Re: Operating on every k-th element of list?

*To*: mathgroup at smc.vnet.net*Subject*: [mg37084] Re: Operating on every k-th element of list?*From*: "Borut L" <gollum at email.si>*Date*: Wed, 9 Oct 2002 05:25:29 -0400 (EDT)*References*: <anufl8$ap2$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Hello, Consider the following approach, whish uses the MapAt command, that is Map with 'mapping-position' control. dummyFun={#,trueFun@#}& list={a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p} spec=Partition[Range[3,Length[list],3],1] MapAt[dummyFun,list,spec] %//Flatten Hope that is what you want, Borut "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??? |