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.