Gsub and sub for replacement in Ruby - Mini Project: Build & Apply

Using gsub and sub for Replacement in Ruby

📖 Scenario: Imagine you have a list of product names with inconsistent formatting. You want to clean up these names by replacing certain words or characters.

🎯 Goal: You will create a Ruby program that uses sub and gsub methods to replace parts of strings. This will help you understand how to change text in Ruby.

📋 What You'll Learn

Create a string variable with a product description

Create a variable to hold the word to replace

Use sub to replace only the first occurrence of a word

Use gsub to replace all occurrences of a word

Print the results to see the changes

💡 Why This Matters

🌍 Real World

Cleaning and formatting text data is common in many jobs, like preparing product descriptions for websites or fixing typos in documents.

💼 Career

Knowing how to replace text efficiently helps in roles like data cleaning, software development, and content management.

1

Create the product description string

Create a string variable called product_description with the value "Fresh apples, apples, and more apples".

Ruby

# Create the product_description string
# Your code here

Hint

Use double quotes to create the string exactly as shown.

2

Create the word to replace

Create a string variable called word_to_replace and set it to "apples".

Ruby

product_description = "Fresh apples, apples, and more apples"
# Create the word_to_replace string
# Your code here

Hint

Remember to use double quotes for the string.

3

Use sub and gsub for replacement

Create two new variables: first_replacement that uses sub on product_description to replace the first occurrence of word_to_replace with "oranges", and all_replacements that uses gsub to replace all occurrences of word_to_replace with "oranges".

Ruby

product_description = "Fresh apples, apples, and more apples"
word_to_replace = "apples"
# Use sub and gsub to replace 'apples' with 'oranges'
# Your code here

Hint

Use sub for the first occurrence and gsub for all occurrences.

4

Print the results

Print the variables first_replacement and all_replacements each on its own line.

Ruby

product_description = "Fresh apples, apples, and more apples"
word_to_replace = "apples"
first_replacement = product_description.sub(word_to_replace, "oranges")
all_replacements = product_description.gsub(word_to_replace, "oranges")
# Print the results
# Your code here

Hint

Use puts to print each result on a new line.

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