getting at individual bits inside byte field: struct module : bitwise operator

[harijay]

Thanks Gabriel , MRAB and Mark

Instead of using the bitwise operator on the unpacked data I was
trying to do a bitwise operator directly after read. This was giving
the unsupported operand error

i.e

# version in my code - wrong
file.read(2) & 0x001f

Instead of

# version in my example and now in my working code
(element1,) = struct.unpack("<H",file.read(2) )
Tonethrufour = element1 & 0x1F

I only realized it after I read your replies and went back to my
actual  code which had the mistake .
Thanks a lot again.

Hari



Thanks for your help and hope I dont commit any more such stupid
mistakes

On Feb 23, 11:27 pm, "Mark Tolonen" <M8R-yft... at mailinator.com> wrote:
> "harijay" <hari... at gmail.com> wrote in message
>
> news:4c7d58a1-830f-4f02-ba07-aa4910f5f4e9 at b16g2000yqb.googlegroups.com...
>
>
>
> > In my last post I had asked about reading data from a binary file
> > using the struct module.
> > Thanks to some excellent help , I have managed to read in
> > successfully
> > most of the header of this binary format that I want to parse.  These
> > are some time-voltage traces from a digital
> > to analog converter for my experiments. The output is according to a
> > format
> > mentioned here : (http://www.dataq.com/support/techinfo/ff.htm)
>
> > I have a question about how to bitmask a bunch of bytes read in from
> > such a binary formatted file .
>
> > For eg the spec says the first two bytes have different parameters in
> > different bits .
> > Byte 1                                                  Byte 0
> > SN16 SD9        SD8     SD7     SD6     SD5     SD4     SD3
> > SD2     SD1     SD0     T4      T3      T2      T1      T0
>
> > I am reading in the two bytes using the following code
>
> > import struct
> > f.seek(0)
> > element1_format = struct.Struct("<H")
> > (element1,) = element1_format.unpack(f.read(2))
>
> > Now element1 has type "str" . How do I apply a bitmask to this to get
> > at information in the component bits .
> > Since the entire file format has many such bitmasked fields and since
> > this is my first venture into binary formats and c-type structs , I
> > wanted to know how to read values inside a byte using python.
> > My few tries at using bitwise operators ( element1 & 0x001f) are
> > frustrated by messages that say " unsupported operand type(s) for &:
> > 'str' and 'int' " .
> > How can I keep my string objects as bits and apply bitmasks to them
> > Any help in this will be greatly appreciated.
> > Thanks
> > hari
>
> Please post a small example that fails as you describe.  From your example,
> element1 should be an int (unpacked from two bytes), and there is no f
> defined.  Without the actual code we can't figure out what you are doing
> wrong.
>
> This works as expected:
>
> >>> import struct
> >>> format = struct.Struct("<H")
> >>> (element1,) = format.unpack('aa') # a two-byte string to unpack
> >>> print hex(element1)
> 0x6161
> >>> print element1 & 0x1f
>
> 1
>
> -Mark