How to slice an array in batches in VBA - arrays

Question

Suppose I have a VBA one dimension array (or dict or collection) with X values. I need to perform an action with these values in batches of Y.
So if X = 55 and Y = 25, I would need to loop 3 times:
Pick values 1 to 25 and perform action
Pick values 26 to 50 and perform action
Pick last 5 values and perform action
Any ideas with good performance will be greatly appreciated :)
Edit:
I came up with the code below. It works although doesn't look very concise
Sub test()
Dim arr As Variant
Dim temparr As Variant
Dim sippno As Integer
Dim loopend As Integer
Dim loopstart As Integer
Dim batchsize As Integer
Dim i As Integer
'Storing main array with all values
arr = Sheet1.Range("A1:A" & Sheet1.Range("A" & Rows.Count).End(xlUp).Row).Value
'Setting count of values, batch size and starting step for loop
sippno = WorksheetFunction.CountA(arr)
loopstart = 1
batchsize = 10
Do Until sippno = 0
If sippno < batchsize Then
loopend = loopstart + sippno - 1
Else
loopend = loopstart + batchsize - 1
End If
ReDim temparr(loopstart To loopend)
For i = loopstart To loopend
temparr(i) = WorksheetFunction.Index(arr, i, 0)
sippno = sippno - 1
Next
loopstart = loopend + 1
'Action to be performed with batch of values stored in second array
Debug.Print WorksheetFunction.TextJoin(", ", True, temparr)
Loop
End Sub

Answer 1

Slicing via Application.Index()
Just for the sake of the art I demonstrate in this late post how to slice a 'vertical' array in one go into several 'flat' arrays in batches of e.g. 10 elements.
This approach benefits from the advanced rearranging features & pecularities of Application.Index()
allowing to pass entire row/column number arrays as arguments; here suffices a vertical array of desired row numbers, e.g. by filtering only rows 11 to 20 via Application.Index(data, Evaluate("Row(11:20)"), 0). .. c.f. see section 2 a)
Further notes:
evaluating a tabular row formula is one quick way to get consecutive row numbers.
transposing the function result changes the array dimension to a 1-dim array
reducing the array boundaries by -1 via ReDim Preserve ar(0 To UBound(ar) - 1) produces a zero-based array (optional)
Option Explicit
Sub splice()
Const batch = 10 ' act in units of 10 elements
With Sheet1
'1) get data (1-based 2-dim array)
Dim lastRow As Long
lastRow = .Cells(Rows.Count, "A").End(xlUp).Row
Dim data: data = .Range("A1:A" & lastRow).Value2
'2) slice
Dim i As Long, nxt As Long, ar As Variant
For i = 1 To UBound(data) Step batch
nxt = Application.min(i + batch - 1, UBound(data))
'2a) assign sliced data to 1- dim array (with optional redim to 0-base)
With Application
ar = .Transpose(.Index(data, Evaluate("row(" & i & ":" & nxt & ")")))
End With
'optional redimming to zero-base
ReDim Preserve ar(0 To UBound(ar) - 1)
'2b) perform some action
Debug.Print _
"batch " & i \ batch + 1 & ": " & _
"ar(" & LBound(ar) & " To " & UBound(ar) & ") ~~> " & _
Join(ar, "|")
Next
End With
End Sub
Slicing a 'flat' 1-dim array
If, however you want to slice a 1-dim array, like e.g. dictionary keys, it suffices to transpose the data input: data = Application.Transpose(...)

Answer 2

Option Explicit
Sub splice()
Const batch = 10
Dim data, ar()
Dim lastrow As Long, n As Long, i As Long
Dim j As Long, r As Long
With Sheet1
lastrow = .Cells(Rows.Count, "A").End(xlUp).Row
data = .Range("A1:A" & lastrow).Value2
End With
i = Int(lastrow / batch)
For n = 0 To i
r = batch
If n = i Then
r = lastrow Mod batch
End If
If r > 0 Then
ReDim ar(r - 1)
For j = 1 To r
ar(j - 1) = data(j + n * batch, 1)
Next
' do something
Debug.Print Join(ar, ",")
End If
Next
End Sub

Answer 3

2d array because to lazy to encode 1d but same idea with 1d:
Sub test()
arr = Sheet3.Range("A1").CurrentRegion.Value2
x = UBound(arr)
y = 5
jj = y
For j = 1 To UBound(arr)
sumaction = sumaction + arr(j, 1)
If (UBound(arr) - jj) < 0 Then
jj = UBound(arr)
sumaction = 0
End If
If j = jj Then
dosomething = sumaction * 2
sumaction = 0
jj = jj + y
End If
Next j
End Sub