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Re: Is it possible to flatten hash table?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg124174] Re: Is it possible to flatten hash table?
  • From: "Oleksandr Rasputinov" <oleksandr_rasputinov at hmamail.com>
  • Date: Wed, 11 Jan 2012 17:21:18 -0500 (EST)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <jejk6c$as0$1@smc.vnet.net>

On Wed, 11 Jan 2012 09:19:08 -0000, Szymon Roziewski  
<szymon.roziewski at gmail.com> wrote:

> Hello,
> I would like to create a hash table for some data.
>
> MakeHash[lon_, lat_, depth_] := Module[{Arr},
>    Arr[lon <> " " <> lat] = depth;
>    Arr
>    ];
>
> BathHashT =
>   MakeHash[ToString[Rnd[#1, 6]], ToString[Rnd[#2, 6]], #3] & @@ # & /@
>     Take[SeaBathCutEnd, 4];
>
> Res = Flatten@BathHashT;
>
> Res["-2.966667 -2.000000"]
>
> As a result of Flatten I get  {Arr$2002701, Arr$2002702, Arr$2002703,
>   Arr$2002704}["-2.966667 -2.000000"],
>
> The function Rnd is to round a float number to 6 decimal digits.
>
> So it seems to be 4 hash table. For each hash there is one table.
> I thought that it could be flatten to one hash containing all data.
> I could do  it in for loop but due to its poor perfmormance I wanted to
> avoid that.

This is the result of Module, which creates a new symbol, Arr$<number>, on  
every invocation. Your code is incomplete because Rnd and SeaBathCutEnd  
are not defined so I am not quite sure what behaviour you see and what  
result you are expecting; however, if you can be certain that there will  
not be any collisions, you can re-use the same symbol:

MakeHash[lon_, lat_, depth_] := (Arr[lon <> " " <> lat] = depth; Arr)

To answer the question directly, it is possible to combine DownValues from  
several symbols as follows:

inputs = MapAt[
  ToString /@ # &,
  RandomInteger[{1000, 2000}, {3, 10}],
  {{1}, {2}}
];

hashes = MapThread[MakeHash, inputs];

DownValues[Arr] =
  Flatten[DownValues /@ hashes] /.
   Verbatim[HoldPattern][sym_[key__]] :>
    HoldPattern[Arr[key]];

(* inputs[[All, 1]] == {"1796", "1556", 1486} *)

In :=
Arr["1796 1556"]

Out =
1486

However, I would not recommend this method over simply avoiding the  
creation of multiple symbols in the first place.



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