Functions and loops in R

Mar 10, 2015 • Karl Broman

These multiple choice questions relate to the lessons on functions in R, loops in R, and conditional statements in R.

I had in mind that these might go at the end of the lesson on conditional statements and might lead to discussion of statements like sum(x <= 10). Maybe there’s too much going on here.

Question 1

Consider the following function.

f <-
function(vec, threshold=10)
    number <- 0
    for(v in vec) {
        if(v <= threshold)
            number <- number + 1


What value would f(c(3, 10, 5, 8, 28), 5) return?

  1. 1
  2. 2
  3. 3
  4. 4

Question 2

Consider the function defined in the previous question.

What value would f(NULL) return?

  1. This will give an error.
  2. 0
  3. 1
  4. NULL

Discussion of Question 2

  1. This is incorrect. I had in mind that a student might thing that a for loop wouldn’t work with an empty set of values.

  2. This is the correct answer. The for loop ends up getting skipped, since vec is empty.

  3. This is incorrect. I had in mind that a student might think that v would take the value NULL and that NULL <= threshold might be interpreted as TRUE.

  4. I guess this is the “fish” answer. Writing these MCQs is hard.