Question

I have some data frames which hold the results of a survey. The first frame lists the question ids (q_id) for each question in the survey:

  q_id
1   q1
2   q2
3   q3

The second data frame holds responses (res) for each subject (s_id) for every question that subject responded to. A subject can skip questions:

  s_id q_id res
1    1   q1   a
2    2   q1   b
3    1   q2   b

What I want to generate is a table which shows the responses to each question, where the columns are the question ids and each row represents a subject. In the above examples, the table would look like this:

  q1 q2  q3
1  a  b  NA
2  b  NA NA

What is the best way to generate such a table?

Answer 1

Assuming that your question data.frame is DQ and your answers DT

You need to make sure that your q_id column in your answers has all the levels available

DT$q_id <- factor(as.character(DT$q_id), levels = levels(DQ$q_id))

then you can use reshape2 and dcast with drop = FALSE to cast as you wish

library(reshape2)
dcast(DT, s_id~q_id, value.var = 'res', drop = FALSE)
  s_id q1   q2   q3
1    1  a    b <NA>
2    2  b <NA> <NA>