[Tutor] Question concerning splitting of arrays

[Kent Johnson]

Derek,

You should look into the numarray package, it can do what you want. See 
http://stsdas.stsci.edu/numarray/numarray-1.1.html/node26.html for some 
examples.

In standard Python, lists of lists are used to represent multi-dimensional 
arrays. Operations on multidimensional arrays are applied sequentially to 
the lists that represent the arrays. As you found out, this is not really a 
good model for full-featured matrix operations. Here is what is happening:
 >>> array = [[11, 12, 13], [21, 22, 23], [31, 32, 33]]

array is a list of three elements. Each element is also a list of three 
elements.

What does this mean?
 >>> array[1][2]
23

array[1][2] is the same as (array[1])[2]. It takes element 1 of array, 
which is the list [21, 22, 23], then takes element 2 of that list, which is 
23. This is what you expect.

What about this?
 >>> array[:][0]
[11, 12, 13]

Again, array[:][0] is the same as (array[:])[0]. array[:] is a copy of all 
of array! So you are copying array, then taking the first element of the 
copy. Not what you wanted at all.

Finally, how about this?
 >>> array[1:][1:]
[[31, 32, 33]]

As before, array[1:][1:] is (array[1:])[1:]. array[1:] is everything but 
the first element of array; this is a list containing two lists:
 >>> array[1:]
[[21, 22, 23], [31, 32, 33]]

Now you take the slice [1:] of that list, and you are dropping the first 
element again. array[1:][1:] is the same as array[2:]!

Kent

At 05:18 PM 9/29/2004 +0930, Weber, Derek wrote:
>Hi all,
>
>I'm actually doing something like translating some matlab code to Python 
>and am wondering whether it's possible to split multi-dimensional arrays 
>as such:
>
> >>> array = [[11, 12, 13], [21, 22, 23], [31, 32, 33]]
> >>> firstColumn = array[:][0]
> >>> lowerRightCorner = array[1:][1:]
> >>> print firstColumn
>[[11], [21], [31]]
> >>> print lowerRightCorner
>[[22, 23], [32, 33]]
>
>I seem to get errors when I do this, so I thought I'd ask people who knew 
>a great deal more about the language than I did to see if there's an easy 
>way to do this, or do I have to write some functions to do it for me?
>
>I think the issue is making the first array reference a range.
>
>Any suggestions would be gratefully accepted. Also, if there's a way to 
>search the archive of the list, I'd do that, but I haven't had any luck 
>with the one referred to by python.org.
>
>Thanks very much.
>
>D.
>
>============================================================
>Derek Weber                  derek.weber at dsto.defence.gov.au
>Information Exploitation Group            Rm: 2.H.06 205Labs
>Defence Science & Technology Organisation Ph: 61 8 8259 7699
>PO Box 1500, Edinburgh SA 5111            Fx: 61 8 8259 5619
>============================================================
>
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