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Tip of the Day
Language: VB5,VB6
Expertise: Advanced
Aug 4, 2001

Swap strings the fast way

Consider the usual way of swapping two strings:

Dim s1 As String, s2 As String
Dim tmp As String

' initialize the strings 
s1 = String$(1000, "-")
s2 = String$(1500, "+")
' do the swap, through a temporary variable
tmp = s1
s1 = s2
s2 = tmp
If you put the above code in a time-critical loop (such as when you are sorting an array) and the strings you're working with are rather long, this code becomes relatively slow, because it has to allocate a new string, copy a lot of characters, and then deallocate the memory for the temporary strings used in the meantime.

A more linear approach consists of just exchanging the contents of the string descriptors for the two s1 and s2 strings, withouth actually moving any character in memory and, above all, without having to allocate and release memory. The following code can be 20+ faster than the previous approach, depending on how long the strings you're processing are:


Private Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (dest As _
    Any, source As Any, ByVal numBytes As Long)

Dim saveAddr As Long 

' save the descriptor of first string
saveAddr = StrPtr(s1)
' copy the s2's descriptor into s1's descriptor (32 bit)
CopyMemory ByVal VarPtr(s1), ByVal VarPtr(s2), 4
' complete the swapping of descriptors
CopyMemory ByVal VarPtr(s2), saveAddr, 4
Francesco Balena
 
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