File I/O
In many practical applications, program input and output are performed by read- ing from or writing to files. Before a file can be accessed from a Fortran program, it must be opened. The following example shows how a plain text file named out.txt may firstly be opened for writing and subsequently opened for reading:
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program fileIO implicit none integer :: istat real :: a,b a=3.142 ! Open a file called out.txt for writing. Its unit number is 101 open(101, file="out.txt", status="unknown", action="write", form="formatted", IOSTAT=istat) ! Check the file was opened successfully: if (istat .NE. 0) then write (*,*) "Error opening file" exit endif write (101, '(f0.3)') a ! Once finished with the file it should be closed: close(101) ! Open the file for reading - a different unit number (102) is used, ! but this is not necessary since we have closed the file ! associated with unit 101 open(102, file="out.txt", status="old", action="read", form="formatted", IOSTAT=istat) ! Check the file was opened successfully: if (istat .NE. 0) then write (*,*) "Error opening file" exit endif ! read the value from file and assign the value to b ! note that a total field width (5 in this case) must be ! supplied to the read function: read (102, '(f5.3)') b write (*,'(f5.3)') b close(102) end program fileIO
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Program Output
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3.142
