Re: Drop elements from list
- To: mathgroup at smc.vnet.net
- Subject: [mg94251] Re: Drop elements from list
- From: SigmundV <sigmundv at gmail.com>
- Date: Tue, 9 Dec 2008 06:57:10 -0500 (EST)
- References: <ghdmqj$flr$1@smc.vnet.net>
Thank you for all the solutions. Some of them I should have come up
with myself. My favorite is probably Fold[Drop[##]&,a,{{7,10},{2,5}}].
I don't know whether it is the fastest or not, but it is easy to
program (as in "characters to type"). Ray Koopman's solution with
Delete is also good (although I would like to point out that
Transpose@Flatten@{Range[2,5],Range[7,10]} does not work as it is, but
Partition[...,1] does the trick) and easy to program as well. The idea
presented by Raffy and dimitris, to pick the relevant elements instead
of deleting, is also good. I have discovered that it generally is
faster to pick explicit elements from a list than to use functions as
Drop or Delete. In that respect I like Raffy's solution, but it seems
that too much time is spent in Complement.
Finally, below is a cell with timings of the different solutions. I
would like to hear your comments on this. The final line is included
to see how much time is spent in Complement.
a = RandomReal[{}, 2000000];
Delete[a, Partition[Range[2, 5]~Join~Range[7, 10]~Join~Range[15, 18],
1]]; // Timing
a[[{1, 6}~Join~Range[11, 14]~Join~Range[19, Length[a]]]]; // Timing
Fold[Drop[##] &, a, {{15, 18}, {7, 10}, {2, 5}}]; // Timing
a[[Complement[Range[Length[a]], Range[2, 5], Range[7, 10], Range[15,
18]]]]; // Timing
Complement[Range[Length[a]], Range[2, 5], Range[7, 10], Range[15,
18]]; // Timing
/Sigmund
On Dec 6, 12:14 pm, SigmundV <sigmu... at gmail.com> wrote:
> Dear group,
>
> Consider a list of length, say, 20:
>
> a = RandomReal[{},20];
>
> How would you then drop/delete elements, say, 2-5 and 7-10 from this
> list? The built-in function Drop does not seem to include such a
> possibility. I should add that I do not have access to Mathematica 7,
> so any solution should work in version 6.
>
> Kind regards,
> Sigmund