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MathGroup Archive 2002

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Operating on every k-th element of list?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg37080] Operating on every k-th element of list?
  • From: AES <siegman at stanford.edu>
  • Date: Tue, 8 Oct 2002 07:17:49 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

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???


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