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list manipulation

  • To: mathgroup at smc.vnet.net
  • Subject: [mg100122] list manipulation
  • From: "Scot T. Martin" <smartin at seas.harvard.edu>
  • Date: Mon, 25 May 2009 06:18:22 -0400 (EDT)
  • References: <200905191101.HAA07313@smc.vnet.net> <gv0gn0$a19$1@smc.vnet.net>

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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