[Numpy-discussion] Creating a 1-d structured array

[josef.pktd at gmail.com]

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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