Is it possible to Print expressions in reverse order?

Question

Let's say I'm debugging a program step by step and want to Print some expressions (using ShowIt, for example).

Is there a way to output the result of Print on top of already printed expressions instead of at the bottom?

Answer 1

This is admittedly messy, but something along these lines might work:

insertBelowEvaluationCell[expr_] := 
  (SelectionMove[EvaluationNotebook[], After, EvaluationCell]; 
   NotebookWrite[EvaluationNotebook[], Cell[BoxData@ToBoxes[expr], "Print"]])

This function moves the insertion point just below the evaluation cell before inserting the text or expression to be printed.

Let's test it:

insertBelowEvaluationCell /@ Range[10]

Problems:

• This messes with the insertion point in the notebook which can be modified interactively as well. Perhaps it's better to write the output into a separate notebook instead.

• It does not work in command line mode (without a front end).

• It's slow (noticeably slower than Print).

Answer 2

This is an arguably even messier solution than Szabolcs', and its performance isn't going to win any awards, but it has some (somewhat dubious) advantages:

1. It allows you to choose whatever target you want for the printing, putting things in an arbitrary notebook, and the output will appear at the current selection in that notebook;
2. After that, printing won't affect the current selection;
3. You can have many different targets for printing;
4. It provides another use for the seemingly useless CellTags feature!

The function in question creates a dummy cell that will eventually contain the output as a side-effect, and returns a closure that you call like Print (more or less; it only takes one argument) in order to direct output to that target cell. The function is here:

Pillsy`UpsideDownPrinter[nb_: InputNotebook[]] :=
 With[{
   tag = ToString@FileHash[
      StringToStream@ToString@{NotebookGet@nb, Date[]}]},
  NotebookWrite[nb, Cell["", CellTags -> tag]];
  Function[content,
   Module[{nbExpr = NotebookGet[nb], pos},
     pos = Position[nbExpr, Cell[___, CellTags -> tag, ___]];
     NotebookPut[
      MapAt[
       Replace[#,
         Cell[stuff_, opts___] :>
          Cell[CellGroup[{Cell[content], Cell[stuff]}], opts]] &,
       nbExpr,
       pos],
      nb]];]]

First, NotebookWrite is used to create a Cell in the target notebook with a unique CellTag (which is what all the FileHash stuff is for), and then we continually modify the target notebook's structure using standard Mathematica structural operations. The stuff with CellGroup allows us to effectively create a "linked list" of generated cells in a (probably futile) stab at maintaining some degree of efficiency.

You use it like so:

printer = Pillsy`UpsideDownPrinter[];

printer["foo!"]; printer["bar!"]; printer["baz!"];

which will give output that looks like

"baz!"
"bar!"
"foo!"