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Re: "?" and "??"
*To*: mathgroup at christensen.cybernetics.net
*Subject*: [mg1774] Re: "?" and "??"
*From*: pehowland at taz.dra.hmg.gb (Paul E. Howland)
*Date*: Wed, 26 Jul 1995 00:57:21 -0400
*Organization*: Defence Research Agency
In article <DBKwDG.2I2 at wri.com>, Scott.A.Hill at williams.edu (Lancelot) writes:
> Also, I'm wondering how I can save several commands as a
>package. I'm running Mathematica off of a UNIX machine, and I've
>figured out how to save programs to my UNIX account and then call them
>back in as needed. Perhaps that is the same thing, eh? Anyway, if
>someone could shed light on how I can define such a file as a
>"package", I'd be interested.
The difference between a package and saving & recalling programs, is that a
package is usually a set of function definitions which extend the capabilities
of Mathematica, where as your program is probably a set of commands that
actually calculate something.
Loading a package doesn't cause Mathematica to necessarily calculate anything,
it just increases its "knowledge" in a certain area. When writing a package,
you normally wrap BeginPackage["mycontext`"] and EndPackage[] around your
function definitions. This causes the active contexts to be only mycontext`
and System`. This prevents you redefining functions defined in other packages.
I thoroughly recommend that you read "Programming in Mathematica" by Roman
Maeder, Addison Wesley, 1991, ISBN-0-201-54877-1. This talks in detail about
writing packages, and includes a skeleton package which shows the basic
commands required to write a package.
A simple package would have a structure something like:
BeginPackage["Example`"]
fn1::usage = "This explains how to use fn1, and also introduces
fn1 outside the Private` context"
Begin["`Private`"]
fn1[a_,b_,c_] := Module[{e}, e=a+b; c+e]
fn2[a_] := Module[{}, Print["This function is in the private
context & hence can't be called from outside it"]]
End[]
EndPackage[]
Roman's book gives more detail about what happens with the ContextPath, how to
protect symbols, etc.
Time spent learning to write your own packages is invaluable. It turns
Mathematica from a sophisticated pocket calculator into an powerful, extensible
computational tool. You can then extend Mathematica to specialise in your own
field of work, although it is worth re-iterating the point that always use
Mathematica's built in functions whenever possible, as these will be
much faster than any package you write.
Hope this helps,
Paul E Howland
Long Range Ground Radar Systems Section tel. +44 (0)1684 895767
LSC2 Division, Room BY209 fax. +44 (0)1684 896315
Defence Research Agency email: PEHOWLAND at DRA.HMG.GB
Malvern, Worcs, WR14 3PS, UK.
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