Turn a SubDataFrame into a DataFrame?

Question

I have a by which creates SubDataFrames. How do I turn these into a DataFrame, preferably without copying?

My original problem is that I cannot add a new column to a SubDataFrame:

# df[:End] = 1:nrow(merged_islands)
# ERROR: LoadError: Cannot assign to non-existent column: End

# insert!(df, length(df), Array(1:nrow(merged_islands)), :End)
# ERROR: LoadError: MethodError: no method matching insert!(::SubDataFrame{Array{Int64,1}}, ::Int64, ::Array{Int64,1}, ::Symbol)

I am guessing converting it into a DataFrame is the easiest way to do it :)

Answer 1

An interesting question. On current master (to be tagged very soon) it is enough to write DataFrame(sdf) where sdf is a SubDataFrame. It will create a copy of all vectors though.

Here is a solution that will create a DataFrame with a view of all vectors contained in SubDataFrame (it should work both on master and on currently tagged release):

function sdf2df(sdf::SubDataFrame)
    p = parent(sdf)
    sel = DataFrames.rows(sdf)
    DataFrame(AbstractVector[view(p[i], sel) for i in 1:ncol(sdf)],
              names(sdf))
end

(I use AbstractVector container type as it will be faster on current master)

You will not be able to add rows to such a DataFrame while it holds at least one view column.

EDIT: as a side note (maybe this was your problem in the end). If you have sdf which is a SubDataFrame whose parent is df which is a DataFrame then if you add columns to df they will be immediately visible in sdf as SubDataFrame only selects rows and inherits all columns from the parent.