Reverse an array in Excel VBA - arrays

Question

I have the following code, which based on the logic it should work.
I want it to be (4,3,2,1), but at the end of the loop I get t=(4,3,3,4)
Sub try()
Dim t As Variant
t = Array(1, 2, 3, 4)
a = UBound(t)
For k = 0 To a
t(k) = t(a - k)
Next k
End Sub
Any ideas?

Answer 1

You have to use a temporary variable to store the stuff before you make the switch else it will be overwritten.
Is this what you are trying?
Sub try()
Dim t As Variant, tmp As Variant
Dim a As Long, b As Long, i As Long
t = Array(1, 2, 3, 4)
a = UBound(t): b = LBound(t)
For i = 0 To ((a - b) \ 2)
tmp = t(i)
t(i) = t(a)
t(a) = tmp
a = a - 1
Next i
For i = 0 To UBound(t)
Debug.Print t(i)
Next i
End Sub

Answer 2

When you do t(k) = t(a - k) you assign t(a-k) to t(k), but then the value stored in t(k) is lost. You need to temporarily store that in another variable (variable x in the following example), then you can swap the values between t(k) and t(a - k) like this:
Sub try()
Dim t As Variant
Dim x As Variant
Dim b As Integer
t = Array(1, 2, 3, 4)
a = UBound(t)
b = (a - 1) / 2
For k = 0 To b
x = t(k)
t(k) = t(a - k)
t(a - k) = x
Next k
End Sub
Notice that you only need to iterate a number of times that is half of your array size (rounded down) otherwise you'd swap back values again and would end up with the same starting array.

Answer 3

If don't want to delete the original array you can
create a copy of the array in reverse creation.
See 1D and 2D array subs:
Option Base 1
Sub CopyArrayinReverseColumns1D()
Dim OriginalArr As Variant
Dim newarr As Variant
Dim a As Long
Dim i As Long
OriginalArr = Array(1, 2, 3, 4)
newarr = Array(0, 0, 0, 0)
a = UBound(OriginalArr)
For i = 1 To a
newarr(i) = OriginalArr(a - i + 1)
Debug.Print newarr(i)
Next
End Sub
Sub CopyArrayinReverseColumns2D()
Dim OriginalArr(2, 4) As Variant
Dim newarr(2, 4) As Variant
Dim a As Long
Dim b As Long
Dim i As Long
Dim n As Long
OriginalArr(1, 1) = 65
OriginalArr(1, 2) = 70
OriginalArr(1, 3) = 75
OriginalArr(1, 4) = 80
OriginalArr(2, 1) = 85
OriginalArr(2, 2) = 90
OriginalArr(2, 3) = 95
OriginalArr(2, 4) = 100
a = UBound(OriginalArr, 1)
b = UBound(OriginalArr, 2)
For i = 1 To a
For n = 1 To b
newarr(i, n) = OriginalArr(a - i + 1, n)
Debug.Print newarr(i, n)
Next
Next
End Sub