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

Data:

df1 <- data.frame(A=c(1,5),
                  B=c(2,7),
                  C=c(3,9))
df2 <- data.frame(A=c(1,2,3,4))

Solution:

df2 <- matrix(rep(df2$A, ncol(df1)), ncol=ncol(df1))
colnames(df2) <- colnames(df1)
rbind(df1,df2)

Result:

  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

Answer 2

By analogy with @useR's excellent Base R answer, here's a tidyverse solution:

library(purrr)

map2_df(df1, df2, c)

  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

Here are a few other (less desirable) options from when I first answered this question.

library(dplyr)

bind_rows(df1, df2 %>% mutate(B=A, C=A))

Or, if we want to dynamically get the number of columns and their names from df1:

bind_rows(df1,
          df2[,rep(1,ncol(df1))] %>% setNames(names(df1)))

And one more Base R method:

rbind(df1, setNames(df2[,rep(1,ncol(df1))], names(df1)))

Answer 3

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.