Re: Incompatible units?
- To: mathgroup at smc.vnet.net
- Subject: [mg97846] Re: Incompatible units?
- From: Mathias Lindner <mathias.lindner at web.de>
- Date: Tue, 24 Mar 2009 05:27:24 -0500 (EST)
- Organization: Chemnitz University of Technology
- References: <firstname.lastname@example.org> <email@example.com>
Sjoerd C. de Vries schrieb: > Hi Mathias, > > Your example is a bit incomplete, or so it seems. What are the > definitions of Tau p and l case? Yes, I know. It wasn't the complete calculation. That is because the complete one is endless ;) You helped me anyway... > > And what is the slash in \Convert doing? And why are you using Convert > for this particular conversion? The division of the two variables is > dimensionless, so no need for Convert. No idea about the slash. In StandardForm I typed Convert. Must have been input during the copy&paste into my newsreader. I use Convert very often and it's annoying. As an example: The result of the multiplication of a frequency and a time has to be dimensionless. Anyway, the shown result is always in Hertz Second. Only with Convert[...,1] the units are avoided. The same applies for Watt/(Volt Amp), Kilo/Milli etc. Any idea what goes wrong here? > > Anyways, your problem is not the Units package, but the use of > Dynamic. Dynamic is a wrapper, much like InputForm or TableForm. It's > used for display in the FrontEnd, but not for calculations in the > kernel. Try for instance this definition: > > g[x_Numeric] := x^2 > > Now enter g[Dynamic[p]]. It displays as g[p]. Now set p to 5 (p=5). > You see the old g[p] change to g and not to 25. This is because the > function doesn't see the number 5, but the object Dynamic[p] which > only displays as 5 in the FrontEnd. It does not represent the value 5 > to the kernel. > > The same happens with your conversion. Because the units are hidden > within the Dynamic wrapper they can't be simplified by the kernel. The > error message is displayed by the FrontEnd and this exposes the units, > but (it should be clear) this is not what the kernel sees! > > Read more about this in the section "Where Should Dynamic Be Placed in > an Expression?" in the tutorial tutorial/IntroductionToDynamic. Thanks for pointing me to this tutorial. After altering the position of Dynamic in this equation Mathematica works as expected. > > Cheers -- Sjoerd Also thanks to Bill Rowe! Avoiding subscripts didn't help in my situation but it's a good advice anyway. I will follow it. Regards, Mathias > > > > On Mar 19, 12:25 pm, Mathias Lindner <mathias.lind... at web.de> wrote: >> Hi >> >> 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[Subscri= > pt[\[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? >> >> Thanks for your help! >> >> Mathias > >