grep and grepl in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the grep() function do in R?

grep() searches for patterns in a character vector and returns the indices of the elements that match the pattern.

beginner

How is grepl() different from grep()?

grepl() returns a logical vector (TRUE or FALSE) indicating if the pattern was found in each element, while grep() returns the indices of matching elements.

beginner

What will grep("cat", c("cat", "dog", "caterpillar")) return?

It will return 1 3 because "cat" is found in the 1st and 3rd elements.

beginner

What does grepl("dog", c("cat", "dog", "caterpillar")) return?

It returns FALSE TRUE FALSE showing which elements contain "dog".

intermediate

Can grep() and grepl() use regular expressions?

Yes, both functions support regular expressions to find complex patterns in text.

What does grep() return when it finds matches?

A The matching text itself

B Logical vector of TRUE/FALSE

C Number of matches

D Indices of matching elements

grep() returns the positions (indices) of elements that contain the pattern.

Which function returns a logical vector indicating matches?

A grepl()

B grep()

C gsub()

D sub()

grepl() returns TRUE or FALSE for each element depending on whether the pattern is found.

What will grepl("a", c("apple", "banana", "cherry")) return?

A [TRUE, TRUE, FALSE]

B [FALSE, TRUE, TRUE]

C [TRUE, FALSE, TRUE]

D [FALSE, FALSE, FALSE]

"a" is in "apple" and "banana" but not in "cherry".

If you want to get the actual matching text, which function should you use?

A grep()

B grepl()

C regmatches() with regexpr()

D grepl() with invert = TRUE

To extract matching text, use regmatches() with regexpr() or gregexpr().

Can grep() and grepl() handle case-insensitive searches?

A No, they are case-sensitive only

B Yes, by setting <code>ignore.case = TRUE</code>

C Only <code>grep()</code> can

D Only <code>grepl()</code> can

Both functions accept ignore.case = TRUE to ignore case differences.

Explain the difference between grep() and grepl() in R.

Think about what each function returns when you search text.

How can you use grep() or grepl() to find if a word exists in a list of strings?

Consider what output you want: positions or logical answers.

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