The information in this article applies to:
- Microsoft Visual Basic Control Creation, Learning, Professional, and
Enterprise Editions for Windows, version 5.0
- Microsoft Visual Basic Standard, Professional, and Enterprise Editions,
16-bit and 32-bit, for Windows, version 4.0
- Microsoft Visual Basic Standard and Professional Editions for Windows,
version 3.0
- Microsoft Access versions 1.0, 1.1, 2.0, 7.0, 97
- Microsoft Excel for Windows 95, version 7.0
- Microsoft Excel 97 for Windows
- Microsoft Word 97 for Windows
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. Thus, 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:
- Allocates temporary memory large enough to hold the result.
- Copies Dest to the start of the temporary area.
- Copies Source to the end of the temporary area.
- De-allocates the old copy of Dest.
- Allocates memory for Dest large enough to hold the result.
- 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" without the quotes,
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
- The code may take a couple of minutes to run.
- GetTickCount returns the number of milliseconds since Windows
was started. Thus, the output is in milliseconds.
- 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).
Keywords : VB4ALL VB4WIN vb5all vb5howto VBKBVBA vbwin GnrlVb kbprg
Technology : kbvba
Version : WINDOWS:1.0 1.1 2.0 3.0 4.0 5.0 7.0
Platform : WINDOWS
Issue type : kbhowto
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