# sequence multiplied by -1

Yingjie Lan lanyjie at yahoo.com
Sat Sep 25 14:11:39 CEST 2010

--- On Sat, 9/25/10, Thomas Jollans <thomas at jollybox.de> wrote:
> for every list l and integer n >= 0:
>     len(l*n) == len(l)*n

Well, this invariance is indeed broken under my proposal.
But it is *already broken* in current python3k.
However, the following invariance is maintained under
my proposal:

len(l*n) == len(l) * abs(n),

which is also broken under current python3k.
if you think len(..) as a mathematical norm,
the above invariance makes perfect sense:

|| a * b || == ||a|| * |b|, b is real

>
> > Simply put, a sequence multiplied by -1 can give a
> reversed sequence.
>
> For that, we have slicing. A negative step value produces a
> reverse slice of
> the list. You can't argue that this makes sense, can you
>
> >>> [1,2,3,4][::-1]
> [4, 3, 2, 1]
> >>> [1,2,3,4][::-2]
> [4, 2]
> >>>

Having more than one way of doing things sometimes is good.
Slicing is a little more complex, what if you want this:

>>> ([1,2,3,4]*2)[::-1]
[4, 3, 2, 1, 4, 3, 2, 1]

under my new proposal, you simply do this:

>>> [1,2,3,4]*-2
[4, 3, 2, 1, 4, 3, 2, 1]

Cheers,

Yingjie