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If I have a long list of form 

{{1,2,3,4},{5,6,7,8}, ... {97,98,99,100}}

how do I edit/convert it by adding the text as shown below 

{{1,2,3,4} -> group1, {5,6,7,8} -> group1, ... {97,98,99,100} -> group3}

(I want to prepare data for machine learning.)

That is, how to add something (not an element) to the end of an existing sub-list. I've discovered how to read in large datasets suitably prepared in a text editor (using ReadList), but I'm wondering how to do the editing within Mathematica.

Surely this sort of editing is possible but I cannot find how to do it in the Documentation. I'm probably mis-understanding something quite simple.

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  • $\begingroup$ Do you want the output to be a list of replacement rules list -> label or a list of just labels (strings)? $\endgroup$
    – J. M.'s missing motivation ♦
    Commented Sep 1, 2015 at 4:29

2 Answers 2

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I assume the multiplicity of group names in your example is not a typo, that is, you might want to name multiple sublists with the same group. Either way, this will do what you want:

(* example list and group names *)
list = Partition[Range@20, 4];
groups = {"group1", "group1", "group2", "group2", "group3"};
Rule @@@ Transpose[{list, groups}]

(* 
   {{1, 2, 3, 4} -> "group1", {5, 6, 7, 8} -> "group1", 
   {9, 10, 11, 12} -> "group2", {13, 14, 15, 16} -> "group2", 
   {17, 18, 19, 20} -> "group3"} 
*)

n.b. - see Guess who it is's comment re: more direct and canonical way of doing last step. DOH on my part...

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    $\begingroup$ Thread[list -> groups] is what I usually do. $\endgroup$
    – J. M.'s missing motivation ♦
    Commented Sep 1, 2015 at 5:21
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    $\begingroup$ @Guesswhoitis.: Ah, yeah, I herp-a-derped on that obvious and better way - was working on something deep, and over-thought above - you know how that goes... $\endgroup$
    – ciao
    Commented Sep 1, 2015 at 5:29
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SeedRandom[1];list = RandomInteger[100, {6, 4}];
i = 1; j = 1; Sequence @@@ ({# -> "group" <> ToString[i++], #2 -> 
          "group" <> ToString[j++]} & @@@ Partition[list, {2}])
(*{{80, 14, 0, 67} -> "group1", {3, 65, 100, 23} -> 
  "group1", {97, 68, 74, 15} -> "group2", {24, 4, 100, 90} -> 
  "group2", {83, 70, 1, 30} -> "group3", {48, 25, 44, 73} -> "group3"}*)
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