[Numpy-discussion] use slicing as argument values?

Chris Barker chris.barker@noaa....
Fri Jul 13 11:18:53 CDT 2012


On Thu, Jul 12, 2012 at 2:32 PM, Chao YUE <chaoyuejoy@gmail.com> wrote:

> numpy ndarray indexing in the function. Like when I call:
>
> func(a,'1:3,:,2:4'), it knows I want to retrieve a[1:3,:,2:4], and
> func(a,'1:3,:,4') for a[1:3,:,4] ect.

why do the string packing/unpacking? why not use an interface much
like the slice() and range() functions?

func(a, ( (start, stop, step),(start, stop, step),(sart, stop, step) ))

or, I agree, jsut pass in the sliced array:

func( a[start:stop:step, start:stop:step, start_stop:step] )

Will the rank of a always be 3? Do you ned to support "step" that
could simplify it a bit.

-Chris






> I am very close now.
>
> #so this function changes the string to list of slice objects.
> def convert_string_to_slice(slice_string):
>     """
>     provide slice_string as '2:3,:', it will return [slice(2, 3, None),
> slice(None, None, None)]
>     """
>     slice_list=[]
>     split_slice_string_list=slice_string.split(',')
>     for sub_slice_string in split_slice_string_list:
>         split_sub=sub_slice_string.split(':')
>         if len(split_sub)==1:
>             sub_slice=slice(int(split_sub[0]))
>         else:
>             if split_sub[0]=='':
>                 sub1=None
>             else:
>                 sub1=int(split_sub[0])
>             if split_sub[1]=='':
>                 sub2=None
>             else:
>                 sub2=int(split_sub[1])
>             sub_slice=slice(sub1,sub2)
>         slice_list.append(sub_slice)
>     return slice_list
>
> In [119]: a=np.arange(3*4*5).reshape(3,4,5)
>
> for this it works fine.
> In [120]: convert_string_to_slice('1:3,:,2:4')
> Out[120]: [slice(1, 3, None), slice(None, None, None), slice(2, 4, None)]
>
> In [121]: a[slice(1, 3, None), slice(None, None, None), slice(2, 4,
> None)]==a[1:3,:,2:4]
> Out[121]:
> array([[[ True,  True],
>         [ True,  True],
>         [ True,  True],
>         [ True,  True]],
>
>        [[ True,  True],
>         [ True,  True],
>         [ True,  True],
>         [ True,  True]]], dtype=bool)
>
> And problems happens when I want to retrieve a single number along a given
> dimension:
> because it treats 1:3,:,4 as 1:3,:,:4, as shown below:
>
> In [122]: convert_string_to_slice('1:3,:,4')
> Out[122]: [slice(1, 3, None), slice(None, None, None), slice(None, 4, None)]
>
> In [123]: a[1:3,:,4]
> Out[123]:
> array([[24, 29, 34, 39],
>        [44, 49, 54, 59]])
>
> In [124]: a[slice(1, 3, None), slice(None, None, None), slice(None, 4,
> None)]
> Out[124]:
> array([[[20, 21, 22, 23],
>         [25, 26, 27, 28],
>         [30, 31, 32, 33],
>         [35, 36, 37, 38]],
>
>        [[40, 41, 42, 43],
>         [45, 46, 47, 48],
>         [50, 51, 52, 53],
>         [55, 56, 57, 58]]])
>
>
> Then I have a function:
>
> #this function retrieves data from ndarray a by specifying slice_string:
> def retrieve_data(a,slice_string):
>     slice_list=convert_string_to_slice(slice_string)
>     return a[*slice_list]
>
> In the list line of the fuction "retrieve_data" I have problem, I get an
> invalid syntax error.
>
>     return a[*slice_list]
>              ^
> SyntaxError: invalid syntax
>
> I hope it's not too long, please comment as you like. Thanks a lot!!!!
>
> Chao
>
>
> 2012/7/12 Benjamin Root <ben.root@ou.edu>
>>
>>
>> On Thu, Jul 12, 2012 at 4:46 PM, Chao YUE <chaoyuejoy@gmail.com> wrote:
>>>
>>> Hi Ben,
>>>
>>> it helps a lot. I am nearly finishing a function in a way I think
>>> pythonic.
>>> Just one more question, I have:
>>>
>>> In [24]: b=np.arange(1,11)
>>>
>>> In [25]: b
>>> Out[25]: array([ 1,  2,  3,  4,  5,  6,  7,  8,  9, 10])
>>>
>>> In [26]: b[slice(1)]
>>> Out[26]: array([1])
>>>
>>> In [27]: b[slice(4)]
>>> Out[27]: array([1, 2, 3, 4])
>>>
>>> In [28]: b[slice(None,4)]
>>> Out[28]: array([1, 2, 3, 4])
>>>
>>> so slice(4) is actually slice(None,4), how can I exactly want retrieve
>>> a[4] using slice object?
>>>
>>> thanks again!
>>>
>>> Chao
>>>
>>
>> Tricky question.  Note the difference between
>>
>> a[4]
>>
>> and
>>
>> a[4:5]
>>
>> The first returns a scalar, while the second returns an array.  The first,
>> though, is not a slice, just an integer.
>>
>> Also, note that the arguments for slice() behaves very similar to the
>> arguments for range() (with some exceptions/differences).
>>
>> Cheers!
>> Ben Root
>>
>>
>>>
>>> 2012/7/12 Benjamin Root <ben.root@ou.edu>
>>>>
>>>>
>>>>
>>>> On Thu, Jul 12, 2012 at 3:38 PM, Chao YUE <chaoyuejoy@gmail.com> wrote:
>>>>>
>>>>> Dear all,
>>>>>
>>>>> I want to create a function and I would like one of the arguments of
>>>>> the function to determine what slicing of numpy array I want to use.
>>>>> a simple example:
>>>>>
>>>>> a=np.arange(100).reshape(10,10)
>>>>>
>>>>> suppose I want to have a imaging function to show image of part of this
>>>>> data:
>>>>>
>>>>> def show_part_of_data(m,n):
>>>>>     plt.imshow(a[m,n])
>>>>>
>>>>> like I can give m=3:5, n=2:7, when I call function
>>>>> show_part_of_data(3:5,2:7), this means I try to do plt.imshow(a[3:5,2:7]).
>>>>> the above example doesn't work in reality. but it illustrates something
>>>>> similar that I desire, that is, I can specify what slicing of
>>>>> number array I want by giving values to function arguments.
>>>>>
>>>>> thanks a lot,
>>>>>
>>>>> Chao
>>>>>
>>>>
>>>>
>>>> What you want to do is create slice objects.
>>>>
>>>> a[3:5]
>>>>
>>>> is equivalent to
>>>>
>>>> sl = slice(3, 5)
>>>> a[sl]
>>>>
>>>>
>>>> and
>>>>
>>>> a[3:5, 5:14]
>>>>
>>>> is equivalent to
>>>>
>>>> sl = (slice(3, 5), slice(5, 14))
>>>> a[sl]
>>>>
>>>> Furthermore, notation such as "::-1" is equivalent to slice(None, None,
>>>> -1)
>>>>
>>>> I hope this helps!
>>>> Ben Root
>>>>
>>>>
>>>> _______________________________________________
>>>> NumPy-Discussion mailing list
>>>> NumPy-Discussion@scipy.org
>>>> http://mail.scipy.org/mailman/listinfo/numpy-discussion
>>>>
>>>
>>>
>>>
>>> --
>>>
>>> ***********************************************************************************
>>> Chao YUE
>>> Laboratoire des Sciences du Climat et de l'Environnement (LSCE-IPSL)
>>> UMR 1572 CEA-CNRS-UVSQ
>>> Batiment 712 - Pe 119
>>> 91191 GIF Sur YVETTE Cedex
>>> Tel: (33) 01 69 08 29 02; Fax:01.69.08.77.16
>>>
>>> ************************************************************************************
>>>
>>>
>>> _______________________________________________
>>> NumPy-Discussion mailing list
>>> NumPy-Discussion@scipy.org
>>> http://mail.scipy.org/mailman/listinfo/numpy-discussion
>>>
>>
>>
>> _______________________________________________
>> NumPy-Discussion mailing list
>> NumPy-Discussion@scipy.org
>> http://mail.scipy.org/mailman/listinfo/numpy-discussion
>>
>
>
>
> --
> ***********************************************************************************
> Chao YUE
> Laboratoire des Sciences du Climat et de l'Environnement (LSCE-IPSL)
> UMR 1572 CEA-CNRS-UVSQ
> Batiment 712 - Pe 119
> 91191 GIF Sur YVETTE Cedex
> Tel: (33) 01 69 08 29 02; Fax:01.69.08.77.16
> ************************************************************************************
>
>
> _______________________________________________
> NumPy-Discussion mailing list
> NumPy-Discussion@scipy.org
> http://mail.scipy.org/mailman/listinfo/numpy-discussion
>



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