Gsub and sub for replacement in Ruby - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to replace all occurrences of 'cat' with 'dog' in the string.

Ruby

text = "cat and cat"
new_text = text.[1]("cat", "dog")
puts new_text

Drag options to blanks, or click blank then click option'

Asub

Bswap

Creplace

Dgsub

Attempts:

3 left

💡 Hint

Common Mistakes

Using sub instead of gsub causes only the first 'cat' to be replaced.

The gsub method replaces all occurrences of the pattern in the string.

2fill in blank

medium

Complete the code to replace only the first occurrence of 'apple' with 'orange'.

Ruby

fruits = "apple, apple, apple"
new_fruits = fruits.[1]("apple", "orange")
puts new_fruits

Drag options to blanks, or click blank then click option'

Asub

Bgsub

Creplace

Dswap

Attempts:

3 left

💡 Hint

Common Mistakes

Using gsub replaces all occurrences instead of just the first.

The sub method replaces only the first occurrence of the pattern.

3fill in blank

hard

Fix the error in the code to replace all digits with '#' characters.

Ruby

text = "Phone: [phone]"
new_text = text.[1](/\d/, "#")
puts new_text

Drag options to blanks, or click blank then click option'

Asub

Bgsub

Creplace

Dswap

Attempts:

3 left

💡 Hint

Common Mistakes

Using sub replaces only the first digit, not all.

To replace all digits, use gsub with the digit pattern.

4fill in blank

hard

Fill both blanks to create a hash mapping words to their lengths, but only for words longer than 4 letters.

Ruby

words = ["apple", "cat", "banana", "dog"]
lengths = {word: word.[1] for word in words if word.[2] 4}
puts lengths

Drag options to blanks, or click blank then click option'

Alength

Bsize

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using < instead of > causes wrong filtering.

Using size instead of length is valid but not the expected answer here.

Use length to get word length and > to filter words longer than 4.

5fill in blank

hard

Fill all three blanks to create a hash with uppercase words as keys and their lengths as values, only for words with length greater than 3.

Ruby

words = ["tree", "sun", "flower", "sky"]
result = {word.[1]: word.[2] for word in words if word.[3] 3}
puts result

Drag options to blanks, or click blank then click option'

Aupcase

Blength

C>

Ddowncase

Attempts:

3 left

💡 Hint

Common Mistakes

Using downcase instead of upcase changes keys incorrectly.

Using < instead of > filters wrong words.

Use upcase to uppercase keys, length for values, and > to filter words longer than 3.

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