[Numpy-discussion] Empty strings not empty?

David Cournapeau cournape@gmail....
Tue Dec 29 20:36:25 CST 2009


On Wed, Dec 30, 2009 at 10:20 AM, Matthew Brett <matthew.brett@gmail.com> wrote:
> Hi,
>
>> x = "¥00"
>> arr = np.array([x])
>> lst = [x]
>>
>> arr[0] == x # False
>> arr[0] == "" # True
>>
>> lst[0] == x # True
>> lst[0] == "" # False
>
> Ah - thanks - got it.
>
>>> It looks to me as though the
>>> buffer contents of [''] is a length 1 string with a 0 byte, and an
>>> array.size of 1 - is that also what you think?  I guess I think that
>>> it should be a length 0 string, with a array.size of 0
>>
>> Array size of 0 would be very weird: it means it would have no items,
>> whereas it actually has one item (which itself has a size 0).
>
> Is this a string-specific thing?

No. I was not very clear: My point was that size 0 array is likely not
what you want. What you want is arrays whose *itemsize" is 0.

>  I mean, you can have size 0 1d
> numeric arrays.  Sorry if I'm being slow, it's late here.
>
> In [70]: np.array([[]]).shape
> Out[70]: (1, 0)
>
> In [71]: np.array([[]]).size
> Out[71]: 0

Yes, you can create array with size 0. But I don't think that's what
you want - you cannot index them normally (even though the array is
2d, you cannot do arr[0][0], so I don't think that's very useful for
your case).

David


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