matplotlib-users

[Matplotlib-users] getting slices out of an array

[Olivier B. <Oli...@id...>]

Hello all,

I'm converting some matlab scripts to matlibplot, and I don't know how
to make some "slicing" efficently with matlibplot. What I want to do is
to translate a matlab code like:

    x =3D [[01, 02, 03, 04, 05, 06];
         [11, 12, 13, 14, 15, 16];
         [21, 22, 23, 24, 25, 26];
         [31, 32, 33, 34, 35, 36];
         [41, 42, 43, 44, 45, 46];
         [51, 52, 53, 54, 55, 56]
         ];
    x(2:4,1:4)
   =20
    ans =3D
   =20
        11    12    13    14
        21    22    23    24
        31    32    33    34
   =20
What I have done now in matplotlib is like:

    x =3D array ([[01, 02, 03, 04, 05, 06],
                [11, 12, 13, 14, 15, 16],
                [21, 22, 23, 24, 25, 26],
                [31, 32, 33, 34, 35, 36],
                [41, 42, 43, 44, 45, 46],
                [51, 52, 53, 54, 55, 56],
                ])
    for i in range (3):
        for j in range (4):
            x2 [i][j] =3D x [i+1][j+0]

so, x2 is now what I want:
    [[11,12,13,14,]
     [21,22,23,24,]
     [31,32,33,34,]]

I will be very happy to make the "slicing" without the loop...
I'm pretty new to matlibplot, so sorry if this is a too simple question.

Thanks in advance for your help.

        Olivier
--=20
   . __    . ___  __.  | Olivier Bornet         Oli...@id...
  / /  `  / /  / /  /  | IDIAP             http://www.idiap.ch/~bornet/
 / /   / / /--/ /--'   | CP 592        http://www.idiap.ch/~bornet/pgp/
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Re: [Matplotlib-users] getting slices out of an array

[Tim L. <ti...@cs...>]

On Thu, 14 Apr 2005, Olivier Bornet <Oli...@id...> wrote...

> Hello all,
> 
> I'm converting some matlab scripts to matlibplot, and I don't know how
> to make some "slicing" efficently with matlibplot. What I want to do is
> to translate a matlab code like:

You'll be pleased to know that the slicing works in almost exactly the
same in way python. Try:

x2 = x[1:4,0:4]

The indexing is slightly different, but I'm sure you can work it out.
 
Tim

> 
>     x = [[01, 02, 03, 04, 05, 06];
>          [11, 12, 13, 14, 15, 16];
>          [21, 22, 23, 24, 25, 26];
>          [31, 32, 33, 34, 35, 36];
>          [41, 42, 43, 44, 45, 46];
>          [51, 52, 53, 54, 55, 56]
>          ];
>     x(2:4,1:4)
>     
>     ans =
>     
>         11    12    13    14
>         21    22    23    24
>         31    32    33    34
>     
> What I have done now in matplotlib is like:
> 
>     x = array ([[01, 02, 03, 04, 05, 06],
>                 [11, 12, 13, 14, 15, 16],
>                 [21, 22, 23, 24, 25, 26],
>                 [31, 32, 33, 34, 35, 36],
>                 [41, 42, 43, 44, 45, 46],
>                 [51, 52, 53, 54, 55, 56],
>                 ])
>     for i in range (3):
>         for j in range (4):
>             x2 [i][j] = x [i+1][j+0]
> 
> so, x2 is now what I want:
>     [[11,12,13,14,]
>      [21,22,23,24,]
>      [31,32,33,34,]]
> 
> I will be very happy to make the "slicing" without the loop...
> I'm pretty new to matlibplot, so sorry if this is a too simple question.
> 
> Thanks in advance for your help.
> 
>         Olivier
> -- 
>    . __    . ___  __.  | Olivier Bornet         Oli...@id...
>   / /  `  / /  / /  /  | IDIAP             http://www.idiap.ch/~bornet/
>  / /   / / /--/ /--'   | CP 592        http://www.idiap.ch/~bornet/pgp/
> / /__.' / /  / /       | CH-1920 Martigny           PGP-key: 0xC53D9218


`-

Re: [Matplotlib-users] getting slices out of an array

[Olivier B. <Oli...@id...>]

Hi,

On Fri, Apr 15, 2005 at 12:32:45AM +1000, Tim Leslie wrote:
> You'll be pleased to know that the slicing works in almost exactly the
> same in way python. Try:
>=20
> x2 =3D x[1:4,0:4]

Cool. :-D
This is exactly what I'm searching for. But this is not really a Python
syntax... The coma in the square brackets is not standard for lists.
Maybe this come from the array type ?

> The indexing is slightly different, but I'm sure you can work it out.

Yes, no problem.

Thanks for your help.

        Olivier
--=20
   . __    . ___  __.  | Olivier Bornet         Oli...@id...
  / /  `  / /  / /  /  | IDIAP             http://www.idiap.ch/~bornet/
 / /   / / /--/ /--'   | CP 592        http://www.idiap.ch/~bornet/pgp/
/ /__.' / /  / /       | CH-1920 Martigny           PGP-key: 0xC53D9218

Re: [Matplotlib-users] getting slices out of an array

[John G. <jn...@eu...>]

Lists are 1-dimensional, so the comma doesn't really make sense.

You can, however, slice lists in python eg

 >>> x = range(100)
 >>> x[10:100:5]
[10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95]

You're right, that the commas come into play when you are dealing with 
the numarray (or numeric) arrays, where you may well have more than one 
dimension.    It is a natural extension of the python slice syntax to 
multiple dimensions.

John


Olivier Bornet wrote:

>Hi,
>
>On Fri, Apr 15, 2005 at 12:32:45AM +1000, Tim Leslie wrote:
>  
>
>>You'll be pleased to know that the slicing works in almost exactly the
>>same in way python. Try:
>>
>>x2 = x[1:4,0:4]
>>    
>>
>
>Cool. :-D
>This is exactly what I'm searching for. But this is not really a Python
>syntax... The coma in the square brackets is not standard for lists.
>Maybe this come from the array type ?
>
>  
>
>>The indexing is slightly different, but I'm sure you can work it out.
>>    
>>
>
>Yes, no problem.
>
>Thanks for your help.
>
>        Olivier
>  
>

Re: [Matplotlib-users] getting slices out of an array

[John H. <jdh...@ac...>]

>>>>> "Olivier" == Olivier Bornet <Oli...@id...> writes:

    Olivier> Cool. :-D This is exactly what I'm searching for. But
    Olivier> this is not really a Python syntax... The coma in the
    Olivier> square brackets is not standard for lists.  Maybe this
    Olivier> come from the array type ?

Yes, this is Numeric/numarray slicing, not list slicing.  When you get
some time, you should read the pdf Numeric or numarray documentation.
It's comprehensive and very good.

Numarray also offers enhanced indexing ( Numeric does not yet) which
is a convenience you probably expect coming from matlab.  Eg, you can
index an array with a sequence of integers (you must be careful that
the sequence is not a tuple however; lists or arrays of ints are ok)

    >>> import numarray as na
    >>> a = na.arange(0.0, 2.0, 0.1)
    >>> ind = [2,5,7]
    >>> a[ind]
    array([ 0.2,  0.5,  0.7])

JDH

Re: [Matplotlib-users] getting slices out of an array

[Olivier B. <Oli...@id...>]

Hello all,

> Yes, this is Numeric/numarray slicing, not list slicing.  When you get
> some time, you should read the pdf Numeric or numarray documentation.
> It's comprehensive and very good.

Thanks for all the answers. I'm now looking at the numarray/numeric
documentation as suggested.

Good day, and thanks again.

        Olivier
--=20
   . __    . ___  __.  | Olivier Bornet         Oli...@id...
  / /  `  / /  / /  /  | IDIAP             http://www.idiap.ch/~bornet/
 / /   / / /--/ /--'   | CP 592        http://www.idiap.ch/~bornet/pgp/
/ /__.' / /  / /       | CH-1920 Martigny           PGP-key: 0xC53D9218

Re: [Matplotlib-users] getting slices out of an array

[Chris B. <Chr...@no...>]

Olivier Bornet wrote:

> This is exactly what I'm searching for. But this is not really a Python
> syntax... The coma in the square brackets is not standard for lists.
> Maybe this come from the array type ?

Yes, it does. I you are doing anything MATLAB-y with Python, you really 
want to know about either numarray or Numeric. numarray is a little bot 
more like MATLAB (it allows array indexing), and Numeric has better 
performance with small arrays, but either will do for most uses. It 
looks like we're on the way to a grand unification of the two anyway.

Make sure you go find the docs f0or the package you choose, and read 
through them, I think you'll like them a lot!

By the way: aside from the looping, when you do:

x = A[i][j]

rather than:

x = A[i,j]

there is a performance hit because the first version is creating a new 
array out of A[i], then indexing into that, rather than just indexing 
directly into the 2-d (or more d) array. This is particularly pronounced 
with numarray, as the array creation overhead is larger than with Numeric.

-Chris







-- 
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