I am using the astype method of numpy's array object to convert an array from character to integer. For the sake of efficiency I am using copy=False, but I noticed that the original array is actually not modified. How is that possible without declaring b as a new array ?
import numpy
a=numpy.array(['0','1'])
b=a.astype(numpy.int32,copy=False)
print a[0], b[0]
There are two obvious problems here. First, the
numpy.array(a)
in
b=numpy.array(a).astype(numpy.int32,copy=False)
already makes a copy. Second, from the docs:
If [the copy parameter] is set to false, and the dtype, order, and subok requirements are satisfied, the input array is returned instead of a copy.
NumPy is simply ignoring copy=False, since the dtype mismatch means it needs to copy.
From the docs, a copy is only avoided if,
copy : bool, optional
By default, astype always returns a newly allocated array. If this is set to false, and the dtype, order, and subok requirements are satisfied, the input array is returned instead of a copy.
Because you have a character array and it needs to convert to an int32, I assume this violates the dtype requirements.
I found a convoluted way to do an in-place type conversion
https://stackoverflow.com/a/4396247/901925
In that example the conversion was from 'int32' to 'float32'. Adapted to our case:
Initial x with a large enough string size:
In [128]: x=np.array(['0','1'],dtype='S4')
In [129]: x.__array_interface__['data']
Out[129]: (173756800, False) # data buffer location
now make a view, and copy values from x to the view:
In [130]: y=x.view(int)
In [131]: y[:]=x
Same data buffer location (same for y)
In [132]: x.__array_interface__['data']
Out[132]: (173756800, False)
Now y is two ints:
In [133]: y
Out[133]: array([0, 1])
x, which is still S4, looks at these bytes in a different way:
In [134]: x
Out[134]:
array([b'', b'\x01'],
dtype='|S4')
So it is possible to perform data type conversions in-place, if byte sizes match, but it is an advanced operation. Both the person asking that question and the one answering it are numpy experts.
And astype,copy=False is mentioned in another answer, but fails for the same reason as here - it can't perform the conversion without changing the original array.
You should read the docs:
If copy is set to false, and the dtype, order, and subok requirements are satisfied, the input array is returned instead of a copy.
So your dtype requirements are not satisified. The array must be copied
a.nbytesandb.nbytes. One is 2, the other 8. You, and numpy, can't squeeze 2 4byte integers into 2 1byte character slots.