在Python中将32位整数转换为4个8位整数数组

Question

如何在Python中有效地将32位整数转换为4个8位整数数组？

目前我有以下代码，这是超慢的：

def convert(int32_val):
    bin = np.binary_repr(int32_val, width = 32) 
    int8_arr = [int(bin[0:8],2), int(bin[8:16],2), 
                int(bin[16:24],2), int(bin[24:32],2)]
    return int8_arr  

例如：

print convert(1)
>>> [0, 0, 0, 1]   

print convert(-1)
>>> [255, 255, 255, 255]

print convert(-1306918380)  
>>> [178, 26, 2, 20]

我需要在无符号32位整数上实现相同的行为。

另外。对于32位整数的大型numpy数组，是否可以将其矢量化？

Answer 1

在我的测试中，仅仅使用python内置的division和module就可以提供6倍的加速。

def convert(i):
    i = i % 4294967296
    n4 = i % 256
    i = i / 256
    n3 = i % 256
    i = i / 256
    n2 = i % 256
    n1 = i / 256
    return (n1,n2,n3,n4)

Answer 2

在Python 3.2及更高版本中，有一个新的int方法^{}也可以使用：

>>> convert = lambda n : [int(i) for i in n.to_bytes(4, byteorder='big', signed=True)]
>>>
>>> convert(1)
[0, 0, 0, 1]
>>>
>>> convert(-1)
[255, 255, 255, 255]
>>>
>>> convert(-1306918380)
[178, 26, 2, 20]
>>>

Answer 3

使用dtype如中所述： http://docs.scipy.org/doc/numpy/reference/generated/numpy.dtype.html

Subdivide int16 into 2 int8‘s, called x and y. 0 and 1 are the offsets in bytes:

np.dtype((np.int16, {'x':(np.int8,0), 'y':(np.int8,1)}))
dtype(('<i2', [('x', '|i1'), ('y', '|i1')]))

或者根据你的情况改编：

In [30]: x=np.arange(12,dtype=np.int32)*1000
In [39]: dt=np.dtype((np.int32, {'f0':(np.uint8,0),'f1':(np.uint8,1),'f2':(np.uint8,2), 'f3':(np.uint8,3)}))

In [40]: x1=x.view(dtype=dt)

In [41]: x1['f0']
Out[41]: array([  0, 232, 208, 184, 160, 136, 112,  88,  64,  40,  16, 248], dtype=uint8)

In [42]: x1['f1']
Out[42]: array([ 0,  3,  7, 11, 15, 19, 23, 27, 31, 35, 39, 42], dtype=uint8)

比较

In [38]: x%256
Out[38]: array([  0, 232, 208, 184, 160, 136, 112,  88,  64,  40,  16, 248])

关于http://docs.scipy.org/doc/numpy/user/basics.rec.html的更多文档

2) Tuple argument: The only relevant tuple case that applies to record structures is when a structure is mapped to an existing data type. This is done by pairing in a tuple, the existing data type with a matching dtype definition (using any of the variants being described here). As an example (using a definition using a list, so see 3) for further details):

x = np.zeros(3, dtype=('i4',[('r','u1'), ('g','u1'), ('b','u1'), ('a','u1')]))

array([0, 0, 0])

x['r'] # array([0, 0, 0], dtype=uint8)

In this case, an array is produced that looks and acts like a simple int32 array, but also has definitions for fields that use only one byte of the int32 (a bit like Fortran equivalencing).

获取4字节的二维数组的一种方法是：

In [46]: np.array([x1['f0'],x1['f1'],x1['f2'],x1['f3']])
Out[46]: 
array([[  0, 232, 208, 184, 160, 136, 112,  88,  64,  40,  16, 248],
       [  0,   3,   7,  11,  15,  19,  23,  27,  31,  35,  39,  42],
       [  0,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0],
       [  0,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0]], dtype=uint8)

---

同样的想法，但更紧凑：

In [50]: dt1=np.dtype(('i4', [('bytes','u1',4)]))

In [53]: x2=x.view(dtype=dt1)

In [54]: x2.dtype
Out[54]: dtype([('bytes', 'u1', (4,))])

In [55]: x2['bytes']
Out[55]: 
array([[  0,   0,   0,   0],
       [232,   3,   0,   0],
       [208,   7,   0,   0],
       [184,  11,   0,   0],
       [160,  15,   0,   0],
       [136,  19,   0,   0],
       [112,  23,   0,   0],
       [ 88,  27,   0,   0],
       [ 64,  31,   0,   0],
       [ 40,  35,   0,   0],
       [ 16,  39,   0,   0],
       [248,  42,   0,   0]], dtype=uint8)

In [56]: x2
Out[56]: 
array([    0,  1000,  2000,  3000,  4000,  5000,  6000,  7000,  8000,
        9000, 10000, 11000])