Here's a general workflow for blocking values of global variables while output is prepared. Before the output is returned, the global variables need to be protected from evaluation. HoldForm is the way to do this and have the variables be typeset in their usual format.

x = 3; y = 7; (* to see if x appears as x *)

(** hold variables to be used in formula/output **)
heldvars = Hold[x, y];
(* use stored variables in heldvars to Block their values *)
heldvars /. Hold[$v___] :> Block[{$v, res},
   (** possibly lengthy code to compute result `res` **)
   res = Column[{x''[t]/Sqrt[x + y] == 5 y}, Spacings -> 0.5];
   (* protect variables with HoldForm before returning *)
   res = res /. {var : Alternatives[$v] :> HoldForm[var]};
   (** format result **)
   TextCell[
    TraditionalForm@res,
    "Text", FontFamily -> "Palatino", FontSize -> 18]
   ]

The lines of code after the comments between the double asterisks (**...**) are user-supplied and would probably change from case to case. The lines of code after the comments between the single asterisks (*...*) should be the same in all use-cases (that I've thought of). One exception might be whether to block the variable res. It might be left global or it might be localized in a Module instead of in Block. Similarly, but more importantly, you shouldn't use $v as a variable in the computation of res, since it was used as a pattern name.

In fact, you can simplify it further by doing this:

Block[{x, y}, 
 TextCell[
  Column[{ToString[x''[t]/Sqrt[x + y] == 5 y , 
     TraditionalForm]  (*I want x to always be a symbol*)}, 
   Spacings -> 0.5] , "Text", FontFamily -> "Palatino", 
  FontSize -> 18]]

You do not need to do any detailed string manipulation and you can put in any arbitrarily complex expression.

Regards

Neil

Ernesto,

I think what you want is to do something like this:

Block[{x}, 
 TraditionalForm@
  TextCell[
   Column[{Row[{ToString[x, TraditionalForm], " = ", 
       5}]    (*I want x to always be a symbol*)}, Spacings -> 0.5], 
   "Text", FontFamily -> "Palatino", FontSize -> 18]]

You can see what the problem is by looking at the FullForm of your expression returned from the Block:

blk = Block[{x}, 
   TraditionalForm@
    TextCell[
     Column[{Row[{x, " = ", 
         5}]    (*I want x to always be a symbol*)}, Spacings -> 0.5],
      "Text", FontFamily -> "Palatino", FontSize -> 18]];

blk // FullForm

The Block clears the local value of x so it can be used as a symbol or reassigned locally as you want to do. However, after it is returned to the front end, it still has the variable form of x inside. This causes Mathematica to reevaluate it in the global context and substitute the value for x. The way to fix this is to "lock down" the symbol x so it doesn't get reevaluated. Since you only seem to want a string in the end that has the traditional form of the variable, I would use the local value of x (a symbol) and convert it to a string in traditional form. At this point it will no longer be reevaluated.

I hope this helps,

Neil

Excellent! Thanks!

So, just to make sure I don't go down the wrong trail, I have some clarifying questions:

• Did you consider using the SlideShow functionality?
• Is the Block expression in Eric.nb the way you usually edit content? Or is that expression itself actually computed from some other input?
• It's not clear to me whether your variables are assigned values in the global context. Could you just make all of your variables Protected? Or do you actually need some of your variables to have values in one context, but then you need to display an expression that uses those variables as raw symbols? I was assuming the latter, but I'm not seeing examples of such conflict (although I haven't gone through Test.nb exhaustively yet).
• Have you gone through the guides/tutorials in the documentation? For example:
  • http://reference.wolfram.com/language/workflowguide/CreatingDocumentsAndPresentations.html
  • http://reference.wolfram.com/language/workflowguide/CreatingAndOrganizingInterfaces.html
  • http://reference.wolfram.com/language/tutorial/CustomInterfaceConstructionOverview.html

Ernesto, I'm very confused. I've looked through Test.nb more thoroughly, and I don't see any reason why any of the textual content can't be just static content. I mean, you could programmatically assemble a notebook with the content, maybe piece together larger content from smaller snippets, but I can't see any reason why it can't just be something like static text cells with embedded cells for the mathematical expressions. I see that you have some controls for navigation, some TabViews and so forth, but none of that seems to depend on any of the symbols being used in the expository content.

I started trying to refactor your example (dDiv, dEx, etc), but when I started matching it up to the content in Test.nb, it just wasn't making any sense. What am I missing? I feel like I've just completely misunderstood your question here. It doesn't seem to have any bearing on the content in Test.nb.

cross-posted here: https://mathematica.stackexchange.com/questions/269308/trouble-understanding-the-local-variables-of-the-block-command

If in a Block its local variables have no value, I don't understand why they are not treated as symbols.

If the variable definitions in the Block have no OwnValues, then they are treated as symbols. This has nothing to do with Block. The problem is that the expression you're creating inside the Block will be outside the Block once the Block is done. Once it's outside the Block, the evaluator continues to do its evaluating, and when it sees the x it will look for OwnValues for that x.

If you're not going to need to evaluate/manipulate the expression later, then you don't need a raw symbol. You can use a string, or you can wrap the symbol in HoldForm. Why would that not work in your case?

Also, SymbolName might be a cleaner way to generate a string from the symbol.

Eric: “If the variable definitions in the Block have no OwnValues, then they are treated as symbols.”

This is valid if x is not global with value.

“You can use a string, or you can wrap the symbol in HoldForm. Why would that not work in your case? Also, SymbolName might be a cleaner way to generate a string from the symbol.”

I have at least 20 lines of code inside each Block that contain many mathematical symbols and I want them not to be evaluated if they are global, since they are use in expressions to read not to evaluate. I have hundreds of Blocks to use HoldForm or SymbolName. It would leave my code unreadable. I cannot use strings because the format of the variables is lost. My suggestion is that if x is a Block local variable and no value is assigned to it, this variable is treated as a symbol.

Ernesto

I appreciate your attention and kindness.