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I would like to transform a table like

{<|"a" -> 1, "b" -> 11, "c" -> 111|>, <|"a" -> 1, "b" -> 12, "c" -> 112|>, <|"a" -> 2, "b" -> 22, "c" -> 111|>}

into an association that has the same keys and as values a list of the values in the original association, i.e.

{<|"a" -> {1, 1, 2}, "b" -> {11, 12, 22}, "c" -> {111, 112, 111}|>}

The way I did that was via

keys = Keys[as[[1]]]
values  = Lookup[as, #] & /@ keys
AssociationThread[keys -> values]

Two questions:

  • Is there a more straightforward way?

  • if everything is wrapped in a Dataset, how can I do it (without reverting to Normal)?

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1 Answer 1

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as={<|"a" -> 1, "b" -> 11, "c" -> 111|>, <|"a" -> 1, "b" -> 12, "c" -> 112|>, <|"a" -> 2, "b" -> 22, "c" -> 111|>};

Merge[as, Identity]

(*<|"a" -> {1, 1, 2}, "b" -> {11, 12, 22}, "c" -> {111, 112, 111}|>*)
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    $\begingroup$ +1. For the second question... if we are given ds = Dataset @ as, then we can use ds[Merge[Identity]]. $\endgroup$
    – WReach
    Jan 11, 2015 at 1:06
  • $\begingroup$ @Algohi Identity seems very poorly documented - is it just acting as a placeholder in this case? $\endgroup$
    – Gordon Coale
    Jan 12, 2015 at 10:22
  • $\begingroup$ Merge[expr,f], the function f is applied to lists of values that share the same key. Identity returns its argument. To understand that check this another solution: Merge[as, {#}[[1]] &]. And also check this: Identity[{1, 1, 2}] $\endgroup$
    – Basheer Algohi
    Jan 12, 2015 at 14:17

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