5
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I have an "internal" notation (since some stuff is equivalent and can be manipulated via patterned expressions in the function F), say

F[c,3,b,2,a,3,c,1]

But for output I'd like the easier readable $c_3b_2a_3c_1$. Happily, I enter

G[] := "";
G[X___, Y_, i_] := "G[X]" <> "Subscript[Y,i]";

Epic fail. Even in a function definition I can't match patterns inside a string since G[X] is exactly that - X is no variable on the RHS here.

G[] := Nothing; (* or something like that *)
G[X___, Y_, i_] := G[X]Subscript[Y,i];

would be a clever hack but alas, the implied multiplication is commutative and ruins the (fixed) ordering. Frankly, even if I would replace the implicit * with a ** and the latter could be printed invisible that would be horrible style.

G[] := "";
G[X___, Y_, i_] := ToString[G[X]] <> ToString[Subscript[Y, i]];

Nice try but no.

How would you do it? I don't care if the output is a string or an expression, it suffices if it looks like above. (Also note that my output is a list of F expressions, so I will map a head replace F->G over the list and define G separately.)

EDIT:

G[] := "";
G[X___, Y_] := ToString[G[X]] <> ToString[Y];

avoiding the subscript trouble gets 4 of 5 points, if my task is impossible.

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Something equivalent to what you tried to do can be done by defining the output format for F:

Format[F[x__], TraditionalForm] := formatF[{}, x] /; EvenQ[Length[{x}]];
formatF[list_List, a_, b_, x___] := formatF[Append[list, Subscript[a, b]], x];
formatF[list_List] := Row[list];

Then, in the traditional form,

enter image description here

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