Extracting 3-byte integers

[Scott David Daniels]

Bob Greschke wrote:
> I have some binary data read from a file that is arranged like
>    <3-byte int> <3-byte int> <3-byte int> etc.
> The "ints" are big-endian and there are 169 of them.  Is there any clever 
> way to convert these to regular Python ints other than (struct) unpack'ing 
> them one at a time and doing the math?

Best way is with scipy (or Numeric or numarray), but for vanilla Python:

     import array, itertools
     ubytes = array.array('B')
     sbytes = array.array('b')
     ubytes.fromfile(binarysource, 169 * 3)
     sbytes.fromstring(ubytes[::3].tostring())

     result = array.array('i', (msb * 256 + mid) * 256 + lsb
                                for msb, mid, lsb
                                in itertools.izip(sbytes,
                                      ubytes[1::3], ubytes[2::3])))

If you want to get fancy, you can replace the last statement with:

     del ubytes[::3]
     uhalf = array.array('H')
     uhalf.fromstring(ubytes.tostring())
     if array.array('H', [1]) != '\x00\x01': uhalf.byteswap()
     result = array.array('i', (msb * 65536 + low for msb, low
                                in itertools.izip(sbytes, uhalf)))


That was fun.

--Scott David Daniels
scott.daniels at acm.org