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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: [mg27404] Re: Extracting units from a list of values
  • From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
  • Date: Sun, 25 Feb 2001 00:53:27 -0500 (EST)
  • Organization: Universitaet Leipzig
  • References: <9750ck$6ke@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hi,

it is a well known fact in theoretical physics, that all
units are equal to 1. 

You should definitly transform the expression to dimensionless
quatities. 

Is the division broken on your machine ? otherwise

expressionWithUnits/(expressionWithUnits /. Meter -> 1)

will do what you whant

Regards
  Jens


Thomas Anderson wrote:
> 
> 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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