How To Improve String Concatenation Performance

MORE INFORMATION

When performing repeated concatenations of the type:

   For I = 1 To N
     Dest = Dest & Source
   Next N
				

the length of time increases proportionally to N-squared. Therefore, 1000 iterations will take about 100 times longer than 100 iterations. This is because Visual Basic does not just add the Source characters to the end of the Dest string; it also performs the following operations:

1. Allocates temporary memory large enough to hold the result.
2. Copies Dest to the start of the temporary area.
3. Copies Source to the end of the temporary area.
4. De-allocates the old copy of Dest.
5. Allocates memory for Dest large enough to hold the result.
6. Copies the temporary data to Dest.

Steps 2 and 6 are very expensive and basically result in the entire concatenated result being copied twice with additional overhead to allocate and de-allocate memory.

This article details a method using the Mid$ statement and pre-allocating memory in larger chunks to eliminate all but step 3 above for most of the concatenation phase.

WARNING: ANY USE BY YOU OF THE CODE PROVIDED IN THIS ARTICLE IS AT YOUR OWN RISK. Microsoft provides this code "as is" without warranty of any kind, either express or implied, including but not limited to the implied warranties of merchantability and/or fitness for a particular purpose.

Step-by-Step Example

1. Type the following code into a module:

         Option Explicit
   
         ' For 16-bit products, uncomment the next three lines by removing the
         ' single quotes and add a single quote to comment out the following
         ' three lines.
         '  Const ConcatStr = "ABC"
         '  Const ccIncrement = 15000
         '  Declare Function GetTickCount Lib "USER" () As Long
   
            Const ConcatStr = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
            Const ccIncrement = 50000
   
            Private Declare Function GetTickCount Lib "KERNEL32" () As Long
   
         Dim ccOffset As Long
   
         Sub StdConcat(ByVal LoopCount As Long)
         Dim BigStr As String, I As Long, StartTick As Long
           StartTick = GetTickCount()
           For I = 1 To LoopCount
             BigStr = BigStr & ConcatStr
           Next I
           Debug.Print LoopCount; "concatenations took";
           Debug.Print GetTickCount() - StartTick; "ticks"
         End Sub
   
         Sub Test_Concat()
           Debug.Print "Using standard concatenation"
           StdConcat 1000
           StdConcat 2000
           StdConcat 3000
           StdConcat 4000
           StdConcat 5000
           Debug.Print
           Debug.Print "Using pre-allocated storage and pseudo-concatenation"
           MidConcat 1000
           MidConcat 2000
           MidConcat 3000
           MidConcat 4000
           MidConcat 5000
         End Sub
   
         Sub Concat(Dest As String, Source As String)
         Dim L As Long
           L = Len(Source)
           If (ccOffset + L) >= Len(Dest) Then
             If L > ccIncrement Then
               Dest = Dest & Space$(L)
             Else
               Dest = Dest & Space$(ccIncrement)
             End If
           End If
           Mid$(Dest, ccOffset + 1, L) = Source
           ccOffset = ccOffset + L
         End Sub
   
         Sub MidConcat(ByVal LoopCount As Long)
         Dim BigStr As String, I As Long, StartTick As Long
           StartTick = GetTickCount()
           ccOffset = 0
           For I = 1 To LoopCount
             Concat BigStr, ConcatStr
           Next I
           BigStr = Left$(BigStr, ccOffset)
           Debug.Print LoopCount; "pseudo-concatenations took";
           Debug.Print GetTickCount() - StartTick; "ticks"
         End Sub
   						

2. In the Debug/Immediate Window, type Test_Concat, and hit the Enter key.
   
   The results will look similar to:

         Using standard concatenation
          1000 concatenations took 2348 ticks
          2000 concatenations took 8954 ticks
          3000 concatenations took 20271 ticks
          4000 concatenations took 35103 ticks
          5000 concatenations took 54453 ticks
   
         Using pre-allocated storage and pseudo-concatenation
          1000 pseudo-concatenations took 82 ticks
          2000 pseudo-concatenations took 124 ticks
          3000 pseudo-concatenations took 165 ticks
          4000 pseudo-concatenations took 247 ticks
          5000 pseudo-concatenations took 289 ticks

Additional Information

1. The code may take a couple of minutes to run.
2. GetTickCount returns the number of milliseconds since Windows was started. Therefore, the output is in milliseconds.
3. Performance improvement ranges from almost 30 times for the 1000-iteration case to almost 200 times for the 5000-iteration case. These times may vary depending on:
   
   
   • The product used.
   • Your system configuration..
   • The size of ccIncrement (larger size favors MidConcat).
   • The number of iterations used (more iterations favors MidConcat).
   • The size of the resultant string (larger size favors MidConcat).