sub and gsub in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of sub() with repeated pattern

What is the output of this R code snippet?

R Programming

text <- "apple apple apple"
result <- sub("apple", "orange", text)
print(result)

A "orange apple apple"

B "orange orange orange"

C "apple apple orange"

D "apple orange apple"

💡 Hint

Remember that sub() replaces only the first match.

The sub() function replaces only the first occurrence of the pattern. So only the first "apple" becomes "orange".

❓ Predict Output

intermediate

Output of gsub() with multiple replacements

What will this R code print?

R Programming

text <- "cat bat rat"
result <- gsub("at", "og", text)
print(result)

A "cat bat rat"

B "cog bog rog"

C "cog bat rat"

D "cat bog rat"

💡 Hint

gsub() replaces all matches in the string.

The gsub() function replaces all occurrences of the pattern. Here, "at" in each word is replaced by "og".

🔧 Debug

advanced

Why does this sub() call not replace anything?

Consider this R code: text <- "Hello World" result <- sub("world", "Earth", text) print(result) Why does the output remain "Hello World"?

R Programming

text <- "Hello World"
result <- sub("world", "Earth", text)
print(result)

A Because sub() requires the pattern to be a regular expression object

B Because sub() only replaces the first word in the string

C Because sub() is case-sensitive and "world" does not match "World"

D Because the replacement string "Earth" is invalid

💡 Hint

Check the capitalization of the pattern and the text.

The sub() function is case-sensitive by default. The pattern "world" does not match "World" because of the capital W.

📝 Syntax

advanced

Identify the syntax error in this gsub() usage

Which option contains a syntax error when trying to replace "dog" with "cat" in the string "dog dog dog"?

R Programming

text <- "dog dog dog"
result <- gsub(pattern, replacement, text)

A gsub("dog", "cat", text)

B sub("dog", "cat", text)

C )txet ,"tac" ,"god"(busg

D gsub("dog", cat, text)

💡 Hint

Check if the replacement argument is a string or a variable.

Option D uses cat without quotes, which is treated as a variable or function, causing an error if undefined.

🧠 Conceptual

expert

Difference in output length between sub() and gsub()

Given the string "a1b2c3d4", what is the length of the string after applying sub("[0-9]", "X", text) and gsub("[0-9]", "X", text) respectively?

A sub() result length: 8, gsub() result length: 8

B sub() result length: 7, gsub() result length: 7

C sub() result length: 8, gsub() result length: 4

D sub() result length: 7, gsub() result length: 8

💡 Hint

Replacing characters with another single character keeps string length the same.

Both sub() and gsub() replace digits with "X" without changing string length. So both results have length 8.

Practice

1. What is the main difference between sub and gsub in R?

easy

A. sub works only with numbers, gsub works only with text.

B. sub replaces all matches, gsub replaces only the first match.

C. sub replaces only the first match, gsub replaces all matches.

D. sub modifies the original string, gsub returns a new string.

Solution

1. Step 1: Understand sub behavior

   sub replaces only the first occurrence of the pattern in the string.

2. Step 2: Understand gsub behavior

   gsub replaces all occurrences of the pattern in the string.

3. Final Answer:

   sub replaces only the first match, gsub replaces all matches. -> Option C

4. Quick Check:

   sub = first match, gsub = all matches [OK]

Hint: Remember: sub = first, gsub = global (all) [OK]

Common Mistakes:

• Confusing which function replaces all matches
• Thinking sub modifies original string in place
• Assuming gsub only replaces first match

2. Which of the following is the correct syntax to replace the first occurrence of "cat" with "dog" in the string "cat and cat" using sub?

easy

A. sub("cat", "dog", "cat and cat")

B. sub("dog", "cat", "cat and cat")

C. gsub("cat", "dog", "cat and cat")

D. replace("cat", "dog", "cat and cat")

Solution

1. Step 1: Identify correct function and parameters

   sub takes pattern, replacement, and the string in that order.

2. Step 2: Check the parameters match the question

   We want to replace "cat" with "dog" in "cat and cat" using sub.

3. Final Answer:

   sub("cat", "dog", "cat and cat") -> Option A

4. Quick Check:

   Correct syntax with sub and parameters [OK]

Hint: sub(pattern, replacement, string) is the order [OK]

Common Mistakes:

• Swapping pattern and replacement arguments
• Using gsub instead of sub for first occurrence
• Using a non-existent function like replace

3. What will be the output of the following R code?

text <- "apple apple apple"
result <- sub("apple", "orange", text)
print(result)

medium

A. "orange orange orange"

B. "orange apple apple"

C. "apple orange apple"

D. "apple apple orange"

Solution

1. Step 1: Understand sub replaces first match only

   The first "apple" in the string is replaced by "orange".

2. Step 2: Apply replacement to the string

   Result becomes "orange apple apple" because only the first "apple" changes.

3. Final Answer:

   "orange apple apple" -> Option B

4. Quick Check:

   sub replaces first match only [OK]

Hint: sub changes only first match, not all [OK]

Common Mistakes:

• Thinking sub replaces all matches
• Confusing sub with gsub
• Expecting all words to change

4. The following code is intended to replace all spaces with underscores in the string "hello world example" but does not work as expected. What is the error?

text <- "hello world example"
new_text <- sub(" ", "_", text)
print(new_text)

medium

A. The print statement is incorrect syntax.

B. The pattern " " is invalid, use "\\s" instead.

C. The replacement string should be "-" not "_".

D. sub replaces only the first space, use gsub to replace all spaces.

Solution

1. Step 1: Identify function behavior

   sub replaces only the first match, so only the first space is replaced.

2. Step 2: Suggest correct function for all replacements

   To replace all spaces, gsub should be used instead of sub.

3. Final Answer:

   sub replaces only the first space, use gsub to replace all spaces. -> Option D

4. Quick Check:

   Use gsub for all matches, sub for first only [OK]

Hint: Use gsub to replace all matches, not sub [OK]

Common Mistakes:

• Using sub when gsub is needed
• Thinking space pattern is invalid
• Changing replacement character unnecessarily

5. You have a vector of strings: c("cat123", "dog456", "bird789"). You want to remove all digits from each string. Which R code correctly does this using gsub?

hard

A. gsub("[0-9]", "", c("cat123", "dog456", "bird789"))

B. sub("[0-9]", "", c("cat123", "dog456", "bird789"))

C. gsub("[^0-9]+", "", c("cat123", "dog456", "bird789"))

D. sub("\\d", "", c("cat123", "dog456", "bird789"))

Solution

1. Step 1: Identify pattern to remove digits

   The pattern "[0-9]" matches any single digit; "\\d+" matches one or more digits.

2. Step 2: Choose function to replace all digits

   Use gsub to replace all digit occurrences, not just the first.

3. Step 3: Check options

   gsub("[0-9]", "", c("cat123", "dog456", "bird789")) uses gsub with "[0-9]" which removes all digits correctly.

4. Final Answer:

   gsub("[0-9]", "", c("cat123", "dog456", "bird789")) -> Option A

5. Quick Check:

   gsub + digit pattern removes all digits [OK]

Hint: Use gsub with digit pattern to remove all digits [OK]

Common Mistakes:

• Using sub instead of gsub, removing only first digit
• Incorrect regex pattern for digits
• Confusing escape sequences in regex