How to slice range of array along a (a-priori unknown) axis?

[Maarten van Reeuwijk]

Hello,

I am using the Numeric package, and I need to strip edge cells off an array
(dimension unknown) in an a-priori unknown direction. I implented this as
follows:

def el_remove_bcell s(var, axis):
""" elementary routine to strip the edge cells of an array for the given
direction.
"""

if axis == 0:
return var[1:-1]
if axis == 1:
return var[:, 1:-1]
else:
return var[:, :,1:-1]

But this only works for at most 3D arrays. It must be possible to program
this fragment without the ifs, for arrays of arbitrary dimension. Is there
a command in the Numerical package I can use for this? It is very important
that this method is very fast, as my arrays normally are in the order of
100 Mb.

TIA, Maarten

--
=============== =============== =============== =============== =======
Maarten van Reeuwijk Thermal and Fluids Sciences
Phd student dept. of Multiscale Physics
www.ws.tn.tudelft.nl Delft University of Technology

[Diez B. Roggisch]

Re: How to slice range of array along a (a-priori unknown) axis?

This should work.

def el_remove_bcell s(var, axis):
if axis == 0:
     return var[1:-1]
else:
return eval("var[%s1:-1]" % ":," * axis)

--
Regards,

Diez B. Roggisch

[Maarten van Reeuwijk]

Re: How to slice range of array along a (a-priori unknown) axis?

> def el_remove_bcell s(var, axis):[color=blue]
> if axis == 0:
> return var[1:-1]
> else:
> return eval("var[%s1:-1]" % ":," * axis)[/color]

Thanks, that does the trick! I love the fact that you can create commands at
runtime :). I also found a command called swapaxes, which is very fast, so
just in case you're interested, here's another way:

def el_remove_bcell s(var, axis):
swap = swapaxes(var, 0, axis)
swap = swap[1:-1]
return swapaxes(swap, 0, axis)

--
=============== =============== =============== =============== =======
Maarten van Reeuwijk Thermal and Fluids Sciences
Phd student dept. of Multiscale Physics
www.ws.tn.tudelft.nl Delft University of Technology

[Peter Otten]

Re: How to slice range of array along a (a-priori unknown) axis?

Maarten van Reeuwijk wrote:
[color=blue]
> I am using the Numeric package, and I need to strip edge cells off an
> array (dimension unknown) in an a-priori unknown direction. I implented
> this as follows:
>
> def el_remove_bcell s(var, axis):
> """ elementary routine to strip the edge cells of an array for the
> given
> direction.
> """
>
> if axis == 0:
> return var[1:-1]
> if axis == 1:
> return var[:, 1:-1]
> else:
> return var[:, :,1:-1]
>
> But this only works for at most 3D arrays. It must be possible to program
> this fragment without the ifs, for arrays of arbitrary dimension. Is there
> a command in the Numerical package I can use for this? It is very
> important that this method is very fast, as my arrays normally are in the
> order of 100 Mb.[/color]

Maybe:
[color=blue][color=green][color=darkred]
>>> def clipdim(a, n):[/color][/color][/color]
.... t = [slice(None)] * N.rank(a)
.... t[n] = slice(1,-1)
.... return a[t]
....[color=blue][color=green][color=darkred]
>>> a[/color][/color][/color]
array([[[ 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]]])[color=blue][color=green][color=darkred]
>>> clipdim(a, 0)[/color][/color][/color]
array([ [[ 9, 10, 11],
[12, 13, 14],
[15, 16, 17]]])[color=blue][color=green][color=darkred]
>>> clipdim(a, 1)[/color][/color][/color]
array([[ [ 3, 4, 5]],
[ [12, 13, 14]],
[ [21, 22, 23]]])[color=blue][color=green][color=darkred]
>>> clipdim(a, 2)[/color][/color][/color]
array([[[ 1],
[ 4],
[ 7]],
[[10],
[13],
[16]],
[[19],
[22],
[25]]])

Peter