Well that's just the point, I wanted to consider group size > 8. On Wed, Jul 21, 2021 at 8:53 AM Andras Deak <deak.and...@gmail.com> wrote: > > On Wed, Jul 21, 2021 at 2:40 PM Neal Becker <ndbeck...@gmail.com> wrote: >> >> In my application I need to pack bits of a specified group size into >> integral values. >> Currently np.packbits only packs into full bytes. >> For example, I might have a string of bits encoded as a np.uint8 >> vector with each uint8 item specifying a single bit 1/0. I want to >> encode them 4 bits at a time into a np.uint32 vector. >> >> python code to implement this: >> >> --------------- >> def pack_bits (inp, bits_per_word, dir=1, dtype=np.int32): >> assert bits_per_word <= np.dtype(dtype).itemsize * 8 >> assert len(inp) % bits_per_word == 0 >> out = np.empty (len (inp)//bits_per_word, dtype=dtype) >> i = 0 >> o = 0 >> while i < len(inp): >> ret = 0 >> for b in range (bits_per_word): >> if dir > 0: >> ret |= inp[i] << b >> else: >> ret |= inp[i] << (bits_per_word - b - 1) >> i += 1 >> out[o] = ret >> o += 1 >> return out >> --------------- > > > Can't you just `packbits` into a uint8 array and then convert that to uint32? > If I change `dtype` in your code from `np.int32` to `np.uint32` (as you > mentioned in your email) I can do this: > > rng = np.random.default_rng() > arr = (rng.uniform(size=32) < 0.5).astype(np.uint8) > group_size = 4 > original = pack_bits(arr, group_size, dtype=np.uint32) > new = np.packbits(arr.reshape(-1, group_size), axis=-1, > bitorder='little').ravel().astype(np.uint32) > print(np.array_equal(new, original)) > # True > > There could be edge cases where the result dtype is too small, but I haven't > thought about that part of the problem. I assume this would work as long as > `group_size <= 8`. > > András > >> >> It looks like unpackbits has a "count" parameter but packbits does not. >> Also would be good to be able to specify an output dtype. >> _______________________________________________ >> NumPy-Discussion mailing list >> NumPy-Discussion@python.org >> https://mail.python.org/mailman/listinfo/numpy-discussion > > _______________________________________________ > NumPy-Discussion mailing list > NumPy-Discussion@python.org > https://mail.python.org/mailman/listinfo/numpy-discussion
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