Merge nth elements from two columns while keeping the original row order in R

I am attempting to merge every nth element from col1, replacing values from that same row in col2 in a new column: col3

df <- data.frame(col1 = c('A', 'B', 'D', 'F', 'C'), col2 = c(2, 1, 2, 3, 1))

> df
  col1 col2
1    A    2
2    B    1
3    D    2
4    F    3
5    C    1

If i was to merge every odd element from col1 with every even element from col2, for example, the output should look something like this:

> df
  col1 col2 col3
1    A    2    A
2    B    1    1
3    D    2    D
4    F    3    3
5    C    1    C

Thanks.

>Solution :

We could do it with an ifelse statement checking if row is even or odd with the modulo operator %%:

library(dplyr)
df %>% 
  mutate(col3 = ifelse((row_number() %% 2) == 0, col2, col1))

  col1 col2 col3
1    A    2    A
2    B    1    1
3    D    2    D
4    F    3    3
5    C    1    C