Microsoft KB Archive/170964

= How To Improve String Concatenation Performance =

Article ID: 170964

Article Last Modified on 10/11/2006

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APPLIES TO


 * Microsoft Visual Basic 5.0 Control Creation Edition
 * Microsoft Visual Basic 5.0 Learning Edition
 * Microsoft Visual Basic 5.0 Professional Edition
 * Microsoft Visual Basic 5.0 Enterprise Edition
 * Microsoft Visual Basic 4.0 Standard Edition
 * Microsoft Visual Basic 4.0 Professional Edition
 * Microsoft Visual Basic 4.0 Professional Edition
 * Microsoft Visual Basic 4.0 16-bit Enterprise Edition
 * Microsoft Visual Basic 4.0 32-Bit Enterprise Edition
 * Microsoft Visual Basic 3.0 Professional Edition
 * Microsoft Visual Basic 3.0 Professional Edition
 * Microsoft Access 1.0 Standard Edition
 * Microsoft Access 1.1 Standard Edition
 * Microsoft Access 2.0 Standard Edition
 * Microsoft Access 95 Standard Edition
 * Microsoft Access 97 Standard Edition
 * Microsoft Excel 95 Standard Edition
 * Microsoft Excel 97 Standard Edition
 * Microsoft Word 97 Standard Edition

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This article was previously published under Q170964



SUMMARY
When concatenating large strings on the order of 50kb or larger (for example, building an HTML table from a database), the length of time to complete can become quite long as the string gets larger. This article demonstrates an alternative to normal concatenation that can improve performance for large strings by 20 times or more.



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
  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   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:


 * 1) * The product used.
 * 2) * Your system configuration..
 * 3) * The size of ccIncrement (larger size favors MidConcat).
 * 4) * The number of iterations used (more iterations favors MidConcat).
 * 5) * The size of the resultant string (larger size favors MidConcat).

