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In Mathematica, I am doing computations involving polynomials with variables like

p[0,1,2]

In the end, I will need to put these equations in to another program. This other program does not allow brackets or commas in variable names. So, I need to send the above variable to

p012

Ultimately, I need my entire list of polynomials to be in input form which I can copy and paste into the other program. I have tried writing something like 

StringVar[i_, j_, k_] := StringForm["p``````", i, j, k]

And then using Simplify to substitute the variables. But, when I use InputForm on the entire list of polynomials I get these funny expressions

StringForm["p``````", 0, 1, 2]

instead of the variables p012 that I would like. How do I make Mathematica treat these as variables?

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3 Answers 3

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I think the simplest approach is to give p a Format:

Format[p[a__Integer], InputForm] := SequenceForm[p,a]

Here is an example polynomial, and what it looks like when converted to InputForm:

poly = 2 p[0,1,2]^2 + p[1,2,3]^3
% //InputForm

enter image description here

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This is a common trap with using StringForm[]. But before discussing the pitfall, let me offer slightly simpler code:

With[{expr = p[0, 1, 2]},
     StringForm["`1``2`", Head[expr], StringJoin[ToString /@ (List @@ expr)]]]
   p012

Looks good so far, but let's see what InputForm[] says:

InputForm[%]
   StringForm["`1``2`", p, "012"]

Certainly not a string!

So, you need an additional ToString[]:

ToString[%] // InputForm
   "p012"

The alternative in newer versions of Mathematica is StringTemplate[], which produces a genuine string. Observe:

With[{expr = p[0, 1, 2]},
     StringTemplate["`head``args`"] @
     <|"head" -> Head[expr], "args" -> StringJoin[ToString /@ (List @@ expr)]|>]
   "p012"
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  • $\begingroup$ ...and you can then use Symbol["p012"] much later if you really want symbols instead of strings. $\endgroup$
    – J. M.'s missing motivation ♦
    Commented Oct 3, 2017 at 1:29
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If I understand what you're trying to do, this is the replacement rule you're looking for:

rule = p[i_, j_, k_] :> ToExpression[StringJoin @@ (ToString /@ {"p", i, j, k})]

You might also consider using Format and/or $PrePrint to have the expressions automatically formatted.

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