Re: List complement operator
- To: mathgroup at smc.vnet.net
- Subject: [mg84978] Re: List complement operator
- From: Szabolcs Horvát <szhorvat at gmail.com>
- Date: Mon, 21 Jan 2008 07:00:26 -0500 (EST)
- References: <fn1ndi$97t$1@smc.vnet.net>
zac wrote:
> Dear group,
>
> I'm loooking for an elegant and efficient way to subtract a list (not
> a set) from an other one.
>
> a = {1, 1, 1, 2, 3, 3, 4, 5, 5, 5, 2, 6, 7, 7};
> b = {1, 2, 2, 3, 4, 7, 8};
>
> Complement[a,b] returns {5, 6}, as expected. However I would like to
> treat each entry as a discrete element, therefore my target result is:
> {1, 1, 3, 5, 5, 5, 6, 7}. Union can not be used, although Sort is
> allowed (ie. whether the result is sorted or not is of no concern).
> Any idea?
Hi Istvan,
This is a quick-and-dirty solution, not particularly efficient if you
have many different elements, but if we're lucky, it might just be fast
enough for your application:
lc[a_, b_] := Join @@ (
Table[#1, {#2}] & @@@ Transpose@With[{u = Union[a, b]},
{u,
Last /@ Sort@Tally@Join[a, u] -
Last /@ Sort@Tally@Join[b, u]}
]
)
The solution just reflects the way I interpreted the problem: Remove as
many 2's from 'a' as are present in 'b'.
--
Szabolcs