Gsub and sub for replacement in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the sub method do in Ruby?

The sub method replaces the first occurrence of a pattern in a string with a specified replacement.

beginner

How is gsub different from sub in Ruby?

gsub replaces all occurrences of a pattern in a string, while sub replaces only the first occurrence.

beginner

Example: What is the result of "hello world".sub("l", "x")?

It returns "hexlo world" because only the first l is replaced by x.

beginner

Example: What is the result of "hello world".gsub("l", "x")?

It returns "hexxo worxd" because all l characters are replaced by x.

intermediate

Can sub and gsub accept a block for dynamic replacement?

Yes, both methods can take a block to compute the replacement dynamically for each match.

Which Ruby method replaces only the first match in a string?

A replace

B gsub

C sub

D match

sub replaces only the first occurrence, while gsub replaces all.

What will "banana".gsub("a", "o") return?

A "bonono"

B "banana"

C "bonana"

D "banano"

All a characters are replaced by o, so the result is "bonono".

Which method can take a block to customize replacements?

A gsub only

B both sub and gsub

C sub only

D neither

Both sub and gsub can accept a block for dynamic replacement.

What does "abcabc".sub(/a/, "x") return?

A "xbcxbc"

B "xxcabc"

C "abcabc"

D "xbcabc"

sub replaces only the first a, so the result is "xbcabc".

If you want to replace all digits in a string with "#", which method do you use?

A gsub

B scan

C match

D sub

gsub replaces all matches, so it is used to replace all digits.

Explain the difference between sub and gsub in Ruby.

Think about how many matches each method changes.

Describe how you can use a block with gsub to replace matches dynamically.

Blocks let you decide replacement based on the match.

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