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MathGroup Archive 2006

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Re: structure array equivalent in Mathematica

  • To: mathgroup at smc.vnet.net
  • Subject: [mg67229] Re: structure array equivalent in Mathematica
  • From: Rolf.Mertig at gmail.com
  • Date: Tue, 13 Jun 2006 01:07:30 -0400 (EDT)
  • References: <e6gdun$ngj$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Here are two possibilities to do what you want:

(* by using a datatype identitifier the order of data can be arbitrary:
*)
observation = {
  DataType["pressure"][10.], DataType["temperature"][20.],
DataType["water_vapor"][30.]
}

(* one possibility to define the "method" - functions : *)
pressure    =
Function[x,Cases[x,DataType["pressure"][_?NumberQ],-1]/.DataType[_][y_]:>y/.{z_}:>z];
temperature =
Function[x,Cases[x,DataType["temperature"][_?NumberQ],-1]/.DataType[_][y_]:>y/.{z_}:>z];
watervapor  =
Function[x,Cases[x,DataType["water_vapor"][_?NumberQ],-1]/.DataType[_][y_]:>y/.{z_}:>z];

{
pressure @ observation
,
temperature @ observation
,
watervapor @ observation
}

data["obs_day"] = {observation, observation, observation};

pressure @ data["obs_day"][[1]]

(* this will also work, by design: *)
pressure @ data["obs_day"]

(* ********************** *)

(* for a lot of data it is more efficient to represent the different
data types by position only: *)
observation2 = {10., 20., 30.};

{
pressure2     = Function[obs, obs[[1]]];
,
temperature2  = Function[obs, obs[[2]]];
,
watervapor2   = Function[obs, obs[[3]]];
}

pressure2 @ observation2
temperature2 @ observation2
watervapor2 @ observation2

data2["obs_day"] = {observation2, observation2, observation2};
pressure2 @ data2["obs_day"][[1]]

---

Regards,

Rolf Mertig
http://www.gluonvision.com
GluonVision GmbH
Berlin

kevin_jazz wrote:
> Like many people I imagine, I'm transitioning to Mathematica from a
> background in another system.
> One of the common data types is the structure array.  Let's say I have
> an observational data set that includes pressure, temperature, and
> water vapor as a function of altitude.  So, in pseudo-code I might
> define a structure as
>
> observation = {pressure: float(100), temperature: float(100),
> water_vapor: float(100)}
>
> I could then access the elements of this observation as
>
> observation.pressure
> observation.temperature, etc.
>
> Furthermore, I could aggregate these observations into a larger list, e.g.
> obs_day = {observation, observation, observation}
> to be accessed as
> obs_day[1].pressure for the first element (assuming 1-index).
>
> Now, the list in Mathematica is quite powerful and I think can be
> set-up in a similar fashion.
>
> So my question is how is the structure array commonly implemented in
> Mathematica or its equivalent?
>
> If there is a previous thread (I looked but didn't find any) on the
> topic or in the Mathematica book or Mathematica Journal that I missed,
> feel free to point me in that direction.
> 
> Many thanks,
> 
> Kevin Bowman
> Jet Propulsion Laboratory


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