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

*To*: mathgroup at smc.vnet.net*Subject*: [mg37085] Re: [mg37080] Operating on every k-th element of list?*From*: Sseziwa Mukasa <mukasa at jeol.com>*Date*: Wed, 9 Oct 2002 05:25:32 -0400 (EDT)*Sender*: owner-wri-mathgroup at wolfram.com

On Tuesday, October 8, 2002, at 07:17 AM, 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??? > > Here is a generalization of what you've asked: f[l_List,c_,k_Integer,p_Integer]:=Flatten[Block[{r=#[[k]] c},Join[Take[#,k-1],{r,#[[k+1]]+ r},Drop[#,k+1]]]&/@Partition[l,p]]/;(Mod[Length[l],p]==0&&k<p) I took the liberty of allowing you to partition the list into sublists of length p independent of k, for your case simply set k = 3 and p = 4. c is the constant you wish to use. Regards, Sseziwa