# How to define a global variable in Mathematica?

I have restricted the context of my notebooks to each individual notebook. So variables in each notebook are local and are not seen in another notebook.

But in two of my notebooks I have two functions that I want to plot simultaneously with the command Show. Please note that I want variables of each notebook to be local except for those two functions. So I need to define two global variables and then plot that two functions in Show. How can I define global variables in Mathematica?

-

You can explicitly define variables in the global context by prefixing their name with Global, for example, Globali = 3 or Globalf[x_]:=x^2. However if you have set the notebook to have private context, you don't have Global in your $ContextPath (in order to prevent interference from other notebooks with non-private context). Therefore in your notebooks the context Global isn't really special, you can use any context, like myShared. Indeed, that's desirable because that way you know that a third notebook will be unlikely to interfere. So you would write in your first notebook mySharedfunction[x_]:=x^2  and then could access that function from the other notebook as mySharedfunction[3] (* ==> 9 *)  Note that you can add your context to the context path by using AppendTo[$ContextPath, "myShared"]


This then allows you to refer to the function above without the prefix, e.g.

function[3]
(*
==> 9
*)


However note that with this, your local symbols may shadow the shared ones (however access as mySharedfunction still works even then), so you have to make sure that you don't use the unprefixed symbol in any way before the prefixed one was created.

You might consider avoiding the issue by using PrependTo instead of AppendTo, but then you are vulnerable to variable injection (including accidental one) from the other notebook. For example, imagine that you have the definitions

a = 3;
PrependTo[$ContextPath, "myShared"]  and then you do in your other notebook Begin["myShared"] function[a_] := a^2 other[a_] := 5 End[]  Let's assume the symbol a had not yet been used in that notebook, then it is created, together with the symbols function and other, in the context myShared, and therefore in your first notebook now hides the local definition of a. That is, if you now evaluate a in your first notebook, the kernel will find myShareda (without a value) first, and therefore use that instead of the local a; of course it won't evaluate to 3. Yet another way to access the symbol without prefix would be the definition function := mySharedfunction  which however only works in a context with evaluation. Especially it should not be used for shared variables because after variable := mySharedvariable  a subsequent assignment like variable = 42  does not change the shared variable, but only the local one (which no longer refers to the shared one). Therefore I think it is a better idea to not do either, but always use the prefixed version. Note that the PrependTo scenario is also an argument against using Begin["myShared"]End[] to simplify definitions in this case: It's too easy to accidentally introduce new symbols in that context which were not intended to be there. Note that another way of sharing functions is to make them into a package and use that package from both notebooks. Ultimately this also boils down to having a shared context, as rcollyer noted in the comments. However a properly written package protects against or at least warns about most problems with hiding. Of course, writing a package might be overkill for your specific situation. - As written, mySharedfunction[a_]:=a^2 is not vulnerable as a is localized due to SetDelayed having the HoldAll attribute. – rcollyer May 24 '12 at 14:28 @rcollyer: No, it is vulnerable because the mere creation of that symbol already hides the local one. And yes, I've actually tested it. After defining the function in notebook 2, a in notebook 1 will no longer evaluate to 3, not because the value of notebook1uniquecontexta has been overridden (that one still has the value 3 associated with it) but because that definition is hidden by the symbol myShareda. Yes, that's a non-obvious effect, and exactly for that reason you should avoid it. – celtschk May 24 '12 at 14:34 Ah, I was executing it the same notebook and wasn't seeing the shadowing for what it was. My apologies. (Incidentally, this is simpler to demonstrate in a single notebook using cell grouping as the default.) That makes for a reasonable argument to 1. be careful with globals, in general, and 2. have the global context be last in $ContextPath to minimize its effects, i.e AppendTo v. PrependTo. I'm disappointed, though, that there isn't a shadowing warning raised. –  rcollyer May 24 '12 at 14:43
@rcollyer: It can't raise a shadowing warning because in that notebook there's no other accessible symbol to be shadowed. If there were, the new symbol wouldn't have been created to begin with. –  celtschk May 24 '12 at 14:49
So, by using PrependTo, you circumvent any checking capability, also. Well, that seals the case, then. Prepend for globals is bad. –  rcollyer May 24 '12 at 14:56