package import problem in mathematica

Question

In mathematica (I am using mma 5.0 ( guess pretty old)), if I type the following as one line:

Needs[\"Graphics`Master`\"]; Animate[Plot[Sin[n x], {x, 0, 2 Pi         


        

Answer 1

As belisarius points out, your question as it stands is a bit v5-centric. The problem, however, still exists in current versions. As an example

Needs["Combinatorica`"]
ToCycles[{3, 4, 1, 2}]

works fine, while (after restarting the kernel),

Needs["Combinatorica`"]; ToCycles[{3, 4, 1, 2}]

fails with an error that

"ToCycles::shdw: Symbol ToCycles appears in multiple contexts {Combinatorica`,Global`}; definitions in context Combinatorica` may shadow or be shadowed by other definitions."

In Mathematica terms, the reason the one-liner doesn't work is that Mathematica tries to resolve all symbols in the line before evaluating Needs (this was a surprise to me). This resolves ToCycles to Global`ToCycles (thus entering this symbol in the symbol table), before Needs gets a chance to load the definition of Combinatorica`ToCycles and add Combinatorica to the $ContextPath. To make the one-liner work, you must use the full name of ToCyles:

Needs["Combinatorica`"]; Combinatorica`ToCycles[{3, 4, 1, 2}]

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To understand the error, you need to know that all Symbols in Mathematica have a full name of the form context`name. A context is similar to a namespace in many other languages. Now, if a symbol (such as ToCycles) is referenced without a context, Mathematica will look through the contexts currently in $ContextPath and see if the symbol is defined in any of those contexts. If not, the symbol is resolved in the current context, $Context which is Global in normal use.
When you load a package, the symbols of that package are defined in a package context (e.g. Combinatorica), and when the package is fully loaded this context is added to the $ContextPath so that you can access the symbols by their short name.
Now, you can see what the error means: Since the Combinatorica has not yet been loaded when the symbols are resolved, ToCycles resolves to Global`ToCycles. After the package loads, Mathematica helpfully checks that all short names are unique, and finds in this case that the short name ToCycles is now defined in two contexts on $ContextPath one thus "shadowing" the other. To refer to a specific of these symbols, you must use the full name, e.g. Combinatorica`ToCycles.

To resolve a shadow conflict, simply Remove the unwanted symbol:

Remove[Global`ToCycles]

Don't know how readable this was, but hope it helps a bit...