How to append group row into dataframe

Question

I have this df1:

A B C
1 2 3
5 7 9

where A B C are columns names.

I have another df2 with one column:

A
1
         


        

Answer 1

For the sake of completeness, here is data.table approach which doesn't require to handle column names:

library(data.table)
setDT(df1)[, lapply(.SD, c, df2$A)]

   A B C
1: 1 2 3
2: 5 7 9
3: 1 1 1
4: 2 2 2
5: 3 3 3
6: 4 4 4

Note that the OP has described df2 to consist only of one column.

There is also a base R version of this approach:

data.frame(lapply(df1, c, df2$A))

  A B C
1 1 2 3
2 5 7 9
3 1 1 1
4 2 2 2
5 3 3 3
6 4 4 4

This is similar to d.b's approach but doesn't required to deal with column names.