Robust line numbers in Mathematica notebook

Question

Is there any way to reference a particular output in Mathematica notebook robustly?

The usual way we refer a particular output is by invoking the command %n where n is the number that is alloted to a particular result based on the order of evaluation. If I re-run the same input, the output reference gets modified to n+1, so one has to refer to as %n+1. Is there a way to refer to particular output by some way that is not susceptible to this evaluation scheme. I know that just assigning a name will do the job. But lets say, I don't want to and just want to do something similar to % yet robust enough that it doesn't change even if I evaluate it again.

Workflow sample :

You can see that output nos 150,151,159 are referenced. Now, if I modify input 150 say, I cannot re-run all the subsequent operations without changing all the output reference nos. In a much bigger project that demands a lot of experimentation, you can see I how frustrating this can be. Given that I don't want to use Set as I have too many things to start naming myself, I thus want a way of referencing the outputs that is somehow just positional.

Answer 1

You can try the following (evaluate the code once at the beginning of your session):

SetOptions[EvaluationNotebook[],
 StyleDefinitions -> Notebook[
   {
    Cell[StyleData[StyleDefinitions -> "Default.nb"]],
    Cell[
     StyleData["Input"],
     ShowCellTags -> True,
     CellProlog :>
      ($Line = PersistentIDs`curLine=
        CurrentValue[EvaluationCell[], {TaggingRules, "ID"}] /. 
          Inherited :> (CurrentValue[EvaluationCell[], {TaggingRules, "ID"}] = 
            Last[DownValues[Out], {{{0}}}][[1, 1, 1]] + 1)),
     CellEpilog :> SetOptions[
       EvaluationCell[],
       CellLabel -> StringTemplate["In[``]:="]@PersistentIDs`curLine
     ]
    ]
   }
  ]
]

Essentially, this freezes the line numbers of any cell evaluated after this point. As the complete state is stored in StyleDefinitions and TaggingRules, this should persist across different sessions, without needing to reevaluate anything.

To unfreeze the line numbers, go select Format>Stylesheet>Default.

Installing

If you intend to use this frequently, you can install the stylesheet for easier reuse. To do so, execute the above code, go to Format>Edit Stylesheet.... Then, go to File>Install, select "Stylesheet" and the notebook that just opened (most likely called Untitled-1 or so) and give it a name. Now, you can apply this to any notebook by choosing Format>Stylesheet>"Name you entered"

How it works

This works by defining CellProlog and CellEpilog of all input cells to do the following (via the stylesheet):

• Before the cell is evaluated: Set $Line to the following (also, save this value in curLine for later):
  • If the Cell is tagged with "ID" (i.e. was already executed), this value
  • Otherwise, take the highest defined Out value +1. Also, assign this value to {TaggingRules,"ID"}
• After the cell is evaluated: Set the CellLabel to curLine (we need to do this in CellEpilog as it gets overridden after CellProlog