Return dynamic array from function VBA - vba

Question

I am trying to create a function that outputs an array.
However, I got the Function call on left-hand side must return Variant or
Object. How can I return a dynamic array from this function?
Public Function Fibonacci_Array(max As Integer) As Integer
Dim result() As Variant
ReDim result(0 To max)
'' Array indices.
Dim i1 As Integer
Dim i2 As Integer
Dim i As Integer
i1 = 0
i2 = 1
'' Array values.
Dim newVal As Long
Dim prev2 As Long
Dim prev As Long
prev2 = 0
prev = 1
'' Loop through
While prev <= max
result(i1) = prev2
result(i2) = prev
newVal = prev + prev2
''Debug.Print newVal
prev2 = prev
prev = newVal
i1 = i1 + 1
i2 = i2 + 1
Wend
'' Problem here.
Fibonacci_Array() = result
End Function

Answer 1

Variant is the most flexible type when it comes to passing arrays to or from functions.
Replace
Public Function Fibonacci_Array(max As Integer) As Integer
by
Public Function Fibonacci_Array(max As Integer) As Variant
Replace
Dim result() As Variant
by
Dim result As Variant
and replace
Fibonacci_Array() = result
by
Fibonacci_Array = result
That will make it compile, but you seem to need a bit of debugging, since when I then type
?Join(Fibonacci_Array(10),", ")
in the Immediate Window, I get:
0, 1, 1, 2, 3, 5, 8, , , ,
(This might be what you want if you want the Fibonacci numbers which are less than max, but then you might want to use a ReDim Preserve to pare the array down to size before returning it. If your intention was to get the first max Fibonacci numbers, the culprit is the line While prev <= max -- it isn't prev that you would want to compare to max).
On Edit I thought it would be fun to write a VBA function which returns the array of all Fibonacci numbers whose size is <= a given max. Since Fibonacci numbers grow rapidly, I decided to use Long rather than Integer, and also to use Binet's formula to calculate the size of the array (possibly +1 for safety) before filling the array, so we don't allocate an array which is much too large:
Function FibNums(max As Long) As Variant
'returns array consisting of all Fibonacci numbers <= max
'max is assumed to be >= 1
Dim i As Long, n As Long, F As Long
Dim Fibs As Variant
'come up with an upper bound on size of array:
n = 1 + Int(Log(Sqr(5) * max) / Log((1 + Sqr(5)) / 2))
ReDim Fibs(1 To n)
Fibs(1) = 1
Fibs(2) = 1
i = 2
Do While Fibs(i) <= max
F = Fibs(i - 1) + Fibs(i)
If F <= max Then
i = i + 1
Fibs(i) = F
Else
Exit Do 'loop is finished
End If
Loop
'at this stage, Fibs contains i numbers
If i < n Then ReDim Preserve Fibs(1 To i)
FibNums = Fibs
End Function
For example:
?Join(Fibnums(100000),", ")
1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368, 75025

Answer 2

Your return type should be the same and you don't need the parenthesis when you assign the value of the function:
Public Function Fibonacci_Array(max As Integer) As Long()
Dim result() As Long
ReDim result(0 To max)
'' Array indices.
Dim i1 As Integer
Dim i2 As Integer
Dim i As Integer
i1 = 0
i2 = 1
'' Array values.
Dim newVal As Long
Dim prev2 As Long
Dim prev As Long
prev2 = 0
prev = 1
'' Loop through
While prev <= max
result(i1) = prev2
result(i2) = prev
newVal = prev + prev2
''Debug.Print newVal
prev2 = prev
prev = newVal
i1 = i1 + 1
i2 = i2 + 1
Wend
'' Problem here.
Fibonacci_Array = result
End Function
Sub a()
Dim b() As Long
b() = Fibonacci_Array(100)
End Sub