r - How to duplicate columns in a data frame

Question

Is there any efficient way, without using for loops, to duplicate the columns in a data frame? For example, if I have the following data frame:

  Var1 Var2
1    1    0
2    2    0
3    1    1
4    2    1
5    1    2
6    2    2

And I specify that column Var1 should be repeated twice, and column Var2 three times, then I would like to get the following:

  Var1 Var1 Var2 Var2 Var2
1    1    1    0    0    0
2    2    2    0    0    0
3    1    1    1    1    1
4    2    2    1    1    1
5    1    1    2    2    2
6    2    2    2    2    2

Any help would be greatly appreciated!

Answer 1

We can replicate the column names (rep), use that as index to duplicate the columns. By default, the data.frame columns can have only unique column names, so it will use make.unique to add .1, .2 as suffix to the duplicate column names in 'df2'. If we don't want that, we can remove the suffix part with sub.

df2 <- df1[rep(names(df1), c(2,3))]
names(df2) <- sub('\..*', '', names(df2))
df2
#  Var1 Var1 Var2 Var2 Var2
#1    1    1    0    0    0
#2    2    2    0    0    0
#3    1    1    1    1    1
#4    2    2    1    1    1
#5    1    1    2    2    2
#6    2    2    2    2    2

Or as @Frank mentioned in the comments, we can also do

`[.noquote`(df1,c(1,1,2,2,2))