program exf7 ! exf7.f90   read and write files

  implicit none   ! must define all integers
  integer :: iostat, stat, dat, i
  integer :: n = 4
  character(len=10) :: aformat = "(a20/)"
  ! format a for characters, f7.3 or e15 for real, i for integer, 1x for space
  
  print *, "exf7.f90  read and write files"
  write(6,fmt="(a20)") "unit 6 is std output"
  ! read(5,fmt="(i5)", number) ! from std input
  write(6, aformat) "unit 5 is std input "

  print *, "writing myfile.dat, then read"
  !  status="new",  status="replace", status="write,  options
  open(unit=11, file="myfile.dat", &
       form="formatted", action="readwrite", iostat=iostat) 
  print *, "iostat=", iostat
  if(iostat /= 0) then
    print *, "can not open file:  myfile.dat for writing"
    stop
  end if
  write(unit=11, fmt="(i1)") n
  print *, "writing ", n
  do i=1,n
    write(unit=11, fmt="(i2)") n*10
    print *, "writing ", i*10
  end do
  close(unit=11, status="keep")

  print *, "reading myfile.dat, that was just written"
  open(unit=11, file="myfile.dat", action="read", iostat=iostat)
  if(iostat /= 0) then
    print *, "can not open file:  myfile.dat for reading"
    stop
  end if
  do
    read(unit=11, fmt="(i1)", iostat=iostat) n
    if(iostat < 0) exit  ! end of file
    if(iostat > 0) cycle ! bad data
    print *, "n=", n
    do i=1,n
      read(unit=11, fmt="(i2)", iostat=iostat) dat
      if(iostat /= 0) print *, "may have bad data"
      if(iostat == 0) print *, "dat=", dat
    end do
  end do
  close(unit=11, status="keep")
  
  print *, "exf7.f90  finished"

end program exf7