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

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.