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MathGroup Archive 2001

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Re: Extracting units from a list of values

  • To: mathgroup at smc.vnet.net
  • Subject: [mg27420] Re: Extracting units from a list of values
  • From: "Allan Hayes" <hay at haystack.demon.co.uk>
  • Date: Sun, 25 Feb 2001 00:53:47 -0500 (EST)
  • References: <9750ck$6ke@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Thomas,

{5, 2 Pi, 3 E Meter^2} /. {_?NumericQ:>1,  _?NumericQ * unit_->unit}

        {1, 1, Meter^2}

--
Allan
---------------------
Allan Hayes
Mathematica Training and Consulting
Leicester UK
www.haystack.demon.co.uk
hay at haystack.demon.co.uk
Voice: +44 (0)116 271 4198
Fax: +44 (0)870 164 0565

"Thomas Anderson" <tga at stanford.edu> wrote in message
news:9750ck$6ke at smc.vnet.net...
> As part of the package I'm working on, one of the functions takes
> a list of measured values as an argument. For maximum flexibility,
> I wish to accept either a list of dimensionless numbers or a list
> of numbers with units, i.e.
>
>    {1, 2, 3}
> or
>    {1 Meter, 2 Meter, 3 Meter}
>
> for example. As part of the argument checking, I want to be able
> to test whether the units are consistent: everything should either
> be dimensionless or have the same units.
>
> The problem boils down to: how can I separate the units from the
> numeric part of the value? I've tried a few things, and so far my
> best attempt has been
>
>    Replace[vals, (_?NumericQ unit_.) :> unit, 1]
>
> where vals is the list of values. This works pretty well:
> dimensionless numbers give a list of "units" of 1, and values like
> "2 Meter" or "1 Elephant^2" give "Meter" and "Elephant^2" respectively.
>
> This method doesn't work, however, with input containing more
> complicated numerical values. For example, {1, 2 Pi, 3 E Meter^2} gets
> transformed into {1, 2, 3 Meter^2}, whereas I want {1, 1, Meter^2} for
> these values. I could apply N[] to the values before extracting the
> units, but then "Meter^2" becomes "Meter^2.", which I don't want.
>
> This isn't a huge problem, since I'm expecting the values to be
> integers
> or real numbers, but I want my code to be as bulletproof as possible.
>
> Thanks in advance for any suggestions.
>
> -Tom Anderson
>  tga at stanford.edu
>
>




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