UNIX for Dummies Questions & Answers

I have a csv flatfile with a few million rows. I need to replace a field (field number is 85) in the file with a sequential number.

As an example, let's assume there are only 4 fields in the file:
A,A,,32
A,A,,27
A,B,,43
C,C,,354

If I wanted to amend the 3rd field in this way my output should be:
A,A,1,32
A,A,2,27
A,B,3,43
C,C,4,354

I'm guessing it will require an awk, but not sure how..

Thanks

Code:

awk -F, -v OFS="," '{ $3=NR } 1' infile > outfile

Cheers mate.. Worked like a dream... I had to start the numbering at a certain number as well (it's for generating a unique id for a csv file to insert into a db), so changed to:
awk -F, -v OFS="," '{ $165=NR+"500" } 1' Infile > Outfile

Thanks....