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Re: How to construct pure expressions

> x[
>    x1[
>       x11[
>          x111[ x1111, x1112, x1113],
>          x112[ x1121, x1122, x1123],
>          x113[ x1131, x1132, x1133]
>       ],


> In general, the list will have following properties:
> * All expressions has a head that starts with the letter x.
> * The number after x correspond to the position list of that element.  
> example, x211 is an element with position {2,1,1}.
> * The number of digits following x correspond to the level of that  
> For example,  x111, x211 are elements in level 3, x1 or x2 is in level  
> x is in level 0 because it has zero digits following x.

Hello Xah,

If you first make a dummy array that has the right structure, and then
replace all the entries and the heads with "x" of the indices, you
can do it with a short command.  Note that all the places where your
example wants x... are atomic, in that they're either bottom-level
entries or they are heads.  In Mathematica, this means we want to target
level {-1} and we want Heads->True.

Example for a 2 x 2 matrix, but slightly modifying the form of the x's
from your example, just as an illustration:

In[1]:= MapIndexed[x @@ DeleteCases[#2, 0] &,
          Table[Null, {3}, {3}], {-1}, Heads->True]
Out[1]= x[][x[1][x[1, 1], x[1, 2], x[1, 3]], x[2][x[2, 1], x[2, 2], x[2,  
>    x[3][x[3, 1], x[3, 2], x[3, 3]]]

That makes it clear what MapIndexed is doing, and we can then just
embellish it by replacing the "x @@ DeleteCases" business with a
pre-defined function that takes care of creating the "x" symbols:

MakeSymbol[base_String, {indices___, 0} | {indices___}] :=
  ToExpression @ StringJoin[base, ToString /@ {indices}]

IndexedExpression[base_String, dims:{___Integer?Positive}] :=
  MapIndexed[MakeSymbol["x", #2]&, Array[Null &, dims], {-1}, Heads->True]

(* I switched to using Array to make the code simpler given the way
your example inputs dimensions *)

Here's a test:

In[4]:= IndexedExpression["x", {3, 3, 3}]

Out[4]= x[x1[x11[x111, x112, x113], x12[x121, x122, x123], 
>     x13[x131, x132, x133]], x2[x21[x211, x212, x213], 
>     x22[x221, x222, x223], x23[x231, x232, x233]], 
>    x3[x31[x311, x312, x313], x32[x321, x322, x323], x33[x331, x332,  

Robby Villegas

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