Re: Units, simplification
- To: mathgroup at smc.vnet.net
- Subject: [mg59818] Re: Units, simplification
- From: "John Jowett" <John.Jowett at cern.ch>
- Date: Tue, 23 Aug 2005 04:51:13 -0400 (EDT)
- Organization: CERN - European Laboratory for Particle Physics
- References: <ddh9a8$cde$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
Yes, I should probably mention that the ToFundamentalSI function in our
Accelerator`ConstantsUnits` package was also inspired by inspecting the
standard package and finding the $ToFundamental rules. Rather than
modifying the standard package, we made a new package since we added a lot
of other things too.
However in the context of this discussion, it's worth mentioning that you
can easily make your own version of a standard package that will still be
there when you upgrade to the next version of Mathematica. I do this,
e.g., for Graphics`Colors` to avoid clashes with the ChemicalElements
package and define all colours as instances of RGBColor.
To do this, just create a directory Graphics in one of the directories that
comes earlier in $Path than the standard packages and put the modified
package file (Colors.m in this case) there. A sensible choice is
ToFileName[{$BaseDirectory, "Applications"}]
or, perhaps,
ToFileName[{$UserBaseDirectory, "Applications"}]
Then this version will override the standard version and there is no need to
touch anything inside the installation directories.
John Jowett
"Bill Rowe" <readnewsciv at earthlink.net> wrote in message
news:ddh9a8$cde$1 at smc.vnet.net...
> On 8/11/05 at 4:54 AM, not_valid at w.pl (KaeL) wrote:
>
>>I admit I am not too experienced mit M. (v. 5.1). My problems
>>regards simplification of unit equations. Consider such a simple
>>entry:
>
>>Simplify[Ampere Meter Meter/Joule Tesla]
>
>>Explanation for non-physicist: this is equal to 1 Usage of
>>"<<Miscellaneous`Units`" does not help here, as this package is
>>rather _conversion_ (SI into cgs, .i.e Tesla into Gauss) oriented.
>>Running "<<Miscellaneous`PhysicalConstants`" does not help either,
>>as this package rather _introduces/defines_ the units. And you
>>would need here sort of relation between units-am I right?
>
>>Any idea how to solve my problem?
>
> Yes. The way I have solved this issue for myself is to edit the Units.m
> package by adding the following two lines:
>
> FundamentalSI::usage = "FundamentalSI[expr] converts expr to fundamental
> SI units (International System)."
>
> and
>
> FundamentalSI[expr_]:=SI[expr]//.$ToFundamental
>
> These two lines define a new function that does what you want. The first
> line should be added to the section of the file that defines usage
> messages. I've put it just ahead of the usage message for SI.
>
> The second line defines the function and needs to go somewhere in the
> Private context. Like the other line, I added it just ahead of the
> definintion for SI[expr_]
>
> Once these two lines are added the following works
>
> In[1]:=
> << "Miscellaneous`Units`";
> FundamentalSI[Ampere Meter Meter/Joule Tesla]
>
> Out[2]=
> 1
>
> Note if you always want these set of fundamental units simply adding
> //.$Fundamental on the end of the definition for SI[expr_] will make SI
> behave in this manner.
>
> There are a couple of consequences of solving the issue in this manner.
>
> I have modified one of the standard distribution packages. So, every
> upgrade to Mathematica will overwrite these changes and you will lose this
> behaviour until you edit the new Units.m file.
>
> Since I have not modified the stub that gets called when doing
> <<Miscellaneous` this new function will not be loaded. That is, to get
> this new functionality as I have modified things you must explictly load
> the Miscellaneous`Units` package using Needs, Get or <<.
>
> Of course this last issue would disapper if you also edit the stub or
> modify the definition for SI[expr_]
>
> There is undoubtedly a way to do this without modifying the Units.m
> package. But since this functionality already exists in the package this
> seemed the most elegant solution to me.
> --
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