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