numpy slice: create view, not copy

[K. Jansma]

Hi,

given an array:

import numpy
a = numpy.arange(100).reshape((10,10))
print a

[[ 0 1 2 3 4 5 6 7 8 9]
[10 11 12 13 14 15 16 17 18 19]
[20 21 22 23 24 25 26 27 28 29]
[30 31 32 33 34 35 36 37 38 39]
[40 41 42 43 44 45 46 47 48 49]
[50 51 52 53 54 55 56 57 58 59]
[60 61 62 63 64 65 66 67 68 69]
[70 71 72 73 74 75 76 77 78 79]
[80 81 82 83 84 85 86 87 88 89]
[90 91 92 93 94 95 96 97 98 99]]


I'd like to create a new array that is a view of a, i.e. it shares data. If
a is updated, this new array should be automatically 'updated'.

e.g. the array west should be a view of a that gives the element at the left
of location i,j in a.
a[i,j] = west[i,j+1]

west can be created using:

a[:,range(-1,a.shape[1]-1)]

As you can see, this also defines periodic boundaries (i.e. west[0,0] = 9)
but it seems that this returns a copy of a, not a view.
How can I change the slice definition in such a way it returns a view?

Thanks in advance,
Karel

 

[Terry Reedy]

given an array:

import numpy
a = numpy.arange(100).reshape((10,10))
print a

Numpy questions are best asked on the numpy mailing list.

 

[Travis Oliphant]

Hi,

given an array:

import numpy
a = numpy.arange(100).reshape((10,10))
print a

[[ 0 1 2 3 4 5 6 7 8 9]
[10 11 12 13 14 15 16 17 18 19]
[20 21 22 23 24 25 26 27 28 29]
[30 31 32 33 34 35 36 37 38 39]
[40 41 42 43 44 45 46 47 48 49]
[50 51 52 53 54 55 56 57 58 59]
[60 61 62 63 64 65 66 67 68 69]
[70 71 72 73 74 75 76 77 78 79]
[80 81 82 83 84 85 86 87 88 89]
[90 91 92 93 94 95 96 97 98 99]]


I'd like to create a new array that is a view of a, i.e. it shares data. If
a is updated, this new array should be automatically 'updated'.

e.g. the array west should be a view of a that gives the element at the left
of location i,j in a.
a[i,j] = west[i,j+1]

west can be created using:

a[:,range(-1,a.shape[1]-1)]

As you can see, this also defines periodic boundaries (i.e. west[0,0] = 9)
but it seems that this returns a copy of a, not a view.
How can I change the slice definition in such a way it returns a view?

You can't get periodic boundary conditions but

a[:,1::]

should give you (part of) the view you are looking for.


-Travis