Inserting a string at a specified line in text file using objective c

Question

Lets say I have a text file with the following data

Username

User ID

Details

suppose, I want to insert a

Answer 1

The only way to do this is to read the file into memory, modify the internal representation of the data, and write it back out.

There are some variations on this process, but in short; files are not like real paper and ink documents: You can't insert content into the middle of a file, only replace what is already there.

Edit: To clarify, you can only grow a file at the end; not in the middle.