[Tutor] reading binary files

[bob gailer]

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