How do I represent and work with n-bit vectors in Python?
In an assignment I am currently working on we need to work with bit vectors, but I am very unsure of how to do this in Python. They should be able to be from 4 bits to 20 bits. I have never worked with bit vector before, but I guess that one would one create arrays of unsigned bytes that you manipulated using the usual AND/OR/XOR operations.
The important restriction here is: I cannot rely on any libraries other than those supplied with standard Python.
I think I know how I would do this in C using arrays of 8 bit unsigned bytes: e.g. to turn the 18th bit of a zeroed array into a one, I would do something like my_bit_array[3] &= 1<<2
But since Python is dynamically typed and does not have a built-in array type, how would I go about doing this in a pythonic way?
And is it possible (how?) to express a bit vector of size 20? I am thinking of perhaps making a 24开发者_开发百科 bit / 3 byte vector and ignoring the 4 bits.
I'm surprised that no one has mentioned int
s (or I guess long
in Python 2). int
s can be arbitrarily large, you can use bitwise operators on them, they're fast, and the code looks like bit twiddling code in C (I consider that to be an advantage).
x = 0 # empty
x |= 1<<19 # set bit 19
x &= ~(1<<19) # clear bit 19
x ^= 1<<19 # toggle bit 19
x = ~x # invert *all* bits, all the way to infinity
mask = ((1<<20)-1) # define a 20 bit wide mask
x &= mask # ensure bits 20 and higher are 0
x ^= mask # invert only bits 0 through 19
(x >> 19) & 1 # test bit 19
(x >> 16) & 0xf # get bits 16 through 20.
I've used this for bitvectors hundreds of bits long.
The library BitVector is a pure-Python library for this purpose, and should suit the needs you specified.
It has lists, which you can populate with bools:
[False] * 20
The bitarray module does this efficiently with booleans.
Use struct module.
A bit dated but I'm going to leave another stdlib option here just for comparison sake. It is also easy to do this using the ctypes module.
For example:
And is it possible (how?) to express a bit vector of size 20 ? I am thinking of perhaps making a 24 bit / 3 byte vector and ignoring the 4 bits.
class Simple(ctypes.LittleEndianStructure):
_pack_ = 1
_fields_ = [
('one', ctypes.c_ubyte, 8),
('two', ctypes.c_ubyte, 8),
('three', ctypes.c_ubyte, 8)
]
s = Simple(0, 2, 256)
bytearray(s) # bytearray(b'\x00\x02\x00')
s = Simple(0, 2, 255)
bytearray(s) # bytearray(b'\x00\x02\xff')
class Simple(ctypes.BigEndianStructure):
_pack_ = 1
_fields_ = [
('one', ctypes.c_ubyte, 8),
('two', ctypes.c_ubyte, 8),
('three', ctypes.c_ubyte, 8)
]
s = Simple(0, 2, 256)
bytearray(s) # bytearray(b'\x00\x02\x00')
s = Simple(0, 2, 255)
bytearray(s) # bytearray(b'\x00\x02\xff')
s.two |= 3
bytearray(s) # bytearray(b'\x00\x03\xff')
or something more straight forward like this:
class bit_vector(Structure):
_fields_ = [('bits', c_uint32, 24),
('unused', c_uint32, 8),
]
bv = bit_vector()
# turn on the 18th bit -- being explicit just to demo it
bv.bits |= int('000000000000000001000000', 2)
bin(bv.bits) # 0b1000000
There is also the pure-Python python-bitstring (with Python 3 support).
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