program ch1101
implicit none
real :: x
character (7) :: which

  open (unit=7,file='input.txt')
  do
    write (unit=6,fmt='('' data set name, or end'')')
    read (unit=5,fmt='(a)') which
    if (which=='end') exit
    open (unit=1,file=which)
    read (unit=1,fmt=100) x
100 format (f6.0)
! ...
    close (unit=1)
  end do
end program ch1101