File Input/Output

Instead of commandline input, a program can read input from a file and/or send output to a file.

The OPEN Statement

Basic format for opening a file for reading or writing:

OPEN(UNIT=number, FILE=file-name, STATUS=status)

• UNIT is a numeric value to reference this file (handle)

  • Number is any positive integer (don’t use 5 or 6)
  • used to refer to the file in the read or write statement instead of using the star (*)

  OPEN(UNIT=12,…)
  
  READ(12,*) ivalue
  

• FILE is the filename designated through a relative or absolute path that is to be read/write

  • give the file-name in single quotes
  • or give a character variable assigned to the file-name

OPEN(UNIT=12,FILE=‘/home/kgoebber/GRADES.DAT’,…)

READ(12,*) grades(1), grades(2), grades(3)

As long as all the grades were on the same line in the file

• Status tells us something about the file (and potentially what we can do with the file)
  • a new file has a status of ‘NEW’
    • never before created; can only be used once; don’t use this file status
  • an old file has a status of ‘OLD’
    • when you know a file has been created or exists
  • The most useful is status of ‘UNKNOWN’
    • when you aren’t sure; will create new or overwrite old
  • More important for WRITE statements than read statements

The Close Statement

It is good coding practice to close the file that you are reading from or writing to once the program is done completing the read/write. This has a couple of benefits in that is frees up the unit number to be used elsewhere in the program and additionally frees up computer memory.

close(unit=#)
  or
close(#)

Writing to a file

• Works very similarly to reading from a file
• Must first “OPEN” the file to write to it

OPEN(UNIT=number,FILE=file-name,STATUS=status)

• Here the status is critical
  • Status of ‘Old’ or ‘Unknown’ will overwrite a file, while ‘new’ will not

WRITE(20,*) mean