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 2012/7/12 Benjamin Root

On Thu, Jul 12, 2012 at 3:38 PM, Chao YUE 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

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On 07/12/2012 04:46 PM, Chao YUE 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

slice is a build in python function and the online docs explain its use (http://docs.python.org/library/functions.html#slice). b[slice(4,5)] will give you something close to b[4], but not quite the same. In [8]: b[4] Out[8]: 5 In [9]: b[slice(4,5)] Out[9]: array([5]) - Jonathan Helmus

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 2012/7/12 Benjamin Root

On Thu, Jul 12, 2012 at 4:46 PM, Chao YUE 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

...

On Thu, Jul 12, 2012 at 3:38 PM, Chao YUE 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

************************************************************************************

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-- *********************************************************************************** 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 ************************************************************************************

On 12/07/2012 23:32, 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.

I don't see the advantage of this approach over directly using the sliced array as an argument of your function, as in func(a[1:3,:,4]). Can you elaborate more why you are going through this route? Cheers, Daniele

On Thu, Jul 12, 2012 at 2:32 PM, Chao YUE 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

...

On Thu, Jul 12, 2012 at 4:46 PM, Chao YUE 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

...

On Thu, Jul 12, 2012 at 3:38 PM, Chao YUE 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

************************************************************************************

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-- Christopher Barker, Ph.D. Oceanographer Emergency Response Division NOAA/NOS/OR&R (206) 526-6959 voice 7600 Sand Point Way NE (206) 526-6329 fax Seattle, WA 98115 (206) 526-6317 main reception Chris.Barker@noaa.gov

Thanks Robert. This is exactly what I want. I have a feeling that there must be something in numpy that can do the job and I didn't know. Thanks again, Chao 2012/7/13 Robert Kern

On Thu, Jul 12, 2012 at 10:32 PM, 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.

[~] |1> from numpy import index_exp

[~] |2> index_exp[1:3,:,2:4] (slice(1, 3, None), slice(None, None, None), slice(2, 4, None))

-- Robert Kern _______________________________________________ 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 ************************************************************************************

Benjamin Root writes: [clip]

a[sl] and a[3:5, 5:14] is equivalent to sl = (slice(3, 5), slice(5, 14)) a[sl] [clip]

which is also equivalent to sl = np.s_[3:5, 5:14]