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

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.

CodePudding user response：

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

CodePudding user response：

In base R, we may also use a row/column indexing

df$col3 <- df[cbind(seq_len(nrow(df)), rep(1:2, length.out = nrow(df)))]

-output

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

CodePudding user response：

base

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

df$col3 <- df$col1
df$col3[c(FALSE, TRUE)] <- df$col2[c(FALSE, TRUE)]

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

^{Created on 2022-03-06 by the reprex package (v2.0.1)}