Negative array indicies and slice()
roy at panix.com
Mon Oct 29 14:52:50 CET 2012
In article <mailman.3009.1351516065.27098.python-list at python.org>,
Andrew Robinson <andrew3 at r3dsolutions.com> wrote:
> Show me an example where someone would write a slice with a negative and
> a positive index (both in the same slice);
> and have that slice grab a contiguous slice in the *middle* of the list
> with orientation of lower index to greater index.
It's possible in bioinformatics. Many organisms have circular
chromosomes. It's a single DNA molecule spliced into a loop. There's
an "origin", but it's more a convenience thing for people to assign some
particular base-pair to be location 0. From the organism's point of
view, the origin isn't anything special (but there *is* a fixed
It's entirely plausible for somebody to want to extract the sub-sequence
from 100 bp (base-pairs) before the origin to 100 bp after the origin.
If you were storing the sequence in Python string (or list), the most
convenient way to express this would be seq[-100:100]. Likewise, if you
wanted the *other* fragment, you would write seq[100:-100].
There is a minor confounding factor here in that biologists number
sequences starting with 1, not 0. At least that was the way when I was
doing this stuff mumble years ago. I don't know what the current
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