Exception handling when the file does not exist

Question

I have a Fortran program that starts with opening and reading data from a .txt file. At the end of the program a new file is written, which replaces the old file (that was originally imported).

However it can occur that the file that needs to be opened does not exists, for that case the variables that should be imported from the .txt file should be 0.

I thought by doing this with the code below, however this does not work and the script is aborted when the file history.txt does not exists.

How can I let the script set default values to my variables when the history.txt file does not exists?

  OPEN(UNIT=in_his,FILE="C:\temp\history.txt",ACTION="read")
  if (stat .ne. 0) then    !In case history.txt cannot be opened (iteration 1)
    write(*,*) "history.txt cannot be opened"
    KAPPAI=0
    KAPPASH=0
    go to 99
  end if
  read (in_his, *) a, b
  KAPPAI=a
  KAPPASH=b
  write (*, *) "KAPPAI=", a, "KAPPASH=", b
  99   close(in_his)  

The file that is imported is pretty simple and looks like:

  9.900000000000006E-003  3.960000000000003E-003

Answer 1

I would use IOSTAT as stated by @Fortranner. I would also set defaults before trying to open the file and I tend not to use goto's. As in:

program test

    implicit none
    integer :: in_his, stat
    real :: KAPPAI, KAPPASH

    in_his  = 7
    KAPPAI  = 0
    KAPPASH = 0

    OPEN(UNIT=in_his, FILE="history.txt",ACTION='read',IOSTAT=stat,STATUS='OLD')
    if (stat .ne. 0) then
            write(*,*) "history.txt cannot be opened"
            stop 1
    end if

    read (in_his, *) KAPPAI, KAPPASH
    close(in_his)

    write (*, *) "KAPPAI=", KAPPAI, "KAPPASH=", KAPPASH

end program test

Answer 2

Another way is to use an inquire statement and check for the existence of the file before you try to open it. This would set a logical variable that could be used in an IF statement to handle the two cases: 1) open file and read values, or 2) set default values w/o opening the file. Or set the default values first, then have the IF statement only handle the case of opening the file and reading the values.