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I want to use n variables $$alpha[[i]]$$ (i goes from 1 to n).

I use these variable inside of a function :

SixJ1[j1_, j2_, j3_, j4_, j5_, j6_] := Module[{j},
  j = {j1, j2, j3, j4, j5, j6};
  Sum[alpha[[k]]*j[[k]]*(j[[k]] + 1)*
     SixJSymbol[{j1, j2, j3}, {j4, j5, j6}] + 
    beta*opVolume[j1, j2, j3, j4, j5, j6], {k, 1, 6}]
  ]

The problem is that when I call this function, mathematica answers me :

"Part specification alpha[[1]] is longer than depth of object."

I think it is because I never initialised thoose variables but I want to keep them symbolic.

How to avoid these errors ?

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  • 2
    $\begingroup$ How about alpha[k]? $\endgroup$ – Michael E2 Jun 14 '17 at 14:15
  • $\begingroup$ I thought that to call an element of a list we needed two bracket : [[ ? $\endgroup$ – StarBucK Jun 14 '17 at 14:16
  • $\begingroup$ Ok, how about alphalist = Array[k, 6]; alphalist[[k]]? (You have to name your variables somehow first.) $\endgroup$ – Michael E2 Jun 14 '17 at 14:17
  • $\begingroup$ In fact your first comment intrigued me. What is the difference between alpha[k] and alpha[[k]]. I know the second is interpreted as en element of a list but I did'nt know we could only use one bracket. What mean exactly the first expression alpha[k] ? $\endgroup$ – StarBucK Jun 14 '17 at 14:20
  • $\begingroup$ Oh in fact it is just the function notation... $\endgroup$ – StarBucK Jun 14 '17 at 14:22
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An alternative:

 alpha = Table[Symbol["alpha" <> ToString[i]], {i, 1, n}];
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Probably the most elegant way that solves your problem, in my opinion, is the following one:

Subscript[alpha, #] & /@ Range[10]

or

Table[Subscript[alpha, i], {i, 10}]

If you want to create a list of "indexed" symbols, you can do it as:

ToExpression["alpha" <> ToString[#]] & /@ Range[10]

Or you can do something like:

alpha[#] & /@ Range[10]

, which is actually same as:

Table[alpha[i], {i, 10}]

[[]] - this is Part[], so you can't really use this for indexing.

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ToExpression[Table[TemplateApply["alpha<*i*>"], {i, 5}]]

{alpha1, alpha2, alpha3, alpha4, alpha5}

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