In Fortran, how to write backward from a file to a

Question

I have an ASCII file that looks like:

____________________________________________
Header1 ...
Header2 ...
Header3 ...
block(1)data1 block(1)data2 block(1)data3
block(1)data4 block(1)data5 block(1)data6
block(2)data1 block(2)data2 block(2)data3
block(2)data4 block(2)data5 block(2)data6
...
block(n)data1 block(n)data2 block(n)data3
block(n)data4 block(n)data5 block(n)data6
____________________________________________

I would like to convert it into an ASCII file that looks like:

____________________________________________
HeaderA ...
HeaderB ...
block(n)data1 block(n)data2 block(n)data3
block(n)data4 block(n)data5 block(n)data6
block(n-1)data1 block(n-1)data2 block(n-1)data3
block(n-1)data4 block(n-1)data5 block(n-1)data6
....
block(1)data1 block(1)data2 block(1)data3
block(1)data4 block(1)data5 block(1)data6
____________________________________________

Data are mainly real numbers, and size of the data set is way too big to use allocatable arrays. So I have somehow to read and write on the fly.

I could not find a way to read-or-write backward in a file.

Answer 1

Well, I sort of have an answer, but it didn't work, perhaps due to compiler bugs or my rudimentary understanding of file positioning in Fortran. My attempt was to open the input file with access = 'stream' and form = 'formatted'. This way I could push the line positions onto a stack and pop them off so they come out in reverse order. Then, walking through the lines in reverse order I could write them into the ourput file.

program readblk
   implicit none
   integer iunit, junit
   integer i, size
   character(20) line
   type LLnode
      integer POS
      type(LLnode), pointer :: next => NULL()
   end type LLnode
   type(LLNODE), pointer :: list => NULL(), current => NULL()
   integer POS, temp(2)

   open(newunit=iunit,file='readblk.txt',status='old',access='stream',form='formatted')
   open(newunit=junit,file='writeblk.txt',status='replace')
   do i = 1, 3
      do
         read(iunit,'(a)',advance='no',EOR=10,size=size) line
         write(junit,'(a)',advance='no') line
      end do
10    continue
      write(junit,'(a)') line(1:size)
   end do
   do
      inquire(iunit,POS=POS)
      allocate(current)
      current%POS = POS
      current%next => list
      list => current
      read(iunit,'()',end=20)
   end do
20 continue
   current => list
   list => current%next
   deallocate(current)
   do while(associated(list))
      temp(2) = list%POS
      current => list%next
      deallocate(list)
      temp(1) = current%POS
      list => current%next
      deallocate(current)
      do i = 1, 2
write(*,*) temp(i)
         read(iunit,'(a)',advance='no',EOR=30,size=size,POS=temp(i)) line
         write(junit,'(a)',advance='no') line
         do
            read(iunit,'(a)',advance='no',EOR=30,size=size) line
            write(junit,'(a)',advance='no') line
         end do
30       continue
      write(junit,'(a)') line(1:size)
      end do
   end do
end program readblk

Here is my input file:

Header line 1
Header line 2
Header line 3
1a34567890123456789012345678901234567890
1b34567890123456789012345678901234567890
2a34567890123456789012345678901234567890
2b34567890123456789012345678901234567890
3a34567890123456789012345678901234567890
3b34567890123456789012345678901234567890

Now with ifort my file positions were printed out as

 214
 256
 130
 172
  44
  88

Note that the first line is at the end of record 3 instead of the beginning of record 4. The output file was

Header line 1
Header line 2
Header line 3
3a34567890123456789012345678901234567890
3b34567890123456789012345678901234567890
2a34567890123456789012345678901234567890
2b34567890123456789012345678901234567890

1a34567890123456789012345678901234567890

With gfortran, the file positions printed out as

 214
 256
 130
 172
  46
  88

This time the first line is at the beginning of record 4 as I would expect. However, the output file had unfortunate contents

Header line 1
Header line 2
Header line 3
3a34567890123456789012345678901234567890
3b34567890123456789012345678901234567890
2a34567890123456789012345678901234567890
2b34567890123456789012345678901234567890
3a34567890123456789012345678901234567890
3b345678901234567890123456789012341a34567890123456789012345678901234567890

I had hoped for a more positive result. I can't tell whether my results are due to poor programming or compiler bugs, but I posted in case someone else could possibly get my pure Fortran solution to work.

Answer 2

way to big to use allocatable arrays.

If the data fits in memory, you can do it. I've tested it out, a file

header(1)
header(2)
header(3)
block(1).data1 block(1).data2 block(1).data3
block(1).data4 block(1).data5 block(1).data6
block(2).data1 block(2).data2 block(2).data3
block(2).data4 block(2).data5 block(2).data6
...
block(9999998).data1 block(9999998).data2 block(9999998).data3
block(9999998).data4 block(9999998).data5 block(9999998).data6
block(9999999).data1 block(9999999).data2 block(9999999).data3
block(9999999).data4 block(9999999).data5 block(9999999).data6

with a file size of 1.2GB could be reversed by this little awk script:

#!/usr/bin/awk
# if line contains word "header", print immediately, move on to next line.
/header/ {print; next}

# move every line to memory.
  {
    line[n++] = $0
  }

# When finished, print them out in order n-1, n, n-3, n-2, n-5, n-4, ...
END {
  for (i=n-2; i>=0; i-=2) {
    print(line[i])
    print(line[i+1])
  }
}

in under 2 minutes.

If this is really not possible, you need to do what @high-performance-mark said and read it in in blocks that are manageable, reverse it in memory, then concatenate them together at the end. Here's my version:

program reverse_order
  use iso_fortran_env, only: IOSTAT_END
  implicit none
  integer, parameter :: max_blocks_in_memory = 10000
  integer, parameter :: max_line_length=100
  character(len=max_line_length) :: line
  character(len=max_line_length) :: data(2, max_blocks_in_memory)

  character(len=*), parameter :: INFILE='data.txt'
  character(len=*), parameter :: OUTFILE='reversed_data.txt'
  character(len=*), parameter :: TMP_FILE_FORMAT='("/tmp/", I10.10,".txt")'
  character(len=len("/tmp/XXXXXXXXXX.txt")) :: tmp_file_name

  integer :: in_unit, out_unit, tmp_unit
  integer :: num_headers, i, j, tmp_file_number

  integer :: ios

! Open the input and output files
  open(newunit=in_unit, file=INFILE, action="READ", status='OLD')
  open(newunit=out_unit, file=OUTFILE, action='WRITE', status='REPLACE')

! Transfer the headers to the output file immediately.
  num_headers = 0
  do
    read(in_unit, '(A)') line
    if (index(line, 'header') == 0) exit
    num_headers = num_headers + 1
    write(out_unit, '(A)') trim(line)
  end do

! We've already read the first data line, so let's rewind and start anew.
  rewind(in_unit)
! move past the headers.
  do i = 1, num_headers
    read(in_unit, *)
  end do


  tmp_file_number = 0

! Read the data from the input line max_blocks_in_memory blocks at a time.
  read_loop : do
    do i = 1, max_blocks_in_memory
      read(in_unit, '(A)', iostat=ios) data(1, i)
      if (ios == IOSTAT_END) then   ! Reached the end of the input file.
        if (i > 1) then         ! Still have final values in memory, write them
                                ! to output immediately.
          do j = i-1, 1, -1
            write(out_unit, '(A)') trim(data(1, j))
            write(out_unit, '(A)') trim(data(2, j))
          end do
        end if
        exit read_loop
      end if
      read(in_unit, '(A)') data(2, i)
    end do

!  Reasd a block of data, write it in reverse order into a temporary file.

    tmp_file_number = tmp_file_number + 1
    write(tmp_file_name, TMP_FILE_FORMAT) tmp_file_number
    open(newunit=tmp_unit, file=tmp_file_name, action="WRITE", status="NEW")
    do j = max_blocks_in_memory, 1, -1
      write(tmp_unit, '(A)') data(1, j)
      write(tmp_unit, '(A)') data(2, j)
    end do
    close(tmp_unit)
  end do read_loop

! Finished with input file, don't need it any more.
  close(unit=in_unit)

! Concatenate all the temporary files in reverse order to the output file.
  do j = tmp_file_number, 1, -1
    write(tmp_file_name, TMP_FILE_FORMAT) j
    open(newunit=tmp_unit, file=tmp_file_name, action="READ", status="OLD")
    do
      read(tmp_unit, '(A)', iostat=ios) line
      if (ios == IOSTAT_END) exit
      write(out_unit, '(A)') trim(line)
    end do
    close(tmp_unit, status="DELETE")  ! Done with this file, delete it after closing.

  end do

  close(unit=out_unit)

end program reverse_order

Answer 3

I would not directly use Fortran, but more a sequence of Linux commands (or Cygwin / GNU utils on Windows). Fortran is possible as well (see second possibility).

An outline (based on OS commands):

• get total number of lines (e/g. wc)
• get first 3 lines out of the file (e.g. with head) to file result file
• process the main part
  • take last but 3 lines (e.g. with tail)
  • run the result through e.g a awk script joining the relevant lines
  • run tac on the result
  • run another awk script splitting the lines
  • append result to result file

Another idea would be (in a programming language):

• create an array with with the beginning file position of each block (so result of ftell).
• move the header to the new file
• run through the array created above, from the end to the beginning
  • do an fseek to the indicated position
  • read relevant number of lines and write them out again