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Re: Incompatible units?

  • To: mathgroup at
  • Subject: [mg97752] Re: Incompatible units?
  • From: Bill Rowe <readnews at>
  • Date: Fri, 20 Mar 2009 02:42:02 -0500 (EST)

On 3/19/09 at 5:25 AM, mathias.lindner at (Mathias Lindner)

>Today I struggled with the units package - again...

>Let me show you what I did:

>Dynamic[Subscript[\[Tau], p]]
>Subscript[l, case]
>\[Lambda] = \Convert[Subscript[l, case]/Dynamic[Subscript[\[Tau], p]], 1]

>This input results in:

>27.4889 Meter Milli
>281 Meter Milli

>Convert::incomp : Incompatible units in (281 Meter Milli)/(27.4889
>Meter Milli) and 1. (281 Meter Milli)/(27.4889 Meter Milli)

>If I don't use dynamic but constant tau_p everything is fine... So
>Mathematica isn't able to to reduce this fraction? What's the reason
>for this behaviour and what can I do about it?

I interpret your "everything is fine" to mean you got the result
you expected/wanted. And it is not at all obvious to me why you
are really attempting to do.

It appears you want to use a subscripted variable. While this
can be done in Mathematica using the Notations package, it is
not as simple as just setting Subscript[\[Tau], p] to some
value. Mathematica sees things with Head Subscript as totally
different than things with Head Symbol. And it is things with
Head Symbol Mathematica treats as variables that can be assigned
values. That is, unless you go to a bit of trouble, Mathematica
does not work as you might expect with subscripts. I strongly
suspect this is the issue you are encountering rather than a
specific issue with the units package.

Try your code using simple symbols without the subscripts. If
this works and you really want subscripted variables, learn how
to use the Notations package.

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