File I/O |
So far we have assumed that the
input/output has been to the standard input or the standard output.
A D V E R T I S E M E N T
It is also
possible to read or write from files which are stored on some external
storage device, typically a disk (hard disk, floppy) or a tape. In Fortran each
file is associated with a unit number, an integer between 1 and 99. Some
unit numbers are reserved: 5 is standard input, 6 is standard output.
Opening and closing a file
Before you can use a file you
have to open it. The command is
open
(list-of-specifiers)
where the most common specifiers
are:
[UNIT=]
u
IOSTAT=
ios
ERR=
err
FILE= fname
STATUS= sta
ACCESS= acc
FORM= frm
RECL= rl
The unit number u is a
number in the range 9-99 that denotes this file (the programmer may chose any
number but he/she has to make sure it is unique).
ios is the I/O status identifier and should be an
integer variable. Upon return, ios is zero if the stement was successful
and returns a non-zero value otherwise.
err is a label which the program will jump to if
there is an error.
fname is a character string denoting the file
name.
sta is a character string that has to be either
NEW, OLD or SCRATCH. It shows the prior status of the file. A scrath file is a
file that is created and deleted when the file is closed (or the program ends).
acc must be either SEQUENTIAL or DIRECT. The
default is SEQUENTIAL.
frm must be either FORMATTED or UNFORMATTED. The
default is UNFORMATTED.
rl specifies the length of each record in a
direct-acccess file.
For more details on these specifiers, see a good Fortran
77 book.
After a file has been opened, you can access it by read and
write statements. When you are done with the file, it should be closed by the
statement
close ([UNIT=]u[,IOSTAT=ios,ERR=err,STATUS=sta])
where, as usual, the parameters in brackets are optional.
Read and write revisited
The only necessary change from our previous simplified
read/write statements, is that the unit number must be specified. But frequently
one wants to add more specifiers. Here is how:
read ([UNIT=]u, [FMT=]fmt, IOSTAT=ios,
ERR=err, END=s)
write([UNIT=]u, [FMT=]fmt, IOSTAT=ios,
ERR=err, END=s)
where most of the specifiers have been described above. The
END=s specifier defines which statement label the program jumps to if it reaches
end-of-file.
Example
You are given a data file with xyz coordinates for a bunch
of points. The number of points is given on the first line. The file name of the
data file is points.dat. The format for each coordinate is known to be
F10.4. Here is a short program that reads the data into 3 arrays x,y,z:
program inpdat
c
c This program reads n points from a data file and
stores them in
c 3 arrays x, y, z.
c
integer nmax, u
parameter (nmax=1000, u=20)
real x(nmax), y(nmax),
z(nmax)
c Open the data file
open (u, FILE='points.dat', STATUS='OLD')
c Read the number of points
read(u,*) n
if (n.GT.nmax) then
write(*,*) 'Error: n = ', n, 'is larger than
nmax =', nmax
goto 9999
endif
c Loop over the data points
do 10 i= 1, n
read(u,100) x(i), y(i), z(i)
10 enddo
100 format (3(F10.4))
c Close the file
close (u)
c Now we should process the data somehow...
c (missing part)
9999 stop
end
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