Gsub and sub for replacement in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Difference between sub and gsub output

What is the output of the following Ruby code?

Ruby

text = "hello hello hello"
result = text.sub("hello", "hi")
puts result

A hi hi hi

B hi hello hello

C hello hi hello

D hello hello hi

💡 Hint

Remember, sub replaces only the first occurrence.

The sub method replaces only the first match of the pattern. So only the first "hello" is replaced with "hi".

❓ Predict Output

intermediate

Using gsub to replace all occurrences

What will this Ruby code print?

Ruby

text = "cat bat rat"
result = text.gsub("at", "og")
puts result

A cat bat rat

B cog bat rat

C gor gob goc

D cog bog rog

💡 Hint

Think about what gsub does to all matches.

The gsub method replaces all occurrences of the pattern. Here, "at" in each word is replaced with "og".

🔧 Debug

advanced

Why does this sub call not replace anything?

Consider this Ruby code:

text = "apple banana apple"
result = text.sub(/Apple/, "orange")
puts result

Why does the output remain unchanged?

A Because the regex is case-sensitive and "Apple" does not match "apple"

B Because sub only replaces the last occurrence

C Because the replacement string is incorrect

D Because sub requires a string pattern, not a regex

💡 Hint

Check the case of the letters in the regex and the string.

The regex /Apple/ is case-sensitive and does not match lowercase "apple". So no replacement happens.

📝 Syntax

advanced

Identify the syntax error in this gsub usage

Which option shows the correct way to replace all digits with "#" in the string "abc123def" using gsub?

A "abc123def".gsub(/\d/, "#")

B "abc123def".gsub(/\d/, #)

C "abc123def".gsub(/\d/, '#')

D "abc123def".gsub(/\d/, "#"

💡 Hint

Look carefully at the quotes and parentheses.

Option A correctly uses double quotes for the string and replacement, and the regex is properly enclosed.

🚀 Application

expert

Using gsub with a block for conditional replacement

What is the output of this Ruby code?

text = "a1b2c3"
result = text.gsub(/\d/) { |digit| (digit.to_i * 2).to_s }
puts result

A a1b2c3

B a12b24c36

C a2b4c6

D a0b0c0

💡 Hint

The block doubles each digit found.

The gsub with a block replaces each digit with its double. So 1→2, 2→4, 3→6.

Practice

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

easy

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

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

C. sub deletes matches, gsub adds new text.

D. sub works only on arrays, gsub works on strings.

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 B

4. Quick Check:

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

Hint: Remember: sub = single replace, gsub = global replace [OK]

Common Mistakes:

• Confusing which method replaces all matches
• Thinking sub works on arrays
• Believing gsub replaces only one 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. "cat and cat".substitute('cat', 'dog')

B. "cat and cat".gsub('cat', 'dog')

C. "cat and cat".replace('cat', 'dog')

D. "cat and cat".sub('cat', 'dog')

Solution

1. Step 1: Identify correct method for first occurrence replacement

   sub replaces only the first match, so it is the correct method here.

2. Step 2: Check method syntax

   The syntax is string.sub(pattern, replacement), so "cat and cat".sub('cat', 'dog') is correct.

3. Final Answer:

   "cat and cat".sub('cat', 'dog') -> Option D

4. Quick Check:

   sub syntax = string.sub(pattern, replacement) [OK]

Hint: Use sub for first match: string.sub('old', 'new') [OK]

Common Mistakes:

• Using gsub instead of sub for first occurrence
• Using non-existent methods like replace or substitute
• Wrong method call syntax

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

text = "hello hello hello"
result = text.sub('hello', 'hi')
puts result

medium

A. hi hello hello

B. hello hi hello

C. hi hi hi

D. hello hello hi

Solution

1. Step 1: Understand what sub does here

   sub replaces only the first occurrence of 'hello' with 'hi'.

2. Step 2: Apply replacement to the string

   The first 'hello' becomes 'hi', the rest remain unchanged, so the string becomes "hi hello hello".

3. Final Answer:

   hi hello hello -> Option A

4. Quick Check:

   sub replaces first match only [OK]

Hint: sub changes only first match, others stay same [OK]

Common Mistakes:

• Thinking sub replaces all matches
• Confusing output with gsub behavior
• Misreading the string content

4. The following code is intended to replace all occurrences of 'apple' with 'orange' in the string, but it only replaces the first one. What is the error?

fruit = "apple banana apple grape"
new_fruit = fruit.sub('apple', 'orange')
puts new_fruit

medium

A. Use gsub instead of sub to replace all occurrences.

B. The string variable should be an array for replacement.

C. The replacement string must be in double quotes.

D. The method sub does not exist in Ruby.

Solution

1. Step 1: Identify the method used

   The code uses sub, which replaces only the first match.

2. Step 2: Correct method for replacing all matches

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

3. Final Answer:

   Use gsub instead of sub to replace all occurrences. -> Option A

4. Quick Check:

   gsub replaces all matches, sub replaces first only [OK]

Hint: Use gsub for all replacements, sub for first only [OK]

Common Mistakes:

• Using sub when gsub is needed
• Thinking sub works on arrays
• Believing method names are case sensitive differently

5. Given the string "2023-04-01 and 2023-05-01", you want to replace only the month part (the two digits after the first dash) with '12' for all dates. Which Ruby code correctly does this using gsub and a regex?

hard

A. "2023-04-01 and 2023-05-01".sub(/-(\d{2})-/, '-12-')

B. "2023-04-01 and 2023-05-01".gsub(/\d{2}-/, '12-')

C. "2023-04-01 and 2023-05-01".gsub(/-(\d{2})-/, '-12-')

D. "2023-04-01 and 2023-05-01".gsub(/-\d{2}/, '-12')

Solution

1. Step 1: Identify the pattern to replace

   The month is the two digits between dashes, so the regex -(\d{2})- matches the dash, two digits, then dash.

2. Step 2: Use gsub to replace all matches

   Using gsub with the pattern and replacement '-12-' replaces the month part in all dates.

3. Final Answer:

   "2023-04-01 and 2023-05-01".gsub(/-(\d{2})-/, '-12-') -> Option C

4. Quick Check:

   gsub with regex replaces all month parts correctly [OK]

Hint: Use gsub with regex matching month pattern for all replacements [OK]

Common Mistakes:

• Using sub instead of gsub for all matches
• Incorrect regex missing dashes
• Replacing wrong parts of the string