In fortran how to read all strings line by line from a .dat file in fortran

Question

I have a .dat file , from which I am trying to read line by line and print it. Can anyone please help about how to do it in fortran?

Thanks in advance!

Data:

REMARK    GENERATED BY TRJCONV
TITLE     UNNAMED t=   0.00000
REMARK    THIS IS A SIMULATION BOX
CRYST1   26.178   26.178   26.178  90.00  90.00  90.00 P 1           1
MODEL        1
ATOM      1  S2  LJ2     1      17.000  15.030  11.630  1.00  0.00            
ATOM      2  S2  LJ2     2      13.290  11.340  15.900  1.00  0.00            
ATOM      3  S2  LJ2     3      17.030  23.070  14.750  1.00  0.00            
ATOM      4  S2  LJ2     4      15.360  14.840   9.480  1.00  0.00            
ATOM      5  S2  LJ2     5      15.780   4.560   9.580  1.00  0.00            
ATOM      6  S2  LJ2     6       5.350  22.490  11.110  1.00  0.00            
ATOM      7  S2  LJ2     7      19.940   3.910  10.840  1.00  0.00            
ATOM      8  S2  LJ2     8      20.380  13.360  15.680  1.00  0.00            
ATOM      9  S2  LJ2     9      18.340   4.200   7.720  1.00  0.00            
ATOM     10  S2  LJ2    10      18.610  16.530   9.910  1.00  0.00            
TER
ENDMDL

Code:

program atom_test

implicit none

 character (LEN=75) ::inputdatafile,outputfile
 real,dimension(100) :: x, y
 integer :: i,n,nframes
character (len=200),dimension(3000):: command

 print *,"Give the datafile name:"
 read *,inputdatafile
outputfile=inputdatafile(1:len(trim(inputdatafile))-4)//"_output.dat"
!write(*,*)outputfile
Open(9, file=inputdatafile, status='old')

call linesFile(inputdatafile,n)
write(*,*)n

do i=1,n
  read(*,*),command(i)
  write (*,*)command(i)

end do
close(9)

end program atom_test
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!! find the number of lines in a file
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
subroutine  linesFile(fileIn,n)
implicit none
 Character(len = 50)   :: fileIn
integer, intent(out) :: n
! locals
integer :: iostatus, unit_read
real    :: dummy


unit_read = 9
open(unit=unit_read, file=fileIn)

n =0
DO
  READ(unit_read, *, IOSTAT=iostatus) dummy
  IF (iostatus < 0) EXIT
    n = n + 1
END DO

 ! WRITE(*, '(i8, a, a)') n, ' bins read from file: ', fileIn
 close(unit_read)

end subroutine  linesFile

Given marked answer works fine. As I am very new in Fortran I have one more question related this. I want to get the elements separately from the ATOM rows in my data file for example: ATOM 1 S2 LJ2 1 17.000 15.030 11.630 1.00 0.00

from here i want to store 1,s2,LJ2,1,17.000,15.030 each in different parameters. In that case I am using this 

/* With the code marked as correct

    ncount=0
 do i = 1, n

       IF (command(i)(1:4).eq.'ATOM') THEN
         ncount=ncount+1
         read(read_unit,*) p, p2, p3,p4,xatom(ncount)
         write(*,*),p

       endif
end do

But it is not working.Can you please give me one suggestion about how to read individually from those lines which starts with ATOM? Thanks in advance!

Answer 1

There are a few things I notice straight away:

1. The line read(*, *) command(i) reads the first element of standard input, not from the file. I think you want to read(9, *).
2. Actually, you'll probably want to read(9, '(A)') because you want to read the whole line, not just until the first element separator (i.e. space or comma).

3. In your subroutine, you open the file again, under the same unit. That is... dangerous if not wrong. Better to just read from the file, then use the rewind command to move the read position back to the beginning of the file.

   program atom_test
   
       implicit none
       integer :: ios
       integer, parameter :: read_unit = 99
       character(len=200), allocatable :: command(:)
       character(len=200) :: line
       integer :: n, i
   
       open(unit=read_unit, file='data.dat', iostat=ios)
       if ( ios /= 0 ) stop "Error opening file data.dat"
   
       n = 0
   
       do
           read(read_unit, '(A)', iostat=ios) line
           if (ios /= 0) exit
           n = n + 1
       end do
   
       print*, "File contains ", n, "commands"
   
       allocate(command(n))
   
       rewind(read_unit)
   
       do i = 1, n
           read(read_unit, '(A)') command(i)
       end do
   
       close(read_unit)
   
       do i = 1, n
           print*, command(i)
       end do
   
   end program atom_test
   

4. If you already dedicate 3000 lines in the beginning, there's no real reason to read everything twice:

   program atom_test
   
       implicit none
       integer :: ios
       integer, parameter :: read_unit = 99
       character(len=200) :: command(3000)
       integer :: n, i
   
       open(unit=read_unit, file='data.dat', iostat=ios)
       if ( ios /= 0 ) stop "Error opening file data.dat"
   
       n = 0
   
       do
           read(read_unit, '(A)', iostat=ios) command(n+1)
           if (ios /= 0) exit
           n = n + 1
       end do
   
       print*, "File contains ", n, "commands"
   
       close(read_unit)
   
       do i = 1, n
           print*, command(i)
       end do
   
   end program atom_test
   

5. In your subroutine to read the number of lines in the file, you try to read a REAL from a file where the first word is often not a number. This might lead to IOSTAT being non-zero, even if you haven't reached the end of the file. Always read a character variable.