Sum all columns from a list

Question

I have a list of column names:

list = {"col1", "col2", "col3"}

and a dataset ds. I want to add a new column to ds that contains the sum of the columns listed in list. As suggested here, I could do:

ds[All, <| #, "colnew" -> #col1 + #col2 + #col3 |> &]

But I cannot list the column names by hand, because the list may be very large, or I might not know the names of the columns a priori.

Answer 1

If the you want to total all columns instead of just those on a select list, then the suggestion given by @Kuba is appropriate:

ds[All, <| #, "colnew" -> Total@# |> &]

If you wish to total only the listed columns, then:

ds[All, <| #, "colnew" -> Total[#[[list]]] |> &]

For example:

list = {"col1", "col2", "col3"};

ds = AssociationThread[list->#]& /@ RandomInteger[10, {4, 3}] // Dataset

ds[All, <| #, "colnew" -> Total[#[[list]]] |> &]

Or, asking for an actual subset of columns:

list2 = {"col1", "col2"};

ds[All, <| #, "colnew" -> Total[#[[list2]]] |> &]

Answer 2

You can programmatically generate <| #, "colnew" -> #col1 + #col2 + #col3 |> & as follows.

list = {"col1", "col2", "col3"};

Tr[Slot /@ list] /. x_ :> (<|#, "colnew" -> x|> &)

Association[#1, "colnew" -> #col1 + #col2 + #col3] &

Answer 3

ds = Dataset[{<|"col1" -> 1, "col2" -> 2|>, <|"col1" -> 3, 
    "col2" -> 4|>, <|"col1" -> 5, "col2" -> 6|>}]

ds[All, Append[#, "colnew" -> Total[#]] &]