Creating a string from an array of strings. Fastest method? - vba

Question

I have an array of strings (A through E) that I want to join into one string ("A B C D E"). Should I loop through the array or use the Join function?
Dim MyArray(5) as String
Dim MyString as String
MyArray(1) = "A"
MyArray(2) = "B"
MyArray(3) = "C"
MyArray(4) = "D"
MyArray(5) = "E"
Which is faster and more advisable?
This?
MyString = MyArray(1)
For i = 2 To 5
MyString = MyString & " " & MyArray(i)
Next
Or this?
MyString = Join(MyArray, " ")

Answer 1

For a 100k array
Sub test()
Dim aArr(1 To 100000) As String
Dim i As Long
Dim sString As String
Dim snTimer As Single
FillArray aArr
snTimer = Timer
For i = 1 To 100000
sString = sString & Space(1) & aArr(i)
Next i
Debug.Print Timer - snTimer
snTimer = Timer
sString = Join(aArr, Space(1))
Debug.Print Timer - snTimer
End Sub
Join is the clear winner
2.050781
0
the reason is that every time you concatenate with & memory has to be reallocated to accommodate the new array (which is all strings are anyway). With Join, you're just copying one array (the source array) to another array (the string) and VBA already knows the size.

Answer 2

If you want to combine many strings efficiently you can define a stringbuilder class.
Running the code below to build up a string of numbers up to a million takes just a fraction of a second (0.3s). Building an array and using Join takes not far off the same time (0.25s), the call to the Join function takes only about 10% of that time.
If the strings are already in an array then it makes sense to use Join but with a small number of strings the difference is unlikely to be noticeable anyway.
Sub JoinStringTest()
Dim i As Long, t As Double
Dim sb As New StringBuilder
Dim sbRet As String
Dim joinRet As String
t = Timer
For i = 1 To 1000000
sb.Append CStr(i)
Next
sbRet = sb.Text
Debug.Print "SB", Timer - t
t = Timer
Dim a(1000000) As String
For i = 1 To 1000000
a(i) = CStr(i)
Next i
joinRet = Join(a, "")
Debug.Print "Join", Timer - t
Debug.Print sbRet = joinRet
End Sub