Convert integers to strings to create output filenames at run time

I have a program in Fortran that saves the results to a file. At the moment I open the file using

OPEN (1, FILE = 'Output.TXT')

However, I now want to run a loop, and save the results of each iteration to the files 'Output1.TXT', 'Output2.TXT', 'Output3.TXT', and so on.

Is there an easy way in Fortran to constuct filenames from the loop counter i?

you can write to a unit, but you can also write to a string

program foo
    character(len=1024) :: filename

    write (filename, "(A5,I2)") "hello", 10

    print *, trim(filename)
end program

Please note (this is the second trick I was talking about) that you can also build a format string programmatically.

program foo

    character(len=1024) :: filename
    character(len=1024) :: format_string
    integer :: i

    do i=1, 10
        if (i < 10) then
            format_string = "(A5,I1)"
        else
            format_string = "(A5,I2)"
        endif

        write (filename,format_string) "hello", i
        print *, trim(filename)
    enddo

end program

A much easier solution IMHO ...................

character(len=8) :: fmt ! format descriptor

fmt = '(I5.5)' ! an integer of width 5 with zeros at the left

i1= 59

write (x1,fmt) i1 ! converting integer to string using a 'internal file'

filename='output'//trim(x1)//'.dat'

! ====> filename: output00059.dat