Programming :: Generating Files With Different Names At Each Loop In Fortran?

I am trying to write a loop to open four main-data-files. At each open of the main-data-file, I want the loop to open another four sub-files and split the data from the mail-data-file according to the creteria and dump into the four files. When the loop goes to the second main-data-file, it shall generate another 4 sub-files with different file name. And the process goes on. That means I will have total of 16 sub files for the four main-data-files. Each four files from the 16 files shall have specific name on it. Based on the code which I given below, I would say when the file at UNIT=100 is opened, 4 new files needed to be opened as well. They are:

maximum_dist_001_064.dat
maximum_dist_065_128.dat
maximum_dist_129_192.dat
maximum_dist_193_256.dat