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Re: How to unflatten an array ?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg112813] Re: How to unflatten an array ?
  • From: Leonid Shifrin <lshifr at gmail.com>
  • Date: Fri, 1 Oct 2010 05:42:56 -0400 (EDT)

Hi Valeri,

It is not clear to me how your sublists are partitioned within the big list
in the unflattened version - always by 3 together, or some other principle?

Your function seems to work by pure accident. Consider random data of your
form instead of your a1:

In[50]:==
rnda ==
Partition[Table[{{RandomInteger[{0,10}],RandomInteger[{0,10}]},RandomIntege=
r[{0,10}]},{9}],3]

Out[50]==
{{{{8,8},7},{{2,1},7},{{3,1},7}},{{{10,9},1},{{8,9},10},{{10,5},8}},{{{9,10=
},6},{{9,8},1},{{7,6},1}}}

In[51]:== rnda2 == Flatten/@Flatten[rnda,1]

Out[51]==
{{8,8,7},{2,1,7},{3,1,7},{10,9,1},{8,9,10},{10,5,8},{9,10,6},{9,8,1},{7,6,1=
}}

In[52]:== unflatten@rnda2

Out[52]==
{{{{2,1},7},{{2,5},f$2163[2,5]},{{2,6},f$2163[2,6]},{{2,8},f$2163[2,8]},{{2=
,9},f$2163[2,9]},{{2,10},f$2163[2,10]}},{{{3,1},7},{{3,5},f$2163[3,5]},{{3,=
6},f$2163[3,6]},{{3,8},f$2163[3,8]},{{3,9},f$2163[3,9]},{{3,10},f$2163[3,10=
]}},{{{7,1},f$2163[7,1]},{{7,5},f$2163[7,5]},{{7,6},1},{{7,8},f$2163[7,8]},=
{{7,9},f$2163[7,9]},{{7,10},f$2163[7,10]}},{{{8,1},f$2163[8,1]},{{8,5},f$21=
63[8,5]},{{8,6},f$2163[8,6]},{{8,8},7},{{8,9},10},{{8,10},f$2163[8,10]}},{{=
{9,1},f$2163[9,1]},{{9,5},f$2163[9,5]},{{9,6},f$2163[9,6]},{{9,8},1},{{9,9}=
,f$2163[9,9]},{{9,10},6}},{{{10,1},f$2163[10,1]},{{10,5},8},{{10,6},f$2163[=
10,6]},{{10,8},f$2163[10,8]},{{10,9},1},{{10,10},f$2163[10,10]}}}

So, it gives a mess here. Either it uses some data format-specific
information in some non-obvious way (but then you had to give us a more
precise format specification for you unflattened list), - and then my
randomly generated data simply do not comply with your format, or your
function is  wrong.

Assuming constant partitioning by 3 sublists in the main unflattened list,
here is my attempt:

In[61]:==
 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[62]:==
a3 ==  Partition[Transpose[{a1fl[[All, {1, 2}]], a1fl[[All, 3]]}], 3]

Out[62]== {{{{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[63]:== a3 ====== a1

Out[63]== True

Regards,
Leonid


On Thu, Sep 30, 2010 at 12:52 PM, Valeri Astanoff <astanoff at gmail.com>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.
>
> --
> Valeri Astanoff
>
>



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