Update column value from hierarchical/nested Dataset

Question

Let's say I have the following nested dataset:

SeedRandom[14];
data=Transpose@Dataset@AssociationThread[{"a","b"},RandomInteger[{0,20},{2,50}]];
data=data[All,<|#,"combine"-><|"c"->Slot@"a"*2,"d"->Slot@"a"-Slot@"b"|>|>&]

I want to selectively apply a function, say f, onto the nested column c and keep the structure of the original dataset. Currently, I used the following to do it.

Dataset[Transpose[  (Transpose@data)[<|#, "combine" -> Normal@data[All, "combine", {"c" -> f}]|> &]  ] ,  MaxItems -> 5]

I found in the Dataset doc that dataset[All,{n->f}] can selectively map f onto the n^th column, which worked for col b.

Dataset[data[All, {"b" -> f}], MaxItems -> 5]

However, when I applied the following. Only the innermost subdataset was updated/returned.

Dataset[data[All, "combine", {"c" -> f}], MaxItems -> 5]

Is it possible to correct this? I also wonder is there any other way to return what I wanted. Thanks.

Answer 1

We can express the desired operation by using nested subquery operators:

data[All, {"combine" -> {"c" -> f}}]