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Re: list manipulation

  • To: mathgroup at smc.vnet.net
  • Subject: [mg100159] Re: [mg100122] list manipulation
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Tue, 26 May 2009 05:12:50 -0400 (EDT)
  • Reply-to: hanlonr at cox.net

data = {
   {element11, element12, element1N, target11, target12, element1N3, 
    element1N4},
   {element21, element22, element2N, target21, target22, element2N3, 
    element2N4}
   };

data //. {
  {s___, target11, target12, e___} :> {s, f[target11, target12], e},
  {s___, target21, target22, e___} :> {s, f[target21, target22], e}
  }

{{element11, element12, element1N, f[target11, target12], element1N3, 
  element1N4}, 
   {element21, element22, element2N, f[target21, target22], 
  element2N3, element2N4}}

data //. (
  {s___, Sequence @@ #, e___} :> {s, f @@ #, e} & /@
   {{target11, 
     target12}, {target21, target22}}
  )


{{element11, element12, element1N, f[target11, target12], element1N3, 
  element1N4}, 
   {element21, element22, element2N, f[target21, target22], 
  element2N3, element2N4}}

% == %%

True


Bob Hanlon

---- "Scot T. Martin" <smartin at seas.harvard.edu> wrote: 

=============
I'm wondering if anyone has bright ideas on the puzzle below. I'm trying 
to take a very large list of the following form:

{{.., .., a, b, ..},{.., .., c, d, ..},..}

into the form

{{.., .., f[a, b], ..},{.., .., f[c, d], ..},..}

It seems to me that there must be some clever and beautiful way to do this 
that I haven't thought of. My code is below with two clumsy solutions.

Suggestions on how to do better?

My code:

In[1]:= (*shown here as minimal to define problem but actual list has \
large N and many entries*)

data = {{element11, element12, element1N, target11, target12,
     element1N3, element1N4}, {element21, element22, element2N,
     target21, target22, element2N3, element2N4}};



In[2]:= (* first clumsy approach *)

Replace[data, list : _List :> {Sequence @@ list[[1 ;; 3]],
Sequence[f[list[[4 ;; 5]]]], Sequence @@ list[[6 ;;]]}, {1}]


Out[2]= {{element11, element12, element1N, f[{target11, target12}],
   element1N3, element1N4}, {element21, element22, element2N,
   f[{target21, target22}], element2N3, element2N4}}



In[3]:= (* second clumsy approach *)

data /. {a : Repeated[_, {3}], b : Repeated[_, {2}],
c : __} -> {a, f[b], c}

Out[3]= {{element11, element12, element1N, f[target11, target12],
   element1N3, element1N4}, {element21, element22, element2N,
   f[target21, target22], element2N3, element2N4}}




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