grep and grepl in R Programming - Mini Project: Build & Apply

Using grep and grepl in R to Find Words in a List

📖 Scenario: Imagine you have a list of fruit names and you want to find which fruits contain the word "apple".

🎯 Goal: You will learn how to use grep and grepl functions in R to search for words inside a list of strings.

📋 What You'll Learn

Create a character vector with fruit names

Create a search word variable

Use grep to find positions of fruits containing the search word

Use grepl to get a logical vector showing which fruits contain the search word

Print the results

💡 Why This Matters

🌍 Real World

Searching text data for specific words or patterns is common in data analysis, such as finding keywords in customer feedback or filtering product names.

💼 Career

Knowing how to use <code>grep</code> and <code>grepl</code> helps in data cleaning, text mining, and preparing data for reports or machine learning.

1

Create a vector of fruit names

Create a character vector called fruits with these exact values: "apple", "banana", "pineapple", "orange", "grape"

R Programming

# Create the fruits vector with the given fruit names
# Your code here

Hint

Use the c() function to combine the fruit names into a vector.

2

Create a search word variable

Create a variable called search_word and set it to the string "apple"

R Programming

fruits <- c("apple", "banana", "pineapple", "orange", "grape")
# Create the search_word variable with the value "apple"
# Your code here

Hint

Just assign the string "apple" to the variable search_word.

3

Use grep and grepl to find fruits containing the search word

Use grep with search_word and fruits to find the positions of fruits containing "apple" and store it in positions. Then use grepl with the same inputs to get a logical vector and store it in matches.

R Programming

fruits <- c("apple", "banana", "pineapple", "orange", "grape")
search_word <- "apple"
# Use grep to find positions of fruits containing search_word
# Use grepl to get logical vector of matches
# Your code here

Hint

Remember grep returns positions (numbers), and grepl returns TRUE or FALSE for each element.

4

Print the results

Print the positions vector and then print the matches vector to show which fruits contain the word "apple".

R Programming

fruits <- c("apple", "banana", "pineapple", "orange", "grape")
search_word <- "apple"
positions <- grep(search_word, fruits)
matches <- grepl(search_word, fruits)
# Print positions and matches
# Your code here

Hint

Use print() to show the vectors.

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