eShopping wrote:
Bob
I am trying to read UNFORMATTED files. The files also occur as
formatted files and the format string I provided is the string used to
write the formatted version. I can read the formatted version OK. I
(naively) assumed that the same format string was used for both files,
the only differences being whether the FORTRAN WRITE statement
indicated unformatted or formatted.
WRITE UNFORMATTED dump memory to disk with no formatting. That is why we
must do some analysis of the file to see where the data has been placed,
how long the floats are, and what "endian" is being used.
I'd like to examine the file myself. We might save a lot of time and
energy that way. If it is not very large would you attach it to your
reply. If it is very large you could either copy just the first 1000 or
so bytes, or send the whole thing thru www.yousendit.com.
At 21:41 03/02/2009, bob gailer wrote:
First question: are you trying to work with the file written
UNFORMATTED? If so read on.
Well, did you read on? What reactions do you have?
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
--
Bob Gailer
Chapel Hill NC
919-636-4239
_______________________________________________
Tutor maillist - Tutor@python.org
http://mail.python.org/mailman/listinfo/tutor
