Fill up and fill down rows

[bkamins]

If you want to update df1 in-place do:

using Impute
for sdf in groupby(df1,:id)
    sdf.a .= Impute.locf(sdf.a)
end

or

transform!(groupby(df1, :id), :a => Impute.locf => :a)

if you want a new data frame:

transform(groupby(df1, :id), :a => Impute.locf => :a)