Create Nested List from tabular data

Question

Im trying to create a nested list from tabular data, then onward into a nested association. I can get part way with the help of this answer but it only works for the first level. Repeated use of this approach doesn't seem to work.

list=  {{"Category", "Detail", "Value"}, {"Cat1", "detail1", 23},
{"Cat1", "detail2", 27}, {"Cat1", "detail3", 13}, {"Cat2", "detail4",15}, 
{"Cat2", "detail5", 35}, {"Cat3", "Detail6", 56}};

list // TableForm

(*  tabular data result *)

First level of data.

Thread[list[[2 ;;, 2]] -> #] & /@ {list[[2 ;;, 3]]} // ToAssociations

 {<|detail1->23,detail2->27,detail3->13,detail4->15,detail5->35,Detail6->56|>} 

Required result

{<| Cat1-> <|detail1->23,detail2->27,detail3->13|>|>,
<| Cat2-> <|detail4->15,detail5->35|>|>,
<| Cat3-> <|Detail6->56|>|>}

Update is it possible to come up with something that works for N levels of nesting?

Eg 3 levels of nesting

list = {{"BigCat", "Category", "Detail", "Value"}, {"BigCat", "Cat1", 
   "detail1", 23}, {"BigCat", "Cat1", "detail2", 27}, {"BigCat", 
   "Cat1", "detail3", 13}, {"BigCat", "Cat2", "detail4", 
   15}, {"BigCat", "Cat2", "detail5", 35}, {"LittleCat", "Cat3", 
   "Detail6", 56}}

Answer 1

From my answer to Handy / flexible directory tree operations with minor modification:

fn[x_List] := GroupBy[x, First -> Rest, fn]

fn[{{x_}}] := x

Tested on your two lists, named list1 and list2 respectively:

fn[Rest @ list1]

<|"Cat1" -> <|"detail1" -> 23, "detail2" -> 27, "detail3" -> 13|>, 
 "Cat2" -> <|"detail4" -> 15, "detail5" -> 35|>, "Cat3" -> <|"Detail6" -> 56|>|>

fn[Rest @ list2]

<|"BigCat" -> <|"Cat1" -> <|"detail1" -> 23, "detail2" -> 27, "detail3" -> 13|>, 
   "Cat2" -> <|"detail4" -> 15, "detail5" -> 35|>|>, 
 "LittleCat" -> <|"Cat3" -> <|"Detail6" -> 56|>|>|>

Answer 2

I guess this is what you want. Using GroupBy and then combining them as Association will do the trick.

GroupBy[Rest@list, First -> Rest, Association[Rule @@@ #] &]

Resulting in

{<| Cat1-> <|detail1->23,detail2->27,detail3->13|>|>,
<| Cat2-> <|detail4->15,detail5->35|>|>,
<| Cat3-> <|Detail6->56|>|>}

Answer 3

If you need a method for any levels of nesting (as mentionned in your update), maybe try something like this:

groupbyNest[x_List] := 
 GroupBy[x, First -> Rest, groupbyNest] /; Last@Dimensions@x > 1

groupbyNest[x_List] := First@Flatten@x /; Last@Dimensions@x == 1

Then for example, given

list = {{"BigCat", "Category", "Detail", "Value"}, {"BigCat", "Cat1", 
   "detail1", 23}, {"BigCat", "Cat1", "detail2", 27}, {"BigCat", 
   "Cat1", "detail3", 13}, {"BigCat", "Cat2", "detail4", 
   15}, {"BigCat", "Cat2", "detail5", 35}, {"LittleCat", "Cat3", 
   "Detail6", 56}}

just

groupbyNest@Rest@list

returns

<|"BigCat" -> <|"Cat1" -> <|"detail1" -> 23, "detail2" -> 27, "detail3" -> 13|>, "Cat2" -> <|"detail4" -> 15, "detail5" -> 35|>|>, "LittleCat" -> <|"Cat3" -> <|"Detail6" -> 56|>|>|>

Answer 4

In general, the transformation you ask for can be done, for example, along the lines I outlined in my answer on Associations (section on example database).

Transforming to a list of associations

To use the solution from tha post verbatim (more or less), we will need an auxiliary function to transform your data to a list of associations first:

toAssocList =
  Composition[
    Map[Association],
    Transpose,
    Map[Thread[First[#] -> Rest[#]] &],
    Transpose
  ]

so that

assocs = toAssocList@list

(* 
   {
     <|"BigCat" -> "BigCat", "Category" -> "Cat1", "Detail" -> "detail1", "Value" -> 23|>, 
     <|"BigCat" -> "BigCat", "Category" -> "Cat1", "Detail" -> "detail2", "Value" -> 27|>, 
     <|"BigCat" -> "BigCat", "Category" -> "Cat1", "Detail" -> "detail3", "Value" -> 13|>, 
     <|"BigCat" -> "BigCat", "Category" -> "Cat2", "Detail" -> "detail4",  "Value" -> 15|>, 
     <|"BigCat" -> "BigCat", "Category" -> "Cat2", "Detail" -> "detail5", "Value" -> 35|>, 
     <|"BigCat" -> "LittleCat", "Category" -> "Cat3", "Detail" -> "Detail6", "Value" -> 56|>
 }
*)

Transformation to the hierarchical structure

Using the function pushUpNested from the mentioned post, we can generate the corresponding tramnsformation:

transform = pushUpNested[{{"BigCat"}, {"Category"}, {"Detail"},{"Value"}}]

(*

 Map[
   Map[
     Map[Map[Identity]@*GroupBy[#1[[Sequence[Key["Value"]]]] &]]@* 
     GroupBy[#1[[Sequence[Key["Detail"]]]] &]
   ]@*
   GroupBy[#1[[Sequence[Key["Category"]]]] &]
 ]@*
 GroupBy[#1[[Sequence[Key["BigCat"]]]] &]

which we can use:

transform@assocs

(* <|"BigCat" -> <|"Cat1" -> <|"detail1" -> <|23 -> {<|"BigCat" -> "BigCat", "Category" -> "Cat1", "Detail" -> "detail1", 
      "Value" -> 23|>}|>, 
 "detail2" -> <|27 -> {<|"BigCat" -> "BigCat", 
      "Category" -> "Cat1", "Detail" -> "detail2", 
      "Value" -> 27|>}|>, 
 "detail3" -> <|13 -> {<|"BigCat" -> "BigCat", 
      "Category" -> "Cat1", "Detail" -> "detail3", 
      "Value" -> 13|>}|>|>, 
 "Cat2" -> <|"detail4" -> <|15 -> {<|"BigCat" -> "BigCat", 
      "Category" -> "Cat2", "Detail" -> "detail4", 
      "Value" -> 15|>}|>, 
 "detail5" -> <|35 -> {<|"BigCat" -> "BigCat", 
      "Category" -> "Cat2", "Detail" -> "detail5", 
      "Value" -> 35|>}|>|>|>, 
"LittleCat" -> <|"Cat3" -> <|"Detail6" -> <|56 -> {<|"BigCat" -> 
       "LittleCat", "Category" -> "Cat3", "Detail" -> "Detail6", 
      "Value" -> 56|>}|>|>|>|>

*)

Cleaning up

It remains now to remove the "leafs" in this construction (the lowest-level values):

Map[First@*Keys, transform@assocs, {3}]

(* 
    <|"BigCat" -> 
        <|
           "Cat1" -> <|"detail1" -> 23, "detail2" -> 27,"detail3" -> 13|>, 
           "Cat2" -> <|"detail4" -> 15, "detail5" -> 35|>
        |>, 
      "LittleCat" -> <|"Cat3" -> <|"Detail6" -> 56|>|>
    |>
*)

Notes

I don't claim this solution is the most efficient, but I think it illustrates a pretty general scheme, and a different, higher-level way to make such transformations - through transform generators. This can be used for much more complex cases. In fact, as I noted in the linked answer, Dataset is using a similar (but much more complex) scheme to generate queries.