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While Export["file.txt", A, "Table"] exports a file of data, A represents a data set, in column form the question is about how to export, or print, a set of data with each data point being numbered. As an example consider the recurrence relation a[n+1]== 2*a[n]+1, a[0]=0 which yields the set of data {1, 1, 3, 7, ...}. Now if the data is exported to a file it looks like

1 1 3 7

What this question is about is how to have the data set exported to a file such that it reads:

0 1 1 1 2 3 3 7

where the first column is the n value of the recurrence relation and the second column is the corresponding data value. As an additional consideration can this two column output be applied to numerical values ? For an example consider the value of Pi calculated by N[Pi, 200]. Can the exported file have an output of the form

0 3 1 1 2 4 3 ... ?

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myA = Transpose[{Range[Length[x]], x}]

Export["file.txt", myA, "Table"]

Incidentally, never use an upper-case letter or name beginning with an upper-case letter for your lists, variables or functions, to avoid potential naming conflicts with Mathematica's internal names.

myA = Transpose[{Range[200], RealDigits[N[π, 200]][[1]]}]
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