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???