Logical indexing in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of logical indexing with NA values

What is the output of the following R code?

R Programming

x <- c(2, NA, 5, 8, NA)
result <- x[!is.na(x) & x > 4]
print(result)

A [1] 5 8

B [1] 2 5 8

C [1] NA NA

D [1] 5 8 NA

💡 Hint

Remember that logical indexing excludes NA when using !is.na().

The code selects elements of x that are not NA and greater than 4. Only 5 and 8 satisfy both conditions.

❓ Predict Output

intermediate

Logical indexing with multiple conditions

What does this R code print?

R Programming

v <- c(10, 15, 20, 25, 30)
subset <- v[v %% 10 == 0 & v > 15]
print(subset)

A [1] 10 20 30

B [1] 20 30

C [1] 20

D [1] 15 25

💡 Hint

Check which numbers are multiples of 10 and greater than 15.

The code selects numbers divisible by 10 and greater than 15. These are 20 and 30.

🔧 Debug

advanced

Identify the error in logical indexing

What error does this R code produce?

R Programming

data <- c(1, 2, 3, 4, 5)
index <- c(TRUE, FALSE, TRUE, FALSE, TRUE, TRUE)
result <- data[index]
print(result)

A [1] 1 2

B Error: subscript out of bounds

C [1] 1 3 5

D Error: logical subscript too long

💡 Hint

Check the length of the logical vector compared to the data vector.

The logical index vector is longer than the data vector, causing the error 'logical subscript too long'.

❓ Predict Output

advanced

Logical indexing with named vectors

What is the output of this R code?

R Programming

scores <- c(John=85, Mary=92, Paul=78, Lisa=90)
high_scores <- scores[scores >= 90]
print(names(high_scores))

A [1] "Mary" "Lisa"

B NULL

C [1] "John" "Mary"

D [1] 92 90

💡 Hint

Logical indexing keeps names of selected elements.

The code selects scores 90 or above and prints their names, which are Mary and Lisa.

❓ Predict Output

expert

Complex logical indexing with NA and recycling

What is the output of this R code?

R Programming

x <- c(NA, 3, NA, 7, 5)
idx <- c(TRUE, FALSE)
result <- x[!is.na(x) & idx]
print(result)

A [1] 3 7 5

B [1] 3 5

C [1] 5

D Error: logical subscript too long

💡 Hint

Logical vector idx is recycled to match length of x.

The logical vector idx is recycled to length 5: TRUE, FALSE, TRUE, FALSE, TRUE. Combined with !is.na(x), positions 2, 4 and 5 are not NA. But idx at position 2 is FALSE, position 4 is FALSE, so only position 5 is selected.