NULL and NA values in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of NA and NULL in vector length

What is the output of the following R code?

R Programming

x <- c(1, 2, NA, NULL, 5)
length(x)

A 4

B 5

C 3

D Error

💡 Hint

Remember that NULL values are removed when combined in vectors.

In R, NULL values are dropped when combined into vectors, so x contains 4 elements: 1, 2, NA, and 5. The length() function returns 4.

❓ Predict Output

intermediate

Comparing NA and NULL with is.na() and is.null()

What is the output of this R code?

R Programming

a <- NA
b <- NULL
is.na(a)
is.na(b)
is.null(a)
is.null(b)

A

[1] TRUE
logical(0)
[1] FALSE
[1] TRUE

B

[1] FALSE
[1] TRUE
[1] TRUE
[1] FALSE

C

[1] TRUE
[1] FALSE
[1] TRUE
[1] FALSE

D

[1] TRUE
[1] TRUE
[1] FALSE
[1] TRUE

💡 Hint

Check how is.na() and is.null() behave differently on NA and NULL.

is.na(NA) returns TRUE because NA is a missing value.
is.na(NULL) returns a logical vector of length 0 because NULL has no elements.
is.null(NA) is FALSE because NA is not NULL.
is.null(NULL) is TRUE.

🔧 Debug

advanced

Why does this NA check fail?

Consider this R code snippet: x <- c(1, 2, NA, 4) if (x == NA) { print("Has NA") } else { print("No NA") } Why does this code not print "Has NA" even though x contains NA?

R Programming

x <- c(1, 2, NA, 4)
if (x == NA) {
  print("Has NA")
} else {
  print("No NA")
}

A Because the vector x does not contain NA values.

B Because NA is treated as zero in comparisons.

C Because comparing with NA using == always returns NA, not TRUE or FALSE.

D Because the if statement only checks the first element of x.

💡 Hint

Think about how NA behaves in logical comparisons.

In R, == NA returns NA, not TRUE or FALSE. The if statement requires a TRUE or FALSE condition, so it fails to detect NA this way. Use is.na() to check for NA values.

🧠 Conceptual

advanced

Difference between NULL and NA in R

Which statement correctly describes the difference between NULL and NA in R?

A Both NULL and NA are identical and can be used interchangeably.

B NA is used to remove elements from a vector, NULL is a missing value marker.

C NULL is a numeric missing value, NA is a character missing value.

D NULL represents absence of a value or object, while NA represents a missing value within an object.

💡 Hint

Think about how NULL and NA behave in vectors and lists.

NULL means no value or no object at all, often used to indicate absence.
NA means a missing value inside an object like a vector or data frame.

❓ Predict Output

expert

Count of NA and NULL in a list

What is the output of this R code?

R Programming

lst <- list(a = 1, b = NA, c = NULL, d = 4)
sum(unlist(sapply(lst, is.na)))
length(lst)

A

1
3

B

1
4

C

0
3

D

0
4

💡 Hint

Remember that NULL elements are kept in lists (length 4). is.na(NULL) returns logical(0), which unlist() drops.

The list lst has 4 elements; NULL is kept unlike in vectors.
sapply(lst, is.na) returns list(FALSE, TRUE, logical(0), FALSE); unlist() drops the empty vector, so sum(c(FALSE, TRUE, FALSE)) = 1.
Length of lst is 4.