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VBA Visual Basic for Applications (Microsoft) FAQ

VBA How To

Concatenate strings more efficiently by taupirho
Posted: 10 Jun 04

If you need to concatenate serious amounts of text (i.e >100000 bytes) together and you're using the regular & or + VBA concatenation operator you'll find that performance can be increased dramatically by using the little known mid$ statement. Here's what the HELP on mid says:

Replaces a specified number of characters in a Variant (String) variable with characters from another string.


Mid(stringvar, start[, length]) = string

The Mid statement syntax has these parts:

Part             Description
stringvar        Required. Name of string variable to    
start            Required; Variant (Long). Character
                 position in stringvar where the
                 replacement of text begins.
length           Optional; Variant (Long). Number of
                 characters to replace. If omitted, all of
                 string is used.
string           Required. String expression that replaces
                 part of stringvar.


The number of characters replaced is always less than or equal to the number of characters in stringvar.

So say I have a string variable 'data_block' with 900000 characters in it and I want to concatenate another string with 100000 characters onto it - say 'data_line'. Here's how you do it with mid

' make sure our data_block variable can accommodate 1000000 characters
' and our data_line can take 100000

data_block = String$(1000000, " ")
data_line = String$(100000, " ")

' Assume we've now filled data_block with 900000 characters and data_line with 100000 characters
' We now concatenate them

Offset = len(data_block) + 1
Mid(data_block, Offset ) = data_line

Again for small strings just use the & operator but the speed increase in using mid$ for large string variables is significant.

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