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Re: How to unflatten an array ?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg112851] Re: How to unflatten an array ?
  • From: Valeri Astanoff <astanoff at gmail.com>
  • Date: Sun, 3 Oct 2010 05:40:58 -0400 (EDT)
  • References: <i84aed$h0e$1@smc.vnet.net> <i86uvo$hpk$1@smc.vnet.net> <i89btu$jf6$1@smc.vnet.net>

On 3 oct, 09:38, Ray Koopman <koop... at sfu.ca> wrote:
> On Oct 2, 2:45 am, Valeri Astanoff <astan... at gmail.com> wrote:
>
>
>
> > Good day,
>
> > I have to apologize for not being very clear about what I wanted.
> > Seems that you, Bob, were the only one to understand what I meant.
> > [I had to replace SplitBy with Split cause I only have version 6]
> > Any way I have to thank everyone who tried and help me.
> > Maybe this test will more explicit than my poor previous example :
> > [...]
>
> I think I understand now what you're doing.
>
> In[1]:= Dimensions[a1 = Table[{{x,y}, RandomInteger[99]},
>                               {x, 0, 99}, {=
y, 0, 99}]]
> Out[1]= {100, 100, 2}
>
> In[2]:= Dimensions[a2 = Flatten /@ Flatten[a1, 1]]
>
> Out[2]= {10000, 3}
>
> In[3]:= 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[4]:= {Timing@Dimensions[a3 = unflatten[a2]], a3 == a1}
>
> Out[4]= {{1.24, {100, 100, 2}}, True}
>
> In[5]:= unflattenBob[arr : {{_, _, _} ..}] := Split[
>          {Most[#],Last[#]}& /@ arr, #1[[1,1]] == #2[[1,1]] =
&]
>
> In[6]:= {Timing@Dimensions[a3 = unflattenBob[a2]], a3 == a1}
>
> Out[6]= {{0.12, {100, 100, 2}}, True}
>
> In[7]:= unflattenRay[arr_] := Partition[Transpose@
>          {arr[[All,{1,2}]],arr[[All,3]]},Length@Union@arr[[All,=
2]]]
>
> In[8]:= {Timing@Dimensions[a3 = unflattenRay[a2]], a3 == a1}
>
> Out[8]= {{0.02, {100, 100, 2}}, True}

Good day,

My "unflatten" was a tortoise, your own is a hare !
This is going to be very useful.
Many thanks (to you and other posters).
--
Valeri


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