Casting array of bytes in numpy to int32-CodePudding

I have a numpy array A of two bytes which I want to combine into an int32. How do I do casting which combines array elements to create different datatype?

CodePudding user response：

Arrays can be 'viewed' with different dtypes, provided the byte counts are correct.

In [265]: arr = np.array([1,11], 'ubyte')
In [266]: arr
Out[266]: array([ 1, 11], dtype=uint8)
In [267]: arr.view('i2')
Out[267]: array([2817], dtype=int16)
In [268]: arr.view('>i2')                # with different endedness
Out[268]: array([267], dtype=int16)
In [269]: arr.view('uint16')
Out[269]: array([2817], dtype=uint16)

int32 requires 4 bytes:

In [270]: arr.view('int32')
Traceback (most recent call last):
  File "<ipython-input-270-4ab2a022f898>", line 1, in <module>
    arr.view('int32')
ValueError: When changing to a larger dtype, its size must be a divisor of the total size in bytes of the last axis of the array.

Similarly if the bytes were obtained from a bytestring

In [271]: astr = arr.tobytes()
In [272]: astr
Out[272]: b'\x01\x0b'
In [273]: np.frombuffer(astr, 'uint8')
Out[273]: array([ 1, 11], dtype=uint8)
In [274]: np.frombuffer(astr, 'uint16')
Out[274]: array([2817], dtype=uint16)
In [275]: np.frombuffer(astr, '<i2')
Out[275]: array([2817], dtype=int16)
In [276]: np.frombuffer(astr, '>i2')
Out[276]: array([267], dtype=int16)

view with a compound dtype

In [279]: arr.view('b')
Out[279]: array([ 1, 11], dtype=int8)
In [280]: arr.view('b,b')
Out[280]: array([(1, 11)], dtype=[('f0', 'i1'), ('f1', 'i1')])

Shape may need adjustment after such a view.