On Thursday, July 12, 2012, Chao YUE wrote:
> Thanks all for the discussion. Actually I am trying to use something like
> 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.
> 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
I won't comment on the wisdom of your approach, but for you very last part,
don't try unpacking the slice list. Also, I think it has to be a tuple,
but I could be wrong on that.
Ben Root
>
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