VB Quicktakes - Faster String Manipulation

This example demonstrates a method for speeding up string concatenation. When adding characters to a string (ex. string1 = string1 & string2) VB is relatively inefficient especially when concatenating a string numerous times such as in a loop. The main reason for this is that VB must allocate memory each time the size of a string variable is increased. The example below demonstrates a more efficient method of concatenation. By allocating spaces to a string and than replacing those spaces with whatever strings are necessary can greatly improve perfomance. Using this method code will run @ 100 times faster when creating a 1 mb text file. The performance boosts grows exponentially with the size of the string being created.

Private Sub cmdTest_Click()
    Const ALLOCATED_SPACE = 10000000
    Dim fnum As Long, currPos
    Dim strTest As String
    currPos = 1

    'sets string length
    strTest = Space(ALLOCATED_SPACE)

    'creates a string listing numbers 1 to 40000
    For i = 0 To 40000
        Mid(strTest, currPos, Len(CStr(i) & " ")) = CStr(i) & " "
        currPos = currPos + Len(CStr(i) & " ")
    'get rid of any extra spaces
    strTest = Trim(strTest)
    fnum = FreeFile()
    Open App.Path & "\Test.txt" For Output As #fnum
    Print #fnum, strTest
    Close #fnum

End Sub

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