First question: are you trying to work with the file written
UNFORMATTED? If so read on.
If you are working with a file formatted (1X, 1X, A8, 1X, 1X, I6, 1X,
1X, A1) then we have a completely different issue to deal with. Do not
read on, instead let us know.
eShopping wrote:
Data format:
TIME 1 F 0.0
DISTANCE 10 F 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0
F=float, D=double, L=logical, S=string etc
The first part of the file should contain a string (eg "TIME"),
an integer (1) and another string (eg "F") so I tried using
import struct
in_file = open(file_name+".dat","rb")
data = in_file.read()
items = struct.unpack('sds', data)
Now I get the error
error: unpack requires a string argument of length 17
which has left me completely baffled!
Did you open the file with mode 'b'? If not change that.
You are passing the entire file to unpack when you should be giving
it only the first "line". That's why is is complaining about the
length. We need to figure out the lengths of the lines.
Consider the first "line"
TIME 1 F 0.0
There were (I assume) 4 FORTRAN variables written here: character
integer character float. Without knowing the lengths of the character
variables we are at a loss as to what the struct format should be. Do
you know their lengths? Is the last float or double?
Try this: print data[:40] You should see something like:
TIME...\x01\x00\x00\x00...F...\x00\x00\x00\x00...DISTANCE...\n\x00\x00\x00
where ... means 0 or more intervening stuff. It might be that the
\x01 and the \n are in other places, as we also have to deal with
"byte order" issues.
Please do this and report back your results. And also the FORTRAN
variable types if you have access to them.
Apologies if this is getting a bit messy but the files are at a remote
location and I forgot to bring copies home. I don't have access to
the original FORTRAN program so I tried to emulate the reading the
data using the Python script below. AFAIK the FORTRAN format line for
the header is (1X, 1X, A8, 1X, 1X, I6, 1X, 1X, A1). If the data
following is a float it is written using n(1X, F6.2) where n is the
number of records picked up from the preceding header.
# test program to read binary data
import struct
# create dummy data
data = []
for i in range(0,10):
data.append(float(i))
# write data to binary file
b_file = open("test.bin","wb")
b_file.write(" %8s %6d %1s\n" % ("DISTANCE", len(data), "F"))
for x in data:
b_file.write(" %6.2f" % x)
You are still confusing text vs binary. The above writes text regardless
of the file mode. If the FORTRAN file was written UNFORMATTED then you
are NOT emulating that with the above program. The character data is
read back in just fine, since there is no translation involved in the
writing nor in the reading. The integer len(data) is being written as
its text (character) representation (translating binary to text) but
being read back in without translation. Also all the floating point data
is going out as text.
The file looks like (where b = blank) (how it would look in notepad):
bbDISTANCEbbbbbb10bFbbb0.00bbb1.00bbb2.00 If you analyze this with
2s8s2si2s1s
you will see 2s matches bb, 8s matches DISTANCE, 2s matches bb, i
matches bbbb. (\x40\x40\x40\x40). The i tells unpack to shove those 4
bytes unaltered into a Python integer, resulting in 538976288. You can
verify that:
>>> struct.unpack('i', ' ')
(538976288,)
Please either assure me you understand or are prepared for a more in
depth tutorial.
b_file.close()
# read back data from file
c_file = open("test.bin","rb")
data = c_file.read()
start, stop = 0, struct.calcsize("2s8s2si2s1s")
items = struct.unpack("2s8s2si2s1s",data[start:stop])
print items
print data[:40]
I'm pretty sure that when I tried this at the other PC there were a
bunch of \x00\x00 characters in the file but they don't appear in
NotePad ... anyway, I thought the Python above would unpack the data
but items appears as
(' ', 'DISTANCE', ' ', 538976288, '10', ' ')
which seems to be contain an extra item (538976288)
Alun Griffiths
--
Bob Gailer
Chapel Hill NC
919-636-4239
_______________________________________________
Tutor maillist - Tutor@python.org
http://mail.python.org/mailman/listinfo/tutor
