grep and grepl in R Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find the indices of elements containing the letter 'a'.

R Programming

words <- c("apple", "banana", "cherry", "date")
indices <- grep([1], words)
print(indices)

Drag options to blanks, or click blank then click option'

A"c"

B"b"

C"a"

D"d"

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to put the pattern inside quotes.

Using the wrong letter in the pattern.

The grep function returns the indices of elements that match the pattern. Here, we look for the letter 'a'.

2fill in blank

medium

Complete the code to check which elements contain the letter 'e' and return TRUE or FALSE.

R Programming

words <- c("apple", "banana", "cherry", "date")
contains_e <- grepl([1], words)
print(contains_e)

Drag options to blanks, or click blank then click option'

A"e"

B"o"

C"a"

D"i"

Attempts:

3 left

💡 Hint

Common Mistakes

Using grep instead of grepl when expecting TRUE/FALSE.

Using a letter not present in any word.

The grepl function returns a logical vector indicating if the pattern is found in each element. Here, we check for 'e'.

3fill in blank

hard

Fix the error in the code to correctly find words containing 'n'.

R Programming

words <- c("apple", "banana", "cherry", "date")
result <- grep([1], words, value = TRUE)
print(result)

Drag options to blanks, or click blank then click option'

An

B"n"

C'n'

Dn"

Attempts:

3 left

💡 Hint

Common Mistakes

Not using quotes around the pattern.

Using mismatched or incomplete quotes.

The pattern must be a string in quotes. Without quotes, R treats it as a variable, causing an error.

4fill in blank

hard

Fill both blanks to create a named logical vector showing which words contain 'a'.

R Programming

words <- c("apple", "banana", "cherry", "date")
result <- setNames(grepl([1], words), [2])
print(result)

Drag options to blanks, or click blank then click option'

A"a"

Bwords

C"words"

Dletters

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around the second blank, which should be a variable.

Using the wrong pattern string.

grepl checks for 'a' in each word. setNames assigns the original words as names to the logical vector.

5fill in blank

hard

Fill all three blanks to create a named vector of words containing 'e' with their indices.

R Programming

words <- c("apple", "banana", "cherry", "date")
indices <- grep([1], words)
selected_words <- words[[2]]
named_vector <- setNames(selected_words, [3])
print(named_vector)

Drag options to blanks, or click blank then click option'

A"e"

Bindices

D"words"

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around indices in the second or third blank.

Using the wrong pattern string.

We search for 'e' with grep, get indices, select words at those indices, and name them with the same indices.

Practice

1. What does the grepl function in R return when searching for a pattern in a character vector?

easy

A. The matching elements themselves

B. The positions of elements matching the pattern

C. A logical vector indicating TRUE or FALSE for each element

D. The count of matches found

Solution

1. Step 1: Understand the purpose of grepl

   grepl checks each element of a vector and returns TRUE if the pattern is found, otherwise FALSE.

2. Step 2: Compare with other functions

   grep returns positions or values, but grepl returns logical TRUE/FALSE values for each element.

3. Final Answer:

   A logical vector indicating TRUE or FALSE for each element -> Option C

4. Quick Check:

   grepl returns logical vector [OK]

Hint: Remember: grepl gives TRUE/FALSE, grep gives positions [OK]

Common Mistakes:

• Confusing grepl with grep output
• Expecting grepl to return matching values
• Thinking grepl returns counts

2. Which of the following is the correct syntax to find the positions of elements containing the word "apple" in vector fruits using grep ignoring case?

easy

A. grep("apple", fruits, ignore.case = TRUE)

B. grepl("apple", fruits, ignore.case = TRUE)

C. grep(fruits, "apple", ignore.case = TRUE)

D. grepl(fruits, "apple", ignore.case = TRUE)

Solution

1. Step 1: Identify correct argument order for grep

   The first argument is the pattern, second is the vector to search.

2. Step 2: Check function name and parameters

   grep("apple", fruits, ignore.case = TRUE) correctly searches for "apple" ignoring case and returns positions.

3. Final Answer:

   grep("apple", fruits, ignore.case = TRUE) -> Option A

4. Quick Check:

   Pattern first, vector second in grep [OK]

Hint: Pattern goes first, vector second in grep [OK]

Common Mistakes:

• Swapping pattern and vector arguments
• Using grepl instead of grep for positions
• Omitting ignore.case parameter

3. What is the output of the following R code?

words <- c("Cat", "dog", "caterpillar", "Dogma")
grep("dog", words, ignore.case = TRUE)

medium

A. [1] TRUE TRUE FALSE FALSE

B. [2] 2 4

C. [1] "dog" "Dogma"

D. [1] 2 4

Solution

1. Step 1: Understand grep output

   grep returns positions of elements matching the pattern.

2. Step 2: Identify matching elements ignoring case

   "dog" matches "dog" (position 2) and "Dogma" (position 4) ignoring case.

3. Final Answer:

   [1] 2 4 -> Option D

4. Quick Check:

   grep returns positions [1] 2 4 [OK]

Hint: grep returns positions, not values or logicals [OK]

Common Mistakes:

• Expecting matching values instead of positions
• Confusing grep with grepl output
• Misreading output format with index labels

4. The following code is intended to return a logical vector indicating which elements contain "blue" ignoring case, but it gives an error. What is the error?

colors <- c("Blue", "red", "green", "BLUE")
grepl(colors, "blue", ignore.case = TRUE)

medium

A. ignore.case is not a valid argument for grepl

B. The pattern and vector arguments are swapped in grepl

C. colors vector contains uppercase letters causing error

D. grepl cannot be used with character vectors

Solution

1. Step 1: Check argument order for grepl

   grepl expects pattern first, then the vector to search.

2. Step 2: Identify swapped arguments in code

   The code uses grepl(colors, "blue", ignore.case = TRUE) which swaps pattern and vector.

3. Final Answer:

   The pattern and vector arguments are swapped in grepl -> Option B

4. Quick Check:

   Pattern first, vector second in grepl [OK]

Hint: Pattern always comes first in grepl [OK]

Common Mistakes:

• Swapping pattern and vector arguments
• Thinking ignore.case is invalid
• Assuming case in vector causes error

5. You have a vector names <- c("Anna", "Annie", "Annette", "Bob", "An"). You want to find all names that start with "Ann" ignoring case. Which code correctly returns the matching names?

hard

A. names[grepl("^Ann", names, ignore.case = TRUE)]

B. grep("Ann", names, ignore.case = TRUE)

C. names[grep("Ann", names)]

D. grepl(names, "^Ann", ignore.case = TRUE)

Solution

1. Step 1: Use grepl with pattern and vector correctly

   To filter names starting with "Ann", use grepl("^Ann", names, ignore.case = TRUE) which returns TRUE for matching elements.

2. Step 2: Subset names vector using logical vector

   Use names[logical_vector] to get matching names.

3. Final Answer:

   names[grepl("^Ann", names, ignore.case = TRUE)] -> Option A

4. Quick Check:

   grepl + logical indexing filters names [OK]

Hint: Use grepl with ^ for start and subset vector [OK]

Common Mistakes:

• Using grep without subsetting vector
• Swapping pattern and vector in grepl
• Not using ^ to match start of string