[Numpy-discussion] Backwards slicing including the first element

Warren Weckesser warren.weckesser@enthought....
Wed Mar 10 08:49:06 CST 2010


Jerome Esteve wrote:
> Dear all,
>
> Is there a way to give an integer value to j when using a[i:j:-1] so 
> that the first element of the array can be included in the slice ?
>
> I would like to use some code like a[i:i-k:-1] to get a slice of 
> length k. 
>
> The numpy documentation seems to suggest that j=-1 should work:
>
> "Assume n is the number of elements in the dimension being sliced. 
> Then, if i is not given it defaults to 0 for k > 0 and n for k < 0 . 
> If j is not given it defaults to n for k > 0 and -1 for k < 0 . 
> If k is not given it defaults to 1."
>
> But a[i:i-k:-1] is empty if i-k is -1. The workaround is a[i::-1][:k], 
> is there something simpler ?

You could use a[i:i-(len(a)+k):-1].  This works because a[-len(a)] is 
the same as a[0].

For example:

-----
In [57]: a
Out[57]: array([10, 11, 12, 13, 14, 15, 16, 17, 18, 19])

In [58]: i = 3

In [59]: for k in range(5):
   ....:     print k, a[i:i-(len(a)+k):-1]
   ....:    
   ....:    
0 []
1 [13]
2 [13 12]
3 [13 12 11]
4 [13 12 11 10]

-----


Warren

>
> Many thanks in advance, Jerome.
>
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