Re: Operating on every k-th element of list?
- To: mathgroup at smc.vnet.net
- Subject: [mg37083] Re: Operating on every k-th element of list?
- From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
- Date: Wed, 9 Oct 2002 05:25:28 -0400 (EDT)
- Organization: Universitaet Leipzig
- References: <anufl8$ap2$1@smc.vnet.net>
- Reply-to: kuska at informatik.uni-leipzig.de
- Sender: owner-wri-mathgroup at wolfram.com
Hi,
something like:
With[{k=3},
Flatten[
{#[[2]] + c*#[[1]], #[[3]]} & /@
Partition[lst, k, k, {1, 1}],
1]
]
??
Regards
Jens
AES wrote:
>
> I want to apply a function to every k-th element of a long list and
> add the result to the k+1 element.
>
> [Actually k = 3 and I just want to multiply myList[[k]] by a
> constant (independent of k) and add the result to myList[[k+1]] for
> every value of k that's divisible by 3.]
>
> Is there a way to do this -- or in general to get at every k-th
> element of a list -- that's faster and more elegant than writing a brute
> force Do[] loop or using Mod[] operators, and that will take
> advantage of native List operators, but still not be too recondite?
>
> I've been thinking about multiplying a copy of myList by a "mask list"
> {0,0,1,0,0,1,..} to generate a "masked copy" and approaches like that.
> Better ways???