# problems with duplicating and slicing an array

Discussion in 'Python' started by Yun Mao, Jan 21, 2005.

1. ### Yun MaoGuest

Hi python gurus,
I have some questions when I'm using python numeric:

1. When I do v = u[:, :], it seems u and v still point to the same
memory. e.g. When I do v[1,1]=0, u[1,1] will be zero out as well.
What's the right way to duplicate an array? Now I have to do v =
dot(u, identity(N)), which is kind of silly.

2. Is there a way to do Matlab style slicing? e.g. if I have
i = array([0, 2])
x = array([1.1, 2.2, 3.3, 4.4])
I wish y = x(i) would give me [1.1, 3.3]
Now I'm using map, but it gets a little annoying when there are two
dimensions.
Any ideas would be deeply appreciated!

Thanks!!!

-Y

Yun Mao, Jan 21, 2005

2. ### Steven BethardGuest

Yun Mao wrote:
> Hi python gurus,
> I have some questions when I'm using python numeric:
>
> 1. When I do v = u[:, :], it seems u and v still point to the same
> memory. e.g. When I do v[1,1]=0, u[1,1] will be zero out as well.
> What's the right way to duplicate an array? Now I have to do v =
> dot(u, identity(N)), which is kind of silly.

Hmm... I don't know Numeric, but in numarray (Numeric's replacement),
you just call the copy method:

py> u = na.arange(6, shape=(2, 3))
py> v = u.copy()
py> v[0,0] = 100
py> u
array([[0, 1, 2],
[3, 4, 5]])
py> v
array([[100, 1, 2],
[ 3, 4, 5]])

> 2. Is there a way to do Matlab style slicing? e.g. if I have
> i = array([0, 2])
> x = array([1.1, 2.2, 3.3, 4.4])
> I wish y = x(i) would give me [1.1, 3.3]

This also works exactly as you'd hope in numarray.

py> i = na.array([0, 2])
py> x = na.array([1.1, 2.2, 3.3, 4.4])
py> x
array([ 1.1, 3.3])

Do you have any reason that you have to use Numeric? Or can you upgrade
to numarray?

Steve

Steven Bethard, Jan 21, 2005

3. ### Guest

Yun Mao wrote:

>I have some questions when I'm using python numeric:

>1. When I do v = u[:, :], it seems u and v still point to the same
>memory. e.g. When I do v[1,1]=0, u[1,1] will be zero out as well.
>What's the right way to duplicate an array? Now I have to do v =
>dot(u, identity(N)), which is kind of silly.

You can use v = 1*u or v = u.copy() to get the type of copy you want.

>2. Is there a way to do Matlab style slicing?

Use the take function.

The output of the following code

from Numeric import array,take
a = array([1,2])
b = a
c = 1*a
d = a.copy()
a[0] = 3
print b
print c
print d
print take(a,[1,0,1])

is

[3 2]
[1 2]
[1 2]
[2 3 2]

, Jan 21, 2005

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