add a categorical variable to data frame based on fixed rows order in r

November 17, 2022

I have a data frame like this:

Q1 <- c("A",NA,"A",NA,"NA","C","D","A","B", NA) 
Q2 <- c("D",NA,"D","C",NA,NA,"A","A","A","A")
Q3 <- c("B","B","C","A",NA,"A","B","D","E",NA)
Q4 <- c("B",NA,"C","C","C","C","D","B",NA,"A")
Q5 <- c("A",NA,"D",NA,NA,"C",NA,"B","B", NA)
Q6 <- c("D",NA,"D","C",NA,NA,"A","A","A","A")
mydf <- data.frame(Q1,Q2,Q3,Q4,Q5,Q6)
mydf

What I want to do is add a new column "subject" to my dataset and assign every 2 rows a category of a subject as below. I know adding a new column is done with mutate but I couldn’t find how to tell R to do that on a fixed rows interval.

    Q1   Q2   Q3   Q4   Q5   Q6    Subject
1     A    D    B    B    A    D   Language
2  <NA> <NA>    B <NA> <NA> <NA>   Language
3     A    D    C    C    D    D   Science
4  <NA>    C    A    C <NA>    C   Science
5    NA <NA> <NA>    C <NA> <NA>   Math 
6     C <NA>    A    C    C <NA>   Math
7     D    A    B    D <NA>    A   Art
8     A    A    D    B    B    A   Art
9     B    A    E <NA>    B    A   History
10 <NA>    A <NA>    A <NA>    A   History

>Solution :

How about this:

library(dplyr)

Q1 <- c("A",NA,"A",NA,"NA","C","D","A","B", NA) 
Q2 <- c("D",NA,"D","C",NA,NA,"A","A","A","A")
Q3 <- c("B","B","C","A",NA,"A","B","D","E",NA)
Q4 <- c("B",NA,"C","C","C","C","D","B",NA,"A")
Q5 <- c("A",NA,"D",NA,NA,"C",NA,"B","B", NA)
Q6 <- c("D",NA,"D","C",NA,NA,"A","A","A","A")
mydf <- data.frame(Q1,Q2,Q3,Q4,Q5,Q6)
mydf
#>      Q1   Q2   Q3   Q4   Q5   Q6
#> 1     A    D    B    B    A    D
#> 2  <NA> <NA>    B <NA> <NA> <NA>
#> 3     A    D    C    C    D    D
#> 4  <NA>    C    A    C <NA>    C
#> 5    NA <NA> <NA>    C <NA> <NA>
#> 6     C <NA>    A    C    C <NA>
#> 7     D    A    B    D <NA>    A
#> 8     A    A    D    B    B    A
#> 9     B    A    E <NA>    B    A
#> 10 <NA>    A <NA>    A <NA>    A

mydf %>% 
  mutate(Subject = rep(c("Language", "Science", 
                         "Math", "art", "History"), 
                       each=2))
#>      Q1   Q2   Q3   Q4   Q5   Q6  Subject
#> 1     A    D    B    B    A    D Language
#> 2  <NA> <NA>    B <NA> <NA> <NA> Language
#> 3     A    D    C    C    D    D  Science
#> 4  <NA>    C    A    C <NA>    C  Science
#> 5    NA <NA> <NA>    C <NA> <NA>     Math
#> 6     C <NA>    A    C    C <NA>     Math
#> 7     D    A    B    D <NA>    A      art
#> 8     A    A    D    B    B    A      art
#> 9     B    A    E <NA>    B    A  History
#> 10 <NA>    A <NA>    A <NA>    A  History

^{Created on 2022-11-17 by the reprex package (v2.0.1)}