Assigning ifelse to data.table

Question

I am a teacher and would like to use the package data.tablein correctly Rto automatically evaluate students' answers in the log file, i.e. add a column with a name correctif the student answers a specific question, this is the correct answer to this question, and 0 otherwise. I can do this easily if each question has only 1 answer, but I worked if the question has several possible answers (the questions and their possible correct answers are stored in another table)

The following is the 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 is the correct use of the data.tablemethod ifelsein j, especially if we are dependent 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, questions 1 and 2 have several 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

, - : . student b , , . correct , , -, , .

> 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

ans log join ing question_table, , .

. .

Answer 1

:

# 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

. - X[Y, on=cols, xvar := z]:

• col X Y, on=c(xcol = "ycol", xcol2 = "ycol2") , 1.9.7+, .(xcol = ycol, xcol2 = ycol2).
• xvar := z X, . by=.EACHI , , X Y z.

. ?data.table .