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Re: How to unflatten an array ?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg112832] Re: How to unflatten an array ?
  • From: Sseziwa Mukasa <mukasa at jeol.com>
  • Date: Sat, 2 Oct 2010 05:45:45 -0400 (EDT)

Here's a faster approach:

In[63]:= First[Timing[
  Partition[ArrayFlatten[{{Transpose[{a4[[All, ;; 2]]}], a4[[All, {3}]]}}], 
    3];]]
Out[63]= 0.118238

On Oct 1, 2010, at 5:43 AM, Sseziwa Mukasa wrote:

> 
> On Sep 30, 2010, at 4:52 AM, Valeri Astanoff wrote:
> 
>> Good day,
>> 
>> Suppose I have a flat array like this :
>> 
>> {{1, 0, 1}, {1, 1, 1}, {1, 2, 3}, {2, 0, 5}...
>> 
>> and I want to get back to this "unflattened" form :
>> 
>> {{{{1, 0}, 1}, {{1, 1}, 1}, {{1, 2}, 3}}, {{{2, 0}, 5}...
>> 
>> What is the most efficient way to do it ?
>> 
>> 
>> All I have found is this :
>> 
>> In[1]:= unflatten[arr : {{_, _, _} ..}] :=
>> Module[{f},
>> Scan[(f[#[[1]], #[[2]]] = #[[3]]) &, arr];
>> Table[{{x, y}, f[x, y]},
>> {x, arr[[All, 1]] // Union},
>> {y, arr[[All, 2]] // Union}]
>> ];
>> 
>> In[2]:= a1 = {{{{1, 0}, 1}, {{1, 1}, 1}, {{1, 2}, 3}},
>> {{{2, 0}, 5}, {{2, 1}, 0}, {{2, 2}, 0}}, {{{3, 0}, 4},
>> {{3, 1}, 5}, {{3, 2}, 2}}, {{{5, 0}, 1}, {{5, 1}, 3},
>> {{5, 2}, 2}}};
>> 
>> In[3]:= a2 = Flatten /@ Flatten[a1, 1]
>> 
>> Out[3]= {{1, 0, 1}, {1, 1, 1}, {1, 2, 3}, {2, 0, 5},
>> {2, 1, 0}, {2, 2, 0}, {3, 0, 4}, {3, 1, 5}, {3, 2, 2},
>> {5, 0, 1}, {5, 1, 3}, {5, 2, 2}}
>> 
>> In[4]:= a3 = unflatten@a2
>> 
>> Out[4]= {{{{1, 0}, 1}, {{1, 1}, 1}, {{1, 2}, 3}},
>> {{{2, 0}, 5}, {{2, 1}, 0}, {{2, 2}, 0}}, {{{3, 0}, 4},
>> {{3, 1}, 5}, {{3, 2}, 2}}, {{{5, 0}, 1}, {{5, 1}, 3},
>> {{5, 2}, 2}}}
>> 
>> In[5]:= a1 == a3
>> 
>> Out[5]= True
>> 
>> 
>> Please help me to something faster for large arrays.
> 
> I would construct the desired result as:
> 
> Partition[{Most[#],Last[#]}&/@a2,3]
> 
> Comparing on a array of 10002 triples the timing of the two algorithms is
> 
> (Debug) In[244]:= a4 = RandomInteger[{-10, 10}, {100002, 3}];
> First[Timing[unflatten[a4];]]
> First[Timing[Partition[{Most[#], Last[#]} & /@ a4, 3];]]
> (Debug) Out[245]= 1.39615
> (Debug) Out[246]= 0.549411
> 
> Regards,
> 	Sseziwa
> 



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