Re: Operating on every k-th element of list?
- To: mathgroup at smc.vnet.net
- Subject: [mg37094] Re: Operating on every k-th element of list?
- From: stagat at mrcsb.com (Bob Stagat)
- Date: Wed, 9 Oct 2002 05:25:46 -0400 (EDT)
- References: <anufl8$ap2$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
Take[list, am, n, sa] gives elements m through n in steps of s.
AES <siegman at stanford.edu> wrote in message news:<anufl8$ap2$1 at smc.vnet.net>...
> 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???