Add 1 blank column every 2 columns

Question

I have a dataframe "df" like this:

col1 col2 col3 col4 col5 col6
 1    2    2    3    5    7
 2    4    6    4    8    2
 5    9    7    3    2    5
 3    4    5    6    8    1

and I would like to create a new dataframe "new_df" in which there is 1 blank column (called "empty") every 2 columns, like this:

empty col1 col2 empty col3 col4 empty col5 col6
 NA    1    2    NA    2    3    NA    5    7
 NA    2    4    NA    6    4    NA    8    2
 NA    5    9    NA    7    3    NA    2    5
 NA    3    4    NA    5    6    NA    8    1

How can I add the blank column in this way? I have tried using:

n = length(df)
empty <- NA

for (i in seq(1,n-2,2))
  {
  new_df <- add_column(df, empty, .before=i)
  }

but it memorizes only the last step, giving me this result:

col1 col2 col3 col4 empty col5 col6
 1    2     2    3   NA    5    7
 2    4     6    4   NA    8    2
 5    9     7    3   NA    2    5
 3    4     5    6   NA    8    1

Answer 1

Another base R solution

tmp1=seq(1,ncol(df),3)
tmp2=!(1:ncol(df) %in% tmp1)

df2=data.frame(matrix(NA,nrow(df),ncol(df)+ncol(df)/2))
df2[tmp2]=df

colnames(df2)[tmp1]=paste0("empty",1:length(tmp1))
colnames(df2)[tmp2]=colnames(df)

  empty1 col1 col2 empty2 col3 col4 empty3 col5 col6
1     NA    1    2     NA    2    3     NA    5    7
2     NA    2    4     NA    6    4     NA    8    2
3     NA    5    9     NA    7    3     NA    2    5
4     NA    3    4     NA    5    6     NA    8    1