[Numpy-discussion] Creating a 1-d structured array
Bruce Southey
bsouthey at gmail.com
Fri May 13 17:03:59 EDT 2011
On 05/13/2011 03:04 PM, josef.pktd at gmail.com wrote:
> On Fri, May 13, 2011 at 3:11 PM, Derek Homeier
> <derek at astro.physik.uni-goettingen.de> wrote:
>> Hi,
>>
>> just a comment since I first thought the solution below might not be
>> what Bruce
>> was looking for, but having realised it's probably what he's been
>> asking for...
>>
>> On 13 May 2011, at 17:20, josef.pktd at gmail.com wrote:
>>
>>> On Fri, May 13, 2011 at 10:58 AM, Bruce Southey<bsouthey at gmail.com>
>>> wrote:
>>>> Hi,
>>>> How do you create a 'single' structured array using np.array()?
>>>> Basically I am attempting to do something like this that does not
>>>> work:
>>>> a=np.array([1,2, 3,4, 5,6], dtype=np.dtype([('foo', int)]))
>>>>
>>>> I realize that this is essentially redundant as if A is an 1-d array
>>>> then a structured array with a named field 'foo' is the same thing
>>>> - A
>>>> would be A['foo'], just shorter.
>> ...
>>> Using a view works, (and direct assignment of dtype)
>>>
>>>>>> a=np.array([1,2, 3,4, 5,6]).view(([('foo', int)]))
>>>>>> a
>>> array([(1,), (2,), (3,), (4,), (5,), (6,)],
>>> dtype=[('foo', '<i4')])
>>>>>> b = a.copy()
>>>>>> b
>>> array([(1,), (2,), (3,), (4,), (5,), (6,)],
>>> dtype=[('foo', '<i4')])
>>>
>>>
>>>>>> a1 = np.array([1,2, 3,4, 5,6]).astype([('foo', int)])
>>> Traceback (most recent call last):
>>> File "<pyshell#36>", line 1, in<module>
>>> a1 = np.array([1,2, 3,4, 5,6]).astype([('foo', int)])
>>> TypeError: expected a readable buffer object
>>>
>>>>>> a1 = np.array([1,2, 3,4, 5,6])
>>>>>> a1.dtype
>>> dtype('int32')
>>>>>> a1.dtype = np.dtype([('foo', int)])
>>>>>> a1
>>> array([(1,), (2,), (3,), (4,), (5,), (6,)],
>>> dtype=[('foo', '<i4')])
>> This is a 1-d structured array, yet a is in fact not the same as
>> a['foo']:
>>
>> >>> a['foo']
>> array([ 1, 2, 3, 4, 5, 6])
>> >>> a['foo'].shape
>> (6,)
>> >>> a.shape
>> (6,)
>> >>> a['foo']+np.arange(6)
>> array([ 1, 3, 5, 7, 9, 11])
>> >>> a+np.arange(6)
>> Traceback (most recent call last):
>> File "<stdin>", line 1, in<module>
>> TypeError: unsupported operand type(s) for +: 'numpy.ndarray' and
>> 'numpy.ndarray'
> funny things with subclasses, I saw this also in an error message in
> another package with a subclass of ndarray,
> should read: 'this ndarray-(sub)class' and 'that ndarray-(sub)class'
>
> for some messages it's just getting used to the "coding"
>
> "TypeError: expected a readable buffer object" translates into "I
> cannot interpret the data as the desired array"
> e.g. list of list or list of tuples
>
>
>> >>> a[0]+1
>> Traceback (most recent call last):
>> File "<stdin>", line 1, in<module>
>> TypeError: unsupported operand type(s) for +: 'numpy.void' and 'int'
>>
>> Thus I am wondering why broadcasting should not be possible in this
>> case,
> Even a 1 column table is still a table (or a list of records), and a 1
> item row is still a row.
>
> Josef
>
>> and if it really isn't, the first error message certainly is not very
>> helpful... (maybe
>> inevitably though, since type() of a structured array is the same as
>> for a "plain"
>> array (unlike for a record array).
>>
>> Cheers,
>> Derek
>>
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Thanks for the replies, especially the dtype example Josef!
So this should be reported as a bug?
Bruce
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