adjacent differences with a list comprehension
fperez528 at yahoo.com
Mon Mar 24 23:46:51 CET 2003
Phil Schmidt wrote:
> Given a list of numbers, such as:
> L = [2, 5, 8, 3, 9, 1]
> I want to generate a list containing the differences between adjacent
> elements, i.e.,
> Ld = [3, 3, -5, 6, -8]
> I don't see how I can do this (easily/elegantly) with list
> Any suggestions? Thanks!
Numeric provides the simplest possible way of writing this:
In : L = array([2, 5, 8, 3, 9, 1])
In : L[1:]-L[:-1]
Out: array([ 3, 3, -5, 6, -8])
Note that in the above, the data is NOT copied when you do L[1:], so even if L
is a multi-megabyte array, you won't incurr a copy penalty. The semantics of
Numeric slicing is different from that of python lists, for very good reasons.
And the loop is done in C, so it's extremely fast.
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