grep and grepl in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of grep with value = TRUE

What is the output of this R code?

vec <- c("apple", "banana", "cherry", "date")
grep("a", vec, value = TRUE)

R Programming

vec <- c("apple", "banana", "cherry", "date")
grep("a", vec, value = TRUE)

A [1] "apple" "banana" "date"

B [1] 1 2 4

C [1] "apple" "banana" "cherry" "date"

D [1] 1 2 3 4

💡 Hint

Remember, value = TRUE returns the matching elements, not their positions.

grep with value = TRUE returns the elements of the vector that contain the pattern. Here, "apple", "banana", and "date" contain the letter 'a'.

❓ Predict Output

intermediate

Output of grepl with logical vector

What is the output of this R code?

vec <- c("cat", "dog", "bird", "cow")
grepl("o", vec)

R Programming

vec <- c("cat", "dog", "bird", "cow")
grepl("o", vec)

A [1] FALSE FALSE FALSE TRUE

B [1] TRUE TRUE FALSE TRUE

C [1] TRUE FALSE FALSE TRUE

D [1] FALSE TRUE FALSE TRUE

💡 Hint

grepl returns TRUE for elements containing the pattern, FALSE otherwise.

Only "dog" and "cow" contain the letter 'o', so their positions are TRUE, others FALSE.

🧠 Conceptual

advanced

Difference between grep and grepl

Which statement correctly describes the difference between grep() and grepl() in R?

A grep() returns the count of matches; grepl() returns the matching strings.

B Both grep() and grepl() return matching values but in different orders.

C grep() returns indices or matching values; grepl() returns a logical vector indicating matches.

D grep() returns a logical vector; grepl() returns indices of matches.

💡 Hint

Think about what each function returns: positions vs logical flags.

grep() returns the indices or matching values (if value=TRUE), while grepl() returns a logical vector indicating which elements matched.

❓ Predict Output

advanced

Output of grep with fixed = TRUE

What is the output of this R code?

vec <- c("a.c", "abc", "a-c", "a.c")
grep("a.c", vec, fixed = TRUE)

R Programming

vec <- c("a.c", "abc", "a-c", "a.c")
grep("a.c", vec, fixed = TRUE)

A [1] 1 4

B [1] 1 2 4

C [1] 1 3 4

D [1] 2 3

💡 Hint

fixed = TRUE treats the pattern as a plain string, not a regular expression.

With fixed = TRUE, the pattern "a.c" matches exactly "a.c" strings at positions 1 and 4.

❓ Predict Output

expert

Count of matches using grepl and sum

What is the value of count after running this R code?

vec <- c("red", "green", "blue", "yellow", "redgreen")
count <- sum(grepl("red", vec))
count

R Programming

vec <- c("red", "green", "blue", "yellow", "redgreen")
count <- sum(grepl("red", vec))
count

A 4

B 2

C 1

D 3

💡 Hint

grepl returns TRUE for each element containing "red"; sum counts TRUE as 1.

The elements "red" and "redgreen" contain "red", so count is 2.