Adding a label to an expression result

Question

To make my notebook easier to read for non-Mathematica literate colleagues (myself included), I'd like to have the output from an assignment expression look somewhat more obvious. Normally an expression assignment would look something like this:

In[1]= myVariable = 1 + 1

Out[1]= 2

But I'd like to get an output that looks more like:

Out[1]= myVariable = 2

I'm sure there are very convoluted ways to get this as an output but is there some really simple thing I can do so that the input is also still fairly easily readable without a heap of other commands wrapped around it?

Answer 1

I'll assume that you want to echo any literal Set operation that occurs in input, even if it is not on a line by itself.

$Pre

This may work for you:

$Pre =
 Function[
   main,
   Unevaluated[main] /. Set -> Function[, Print@HoldForm[# = #2]; # = #2, HoldFirst],
   HoldAll
 ];

Now:

{a = 2 + 2, b = 10/2, c = Sqrt[9]};

a = 4

b = 5

c = 3

{a, b, c}

{4, 5, 3}

---

echo function

Alternatively, since I cannot imagine the echo being practical for every Set you might do it like this:

$Pre =.  (* clear the prior definition for $Pre *)

echo =
 Function[
   main,
   Unevaluated[main] /. Set -> Function[, Print@HoldForm[# = #2]; # = #2, HoldFirst],
   HoldAll
 ];

Then:

{a = 2 + 2, b = 10/2, c = Sqrt[9]} // echo

---

CellEvaluationFunction

If you want to echo some but not all Sets, and you do not want to have // echo appear, you could perhaps use CellEvaluationFunction for individual Cells.

As an example, this code generates a new input Cell into which any Set that is typed will be echoed:

CellPrint[ExpressionCell[Placeholder[], "Input", CellEvaluationFunction ->
  (ReleaseHold[
     MakeExpression@# /. 
      Set -> Function[, Print@HoldForm[# = #2]; # = #2, HoldFirst]
   ] &)
]]

The cell that is created can be copied and pasted to make more, or you can use the Option Inspector to set the value of CellEvaluationFunction for existing cells.

Answer 2

Presuming you only want to this special output to come from computations that bind a variable to the value of the computation, here is one way it can be done by $Pre and $Post:

SetAttributes[saveSet, HoldAll];
saveSet[form : Set[var_, _]] := (lastSet = ToString@Unevaluated@var; form);
saveSet[form : ___] := (lastSet =.; form)

$Pre = saveSet;

$Post = (If[ValueQ@lastSet, Row[{lastSet, " = ", #}], #]) &;

After these definitions are evaluated, computations using Set (=) show up as:

y = 42^2 + 1

y = 1765

but other expression evaluations will printout normally:

x == y

False

The downside of this is that, when these definitions are in effect, % becomes unusable after a Set evaluation. Condsider

y = 42^2 + 1

y = 1765

% - 1

-1+y = 1765

Answer 3

I think the simplest and most flexible method is to just add print statements before each operation that you want to label:

Print["myVariable ="];
myVariable=1+1

myVariable =

2

Print["{M, r, T} ="];
{m=10,r=6/2,t=1/2}

{M, r, T} =

{10,3,1/2}

This method allows you to label expressions without the limitations of variable naming in Mathematica. For example, the following uses an ASCII variable name for a stress tensor component but labels it with the Greek characters and subscripts:

Print["Subscript[\[Tau], \[Phi],\[Theta]] ="];
shearPhiTheta=m/(2 \[Pi] *r^2*t)

$\tau _{\phi ,\theta }\text{ =}$

$\frac{10}{9 \pi }$