appending to rows

Hi guys
I was wondering if you could help me
 
I have 2 files.
One file has 3 data rows, A, B and C
The file has 1 data row D.
 
I need to append row D to rows A and C in the first file, based on some
condition.
Any ideas how I could go about this ?
 
Another alternative would be for me to open up a new file and write
row 1 : AD
row 2 : B
row 3 : CD
 
Is the second approach better in terms of performance than the first
approach as I will have more than 1 million rows to deal with ?
 
Thanks in advance
Tony
 
 

• appending to rows by Ho, Tony
• RE: appending to rows by Hanson, Robert
• Re: appending to rows by John W. Krahn