Research Note #25 - Reading a Text File Line-by-Line with Fortran

Again, this is actually a very simple task which I usually deal with ... and forget, most of the time! That's why this post exists. The following is a basic Fortran code to enable you reading the contents of a text file, line-by-line. 

As an example, let's make a text file (named 'list.txt') containing a list of files with their complete paths on Linux:

$ ls ../gcom-c/aerosol/0524/pl/* > list.txt

$ cat list.txt

../gcom-c/aerosol/0524/pl/GC1SG1_20190901D01D_T0524_L2SG_ARPLK_1001.h5

../gcom-c/aerosol/0524/pl/GC1SG1_20190902D01D_T0524_L2SG_ARPLK_1001.h5

../gcom-c/aerosol/0524/pl/GC1SG1_20190903D01D_T0524_L2SG_ARPLK_1001.h5

../gcom-c/aerosol/0524/pl/GC1SG1_20190904D01D_T0524_L2SG_ARPLK_1001.h5

Now, this is the Fortran code to read file 'list.txt':

PROGRAM READTEXT

IMPLICIT NONE

CHARACTER(LEN=8) :: listfile

CHARACTER(LEN=70) :: filecontents

INTEGER :: io

listfile="list.txt"

OPEN(10, FILE=listfile, FORM="FORMATTED", STATUS="OLD",&

        ACTION="READ")

DO

   READ(10,"(A70)",IOSTAT=io) filecontents

   IF (io/=0) EXIT

   PRINT *, filecontents

END DO

CLOSE(10)

END PROGRAM READTEXT

How it works?

The program will open file 'list.txt', then read the first line of the file and put the (string) contents into variable 'filecontents'. It will then print the contents of variable onto the screen. Since there's a DO.. END DO statement, this process will repeat, with the next line being read, replacing the contents of variable 'filecontents'. This looping process will end once there are no more lines found in the file, invoking IOSTAT to a value other than 0 (0 means no I/O error occurred, other values mean there are I/O errors), then program will exit. That's all!

Some important notices:

• The absence of 'DO ... END DO' will make the program only reads the first line of the file.
• The absence of 'IF (io/=0) EXIT' will make the program runs endlessly, with the last line of the file kept being showed on the screen.
• If the output format is not determined (for example, FMT=* instead of A70), the program will not correctly show the file's contents. The (character) length of this format should be same or more than the one at variable declaration. Why 70? Because in this case, the full path of file list (in one line) has 70 characters.