How to convert expression with Subscript to string

Question

I've tried ToString[], but it doesn't work properly with subscripts:

How to convert expr containing subscript to a string with exactly the same FullForm that "expr" has?

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UPDATE by xzczd

There actually exists a more general issue behind the specific problem above, that is, how to convert an expression containing 2D format into corresponding string that retains the format programmatically? For example, how to convert

{a^b + 1, 1.23*10^2, 2 Subscript[a, b]}
(* Please Press Ctrl+Shift+N to convert it to standard form *)

to

{"\!\(\*SuperscriptBox[\(a\), \(b\)]\)+1", "1.23\[Times]\!\(\*SuperscriptBox[\(10\), \(2\
\)]\)", "2\!\(\*SubscriptBox[\(a\), \(b\)]\)"}

The attempt is easy to achieve by hand: we just need to place quotes around the expression and place a \[Times] between 1.23 and 10^2, but how to do this with program? ToString won't work, as shown above.

Answer 1

how to convert a expression containing 2D format into corresponding string that retains the format programmatically? For example, how to convert

{a^b + 1, 1.23*10^2, 2 Subscript[a, b]}
(* Please Press Ctrl+Shift+N to convert it to standard form *)

to

{"\!\(\*SuperscriptBox[\(a\), \(b\)]\)+1", "1.23\[Times]\!\(\*SuperscriptBox[\(10\), \(2\
\)]\)", "2\!\(\*SubscriptBox[\(a\), \(b\)]\)"}

The easiest way is to prevent evaluation of the expressions what can be done with Unevaluated and HoldAll. First, define the function

toString = Function[expr, ToString[Unevaluated@expr, StandardForm], HoldAll];

Now you can simply apply this function to an expression:

You can Map it over the list for obtaining separate string for every expression in the list:

Or you can make this function Listable and it will thread over the lists automatically:

toString = Function[expr, ToString[Unevaluated@expr, StandardForm], {HoldAll, Listable}];

Answer 2

Obtaining the box form from FullForm[ToBoxes["t1"]] (that's t subscript 1) - the box form can be used in a function like so:-

make[x_, n_] := 
 ToExpression[
  StringJoin["\"\\!\\(\\*SubscriptBox[\\(", ToString[x], "\\), \\(", 
   ToString[n], "\\)]\\)\""]]

make[t, 1]

make[t, 1] == "t1" (* again, t subscript 1 *)

True

To convert a subscripted variable another function can be used:-

f[x_] := Module[{a, b},
  a = x[[1]];
  b = x[[2]];
  make[a, b]]

f[t1]

"t1"

To convert an expression such as the following more manipulations will be needed.

expr = s + Subscript[t, 1] + u (* Hold can be used to preserve the order *)

s + u + t1

The problem is a matter of manipulating the output from this ...

ToBoxes[expr]

RowBox[{"s", "+", "u", "+", SubscriptBox["t", "1"]}]

... to the output here:

ToBoxes["s+u+t1"] (* expr with t subscript 1 pasted into quotes*)

(* "\"s+u+\\!\\(\\*SubscriptBox[\\(t\\), \\(1\\)]\\)\"" *)

For this example the manipulation can be done like so:

z = ToBoxes[expr];

ToExpression[
 StringJoin["\"",
  First[z] /. SubscriptBox[a_, b_] :>
    StringJoin["\\!\\(\\*SubscriptBox[\\(", a, "\\), \\(", b, "\\)]\\)"],
  "\""]]

"s+u+t1"

The output here includes t subscript 1.