Several weeks ago I posted a version of this and I thought I had it solved
with help from this DG - but not so!

xx(j,k) calculates the crossproduct of two 3-dimensional vectors. the
result is returned in either a row or column on the work sheet as selected by
the user. Normally the function works ok. It even works ok in
"=xx(xx(i,j),xx(k,l)) when the selection is a row but crashes at line 9, on
the third pass (Va and Vb are then Variants) when the selection is a column.
I'm using Option Base 1.

Function XX(Va, Vb)
Debug.Print TypeName(Va), TypeName(Vb)
Static r As Integer, c As Integer

r = Selection.Rows.Count
c = Selection.Columns.Count

Dim CP() As Double
If r c Then
ReDim CP(3, 1)
8 MsgBox "Got to line 8 OK"
9 CP(1, 1) = Va(2) * Vb(3) - Va(3) * Vb(2)
10 MsgBox "Got to line 10 OK"
CP(2, 1) = Va(3) * Vb(1) - Va(1) * Vb(3)
CP(3, 1) = Va(1) * Vb(2) - Va(2) * Vb(1)
Debug.Print r, CP(1, 1), CP(2, 1), CP(3, 1)
Else
ReDim CP(1, 3)
11 CP(1, 1) = Va(2) * Vb(3) - Va(3) * Vb(2)
CP(1, 2) = Va(3) * Vb(1) - Va(1) * Vb(3)
CP(1, 3) = Va(1) * Vb(2) - Va(2) * Vb(1)
Debug.Print r, CP(1, 1), CP(1, 2), CP(1, 3)
End If
XX = CP
End Function

Can anyone help?

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Thanks for your help