Random access problems.

for random access files you have to write the entire file with blank records up to the max user number. Then as you get records, you seek that position (record size * number) and write the record in that spot. When they query, they provide the record number and you seek that position, read it, and spit it back to the user. If they seek a blanked record you can display it or note that its invalid ID number and message the user...

Yea Danny has told u the good method which need a knowledge about "filestructure" that's why I told u : "or maybe in a new file" .. otherwise if your file isn't too long you may make the traditional method which could be done using one of those gates :
-get the content of the whole file in a big variable, store in the file "output-method" the first line, then finally restore "append-method" the variable container in the file back.
-make a temporary comy of the file , clear the original file and insert in it the first line , append what exists in the temporary file to the original file , delete the temporary file !!!
... this method is very old , it works but not effective .

Now I can tell u a new method to insert this number : put it dynamically at the end of your file :
*each time the file is opened for appending data , first you must read the last line [which contains the number record] , delete the last line , and append your data then finally rewrite the record at the end (this is why I called it dynamically) ...
**just you need a good technice to read the last line but it's easy .