Why strings are mutable in Ruby - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to change the string content.

Ruby

str = "hello"
str[1] " world"
puts str

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

A<<

B+=

C==

D===

Attempts:

3 left

💡 Hint

Common Mistakes

Using + which creates a new string instead of modifying.

In Ruby, the << operator appends to the original string, showing mutability.

2fill in blank

medium

Complete the code to replace part of the string.

Ruby

str = "hello"
str[1] 0, 5, "hi"
puts str

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

A[]=

Breplace

Cslice

Dsub

Attempts:

3 left

💡 Hint

Common Mistakes

Using sub which returns a new string without modifying original.

The []= method replaces part of the string in place, showing mutability.

3fill in blank

hard

Fix the error in the code to mutate the string.

Ruby

str = "hello"
str.[1](" world")
puts str

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

Asub

Bconcat

Cgsub

Dreplace

Attempts:

3 left

💡 Hint

Common Mistakes

Using sub or gsub which return new strings without mutating.

The concat method appends to the string in place, mutating it.

4fill in blank

hard

Fill both blanks to create a mutable string and modify it.

Ruby

str = [1]"hello"
str[2] " world"
puts str

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

A"hello"

BString.new(

C<<

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using + which creates a new string instead of modifying.

Using String.new creates a mutable string object, and << appends to it.

5fill in blank

hard

Fill all three blanks to replace part of the string and show mutability.

Ruby

str = "hello"
str[1] 0, 5, [2]
puts str
puts str[3] "!"

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

A[]=

B"hi"

C<<

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using + which creates new strings instead of mutating.

The []= method replaces part of the string, and << appends to it, both mutating the string.

Practice

1. Why are strings mutable in Ruby?

easy

A. Because Ruby allows changing string content without creating new objects

B. Because strings are stored as numbers internally

C. Because Ruby does not support string variables

D. Because strings are immutable in Ruby

Solution

1. Step 1: Understand mutability concept in Ruby strings

   Ruby strings can be changed after creation, meaning their content can be updated without making a new string object.

2. Step 2: Identify why this is useful

   This allows efficient text updates and memory use, as you don't need to create new strings for every change.

3. Final Answer:

   Because Ruby allows changing string content without creating new objects -> Option A

4. Quick Check:

   Ruby strings are mutable = Because Ruby allows changing string content without creating new objects [OK]

Hint: Mutability means you can change strings directly [OK]

Common Mistakes:

• Confusing mutability with immutability
• Thinking strings are stored as numbers
• Believing Ruby strings cannot be changed

2. Which Ruby code correctly changes a string's content?

easy

A. str = "hello"; str.freeze

B. str = "hello"; str[0] = 'H'

C. str = "hello"; str.upcase

D. str = "hello"; str = str + '!'

Solution

1. Step 1: Check each code for mutability

   str = "hello"; str[0] = 'H' changes the first character directly, showing string mutability.

2. Step 2: Compare other options

   str = "hello"; str = str + '!' creates a new string; str = "hello"; str.upcase returns a new uppercase string without changing the original; str = "hello"; str.freeze freezes string making it immutable.

3. Final Answer:

   str = "hello"; str[0] = 'H' -> Option B

4. Quick Check:

   Direct index assignment changes string = str = "hello"; str[0] = 'H' [OK]

Hint: Use index assignment to change string characters [OK]

Common Mistakes:

• Using freeze which prevents changes
• Thinking concatenation changes original string
• Confusing upcase method with direct assignment

3. What is the output of this Ruby code?

str = "cat"
str[1] = 'o'
puts str

medium

A. cat

B. c t

C. cot

D. Error

Solution

1. Step 1: Understand string indexing and assignment

   str[1] = 'o' changes the character at index 1 (second character) from 'a' to 'o'.

2. Step 2: Apply the change and print

   Original string "cat" becomes "cot" after the change, so puts prints "cot".

3. Final Answer:

   cot -> Option C

4. Quick Check:

   Changing index 1 to 'o' = cot [OK]

Hint: Index 1 means second character in string [OK]

Common Mistakes:

• Expecting original string unchanged
• Confusing index positions starting at 1
• Thinking it causes an error

4. Find the error in this Ruby code that tries to change a string:

str = "dog"
str.freeze
str[0] = 'c'
puts str

medium

A. Cannot modify frozen string error

B. Syntax error on line 2

C. No error, output is "cog"

D. puts method is undefined

Solution

1. Step 1: Understand freeze method effect

   freeze makes the string immutable, so any modification causes an error.

2. Step 2: Identify error on modification attempt

   str[0] = 'c' tries to change the frozen string, causing a runtime error "can't modify frozen String".

3. Final Answer:

   Cannot modify frozen string error -> Option A

4. Quick Check:

   freeze prevents changes = error [OK]

Hint: freeze stops string changes, causing error on modification [OK]

Common Mistakes:

• Ignoring freeze effect
• Expecting string to change after freeze
• Thinking freeze causes syntax error

5. You want to update multiple characters in a Ruby string efficiently. Which approach best uses string mutability?

str = "hello"
# Update 'e' and 'o' to 'a' and 'u'

hard

A. str.freeze; str[1] = 'a'

B. str = str.sub('e', 'a').sub('o', 'u')

C. str = "hallu"

D. str[1] = 'a'; str[4] = 'u'

Solution

1. Step 1: Identify direct string mutation

   str[1] = 'a'; str[4] = 'u' changes characters by index directly, using mutability efficiently without creating new strings.

2. Step 2: Compare other options

   str = str.sub('e', 'a').sub('o', 'u') creates new strings with sub; str = "hallu" assigns a new string; str.freeze; str[1] = 'a' freezes string then tries to change causing error.

3. Final Answer:

   str[1] = 'a'; str[4] = 'u' -> Option D

4. Quick Check:

   Direct index assignment updates string efficiently [OK]

Hint: Change characters by index for efficient updates [OK]

Common Mistakes:

• Using sub which creates new strings
• Assigning new string instead of mutating
• Freezing string before modification