ifelse assignment in data.table

Question

I am a teacher, and would like to correctly use the data.table package in R to automatically grade student answers in a log file, i.e. add a column called correct if the student answer to a particular question, is the correct answer to that question, and 0 otherwise. I can do this easily if each question has only 1 answer, but I am getting tripped up if a question has multiple possible answers (questions and their possible correct answers are stored in another table)

Below is a MWE:

set.seed(123)
question_table <- data.table(id=c(1,1,2,2,3,4),correct_ans=sample(1:4,6,replace = T))
log <- data.table(student=sample(letters[1:3],10,replace = T),
                  question_id=c(1,1,1,2,2,2,3,3,4,4), 
                  student_answer= c(2,4,1,3,2,4,4,5,2,1))

My question lies in what is the correct data.table way to use ifelse in j, especially if we depend on another table?

log[,correct:=ifelse(student_answer %in% 
                          question_table[log$question_id %in% id]$correct_ans,1,0)]

As can be seen below, question 1 and 2 both have multiple possible correct answers.

> question_table
   id correct_ans
1:  1           2
2:  1           4
3:  2           2
4:  2           4
5:  3           4
6:  4           1

While the correct column is calculated without errors, something isn't right: e.g. when student b answers question, he is awarded a correct score, even though he answered incorrectly. Only some entries of the correct column are off, which leads me to believe there is something i am not getting with how variables have are scoped.

> log
    student question_id student_answer correct
 1:       b           1              2       1
 2:       c           1              4       1
 3:       b           1              1       1   <- ?
 4:       b           2              3       0
 5:       c           2              2       1
 6:       b           2              4       1
 7:       c           3              4       1
 8:       b           3              5       0
 9:       a           4              2       1   <- ?
10:       c           4              1       1

I considered making a helper column with the correct ans in the log table by joining with question_table, but that does not work since the key is not unique in the latter.

Any and all help would be appreciated. Thanks in advance.

Answer 1

You can use a join:

# initialize to zero
log[, correct := 0L ]

# update to 1 if matched
log[question_table, on=c(question_id = "id", student_answer = "correct_ans"),
   correct := 1L ] 

    student question_id student_answer correct
 1:       b           1              2       1
 2:       c           1              4       1
 3:       b           1              1       0
 4:       b           2              3       0
 5:       c           2              2       1
 6:       b           2              4       1
 7:       c           3              4       1
 8:       b           3              5       0
 9:       a           4              2       0
10:       c           4              1       1

How it works. The syntax for an update join is X[Y, on=cols, xvar := z]:

• If col names differ between X and Y, use on=c(xcol = "ycol", xcol2 = "ycol2") or, in version 1.9.7+, .(xcol = ycol, xcol2 = ycol2).
• xvar := z will only operate on the rows of X that are matched. Sometimes, it is also useful to use by=.EACHI here, depending on how many rows of X are matched by each in Y and how complicated the expression for z is.

See ?data.table for full documentation on the syntax.