ByVal ptrToSafeArray, LenB(uSAFEARRAY)
    ptrToArrayData = uSAFEARRAY.pvData
    'determine byte size of source elements
    cbElements = uSAFEARRAY.cbElements

    'get pointer to destination array Safearray
    ptrToArrayVar = VarPtr(slicedArray) + 8 'Variant reserves first 8bytes
    CopyMemory ptrToSafeArray, ByVal ptrToArrayVar, PTR_LENGTH
    CopyMemory uSAFEARRAY, ByVal ptrToSafeArray, LenB(uSAFEARRAY)
    ptrToArrayData2 = uSAFEARRAY.pvData

    ptrCursor = ptrToArrayData + ((idx - 1) * cbElements)
    For i = LBound(slicedArray, 1) To UBound(slicedArray, 1)

        CopyMemory ByVal ptrToArrayData2, ByVal ptrCursor, cbElements
        ptrCursor = ptrCursor + (cbElements * atsBound1)
        ptrToArrayData2 = ptrToArrayData2 + cbElements
    Next i

End Sub

Example usage:

Sub exampleUsage()
Dim sourceArr() As Variant
Dim destArr As Variant
Dim sliceIndex As Long

    On Error GoTo Err:

    sourceArr = Sheet1.Range("A1:D10000").Value2
    sliceIndex = 2 'Slice column 2 / slice row 2

    'Build target array
    ReDim destArr(20 To 10000) '1D array from row 20 to 10000
'    ReDim destArr(1 To 10000) '1D array from row 1 to 10000
'    ReDim destArr(20 To 10000, 1 To 1) '2D array from row 20 to 10000
'    ReDim destArr(1 To 10000, 1 To 1) '2D array from row 1 to 10000

    'Slice Column
    SliceColumn sliceIndex, sourceArr, destArr

    'Slice Row
    ReDim destArr(1 To 4)
    SliceRow sliceIndex, sourceArr, destArr

Err:
    'Tidy Up See ' http://stackoverflow.com/questions/16323776/copy-an-array-reference-in-vba/16343887#16343887
    FillMemory destArr, 16, 0

End Sub

Timings were on an old dual core CPU using the following test

Sub timeMethods()
Const trials As Long = 10
Const rowsToCopy As Long = 1048576
Dim rng As Range
Dim Arr() As Variant
Dim newArr As Variant
Dim newArr2 As Variant
Dim t As Long, t1 As Long, t2 As Long, t3 As Long
Dim i As Long

    On Error GoTo Err

    'Setup Conditions 1time only
    Sheet1.Cells.Clear
    Sheet1.Range("A1:D1").Value = Split("A1,B1,C1,D1", ",") 'Strings
'    Sheet1.Range("A1:D1").Value = Split("1,1,1,1", ",") 'Longs
    Sheet1.Range("A1:D1").AutoFill Destination:=Sheet1.Range("A1:D" & rowsToCopy), Type:=xlFillDefault

    'Build source data
    Arr = Sheet1.Range("A1:D" & rowsToCopy).Value
    Set rng = Sheet1.Range("A1:D" & rowsToCopy)

    'Build target container
    ReDim newArr(1 To rowsToCopy)
    Debug.Print "Trials=" & trials & " Rows=" & rowsToCopy
    'Range
    t3 = 0
    For t = 1 To trials
        t1 = GetTickCount

            For i = LBound(newArr, 1) To UBound(newArr, 1)
                newArr(i) = rng(i, 2).Value2
            Next i

        t2 = GetTickCount
        t3 = t3 + (t2 - t1)
        Debug.Print "Range: " & t2 - t1
    Next t
    Debug.Print "Range Avg ms: " & t3 / trials

    'Array
    t3 = 0
    For t = 1 To trials
        t1 = GetTickCount

            For i = LBound(newArr, 1) To UBound(newArr, 1)
                newArr(i) = Arr(i, 2)
            Next i

        t2 = GetTickCount
        t3 = t3 + (t2 - t1)
        Debug.Print "Array: " & t2 - t1
    Next t
    Debug.Print "Array Avg ms: " & t3 / trials

    'Index
    t3 = 0
    For t = 1 To trials
        t1 = GetTickCount

            newArr2 = WorksheetFunction.Index(rng, 0, 2) 'newArr2 2d

        t2 = GetTickCount
        t3 = t3 + (t2 - t1)
        Debug.Print "Index: " & t2 - t1
    Next t
    Debug.Print "Index Avg ms: " & t3 / trials

    'CopyMemBlock
    t3 = 0
    For t = 1 To trials
        t1 = GetTickCount

            SliceColumn 2, Arr, newArr

        t2 = GetTickCount
        t3 = t3 + (t2 - t1)
        Debug.Print "CopyMem: " & t2 - t1
    Next t
    Debug.Print "CopyMem Avg ms: " & t3 / trials

Err:
    'Tidy Up
    FillMemory newArr, 16, 0


End Sub

Answer 6

Lance's solution has a bug in that it does not respect an offset start value with a sub-arry of unspecified length, I also found how it works quite confusing. I offer a (hopefully) more transparent solution below.

Public Function GetSubTable(vIn As Variant, Optional ByVal iStartRow As Integer, Optional ByVal iStartCol As Integer, Optional ByVal iHeight As Integer, Optional ByVal iWidth As Integer) As Variant
    Dim vReturn As Variant
    Dim iInRowLower As Integer
    Dim iInRowUpper As Integer
    Dim iInColLower As Integer
    Dim iInColUpper As Integer
    Dim iEndRow As Integer
    Dim iEndCol As Integer
    Dim iRow As Integer
    Dim iCol As Integer

    iInRowLower = LBound(vIn, 1)
    iInRowUpper = UBound(vIn, 1)
    iInColLower = LBound(vIn, 2)
    iInColUpper = UBound(vIn, 2)

    If iStartRow = 0 Then
        iStartRow = iInRowLower
    End If
    If iStartCol = 0 Then
        iStartCol = iInColLower
    End If

    If iHeight = 0 Then
        iHeight = iInRowUpper - iStartRow + 1
    End If
    If iWidth = 0 Then
        iWidth = iInColUpper - iStartCol + 1
    End If

    iEndRow = iStartRow + iHeight - 1
    iEndCol = iStartCol + iWidth - 1

    ReDim vReturn(1 To iEndRow - iStartRow + 1, 1 To iEndCol - iStartCol + 1)

    For iRow = iStartRow To iEndRow
        For iCol = iStartCol To iEndCol
            vReturn(iRow - iStartRow + 1, iCol - iStartCol + 1) = vIn(iRow, iCol)
        Next
    Next

    GetSubTable = vReturn
End Function