The Call Stack (scoping) in R

The questions relate to the call stack section in the lesson Creating Functions.

The two questions probe understanding of how variables are used across environments in R

Question 1

After running the following code, what will R return (answer without running)?

a<-10
b<-20
function1<-function(){
	a<-1
	b<-2
	c(a,b)
}

c(function1(),a,b)

1. 10,20,10,20
2. 1,2,1,2
3. 10,20,1,2
4. 1,2,10,20

Question 2

After running the following code, what will R return (answer without running)?

rm(a,b)
a<-10
b<-20
function2<-function(){
	a<-1
	c(a,b)
}
c(function2(),a,b)

1. 10,20,10,20
2. 1,2,10,20
3. R will produce an error - Error: object ‘b’ not found
4. 1,20,10,20

Discussion of Question 2

1. This will test the understanding of the function first looking in its own environment for variables

2. This answer is a red herring using the previous question - if the student has not noticed b was not defined in the new function, they will choose this

3. This tests the students understanding of how R searches for variables - if they dont understand it goes up in environments, they will choose this

4. This is the correct answer.