[Numpy-discussion] List with numpy semantics

josef.pktd@gmai... josef.pktd@gmai...
Tue Nov 2 21:40:24 CDT 2010

On Tue, Nov 2, 2010 at 10:31 PM,  <josef.pktd@gmail.com> wrote:
> On Tue, Nov 2, 2010 at 10:21 PM,  <josef.pktd@gmail.com> wrote:
>> On Tue, Nov 2, 2010 at 10:02 PM, Nikolaus Rath <Nikolaus@rath.org> wrote:
>>> Gerrit Holl <gerrit.holl@gmail.com> writes:
>>>> On 31 October 2010 17:10, Nikolaus Rath <Nikolaus@rath.org> wrote:
>>>>> Hello,
>>>>> I have a couple of numpy arrays which belong together. Unfortunately
>>>>> they have different dimensions, so I can't bundle them into a higher
>>>>> dimensional array.
>>>>> My solution was to put them into a Python list instead. But
>>>>> unfortunately this makes it impossible to use any ufuncs.
>>>>> Has someone else encountered a similar problem and found a nice
>>>>> solution? Something like a numpy list maybe?
>>>> You could try a record array with a clever dtype, maybe?
>>> It seems that this requires more cleverness than I have... Could you
>>> give me an example? How do I replace l in the following code with a
>>> record array?
>>> l = list()
>>> l.append(np.arange(3))
>>> l.append(np.arange(42))
>>> l.append(np.arange(9))
>>> for i in range(len(l)):
>>>   l[i] += 32
>> Depending on how you want to use it, it might be more convenient to
>> use masked arrays or fill with nan (like pandas and larry) to get a
>> rectangular array. it might be more convenient for some things, but if
>> the sizes differ a lot then it might not be more efficient.
> another option I sometimes use (e.g. for unbalanced panel data), is to
> just stack them on top of each other into one long 1d array, and keep
> track which is which, e.g. keeping the (start-end) indices or using an
> indicator array. For example, with an integer label array np.bincount
> is very fast to work with it.
> This is mainly an advantage if there are many short arrays and many
> operations have to applied to all of them.

And the third option, also often used with panel data, is to stack
them sparse, like an unbalanced kronecker product, on top but next to
each other and fill the empty space with zeros.

(typing is faster than thinking)

> Josef
>> Josef
>>> Thanks,
>>>   -Nikolaus
>>> --
>>>  »Time flies like an arrow, fruit flies like a Banana.«
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