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Monday, July 31, 2006

ASP Tips: Tip 14: Avoid String Concatenation in Loops

Many people build a string in a loop like this:

s = "<table>" & vbCrLf
For Each fld in rs.Fields
s = s & " <th>" & fld.Name & "</th> "

While Not rs.EOF
s = s & vbCrLf & " <tr>"
For Each fld in rs.Fields
s = s & " <td>" & fld.Value & "</td> "
s = s & " </tr>"

s = s & vbCrLf & "</table>" & vbCrLf
Response.Write s

There are several problems with this approach. The first is that repeatedly concatenating a string takes quadratic time; less formally, the time that it takes to run this loop is proportional to the square of the number of records times the number of fields. A simpler example should make this clearer.

s = ""
For i = Asc("A") to Asc("Z")
s = s & Chr(i)

On the first iteration, you get a one-character string, "A". On the second iteration, VBScript has to reallocate the string and copy two characters ("AB") into s. On the third iteration, it has to reallocate s again and copy three characters into s. On the Nth (26th) iteration, it has to reallocate and copy N characters into s. That's a total of 1+2+3+...+N which is N*(N+1)/2 copies.

In the recordset example above, if there were 100 records and 5 fields, the inner loop would be executed 100*5 = 500 times and the time taken to do all the copying and reallocation would be proportional to 500*500 = 250,000. That's a lot of copying for a modest-sized recordset.

In this example, the code could be improved by replacing the string concatenation with Response.Write() or inline script (<% = fld.Value %>). If response buffering is turned on (as it should be), this will be fast, as Response.Write just appends the data to the end of the response buffer. No reallocation is involved and it's very efficient.

In the particular case of transforming an ADO recordset into an HTML table, consider using GetRows or GetString.

If you concatenate strings in JScript, it is highly recommended that you use the += operator; that is, use s += "some string", not s = s + "some string".

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