String concatenation and << in Ruby - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to concatenate two strings using the + operator.

Ruby

greeting = "Hello, " [1] "world!"

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

A*

B<<

C+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using << instead of + will modify the original string instead of creating a new one.

Using * or - operators will cause errors or unexpected results.

The + operator joins two strings together to make one string.

2fill in blank

medium

Complete the code to append a string to another string using the << operator.

Ruby

name = "Ruby"
name [1] " is fun!"

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

A<<

B+

C*

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using + creates a new string instead of modifying the original.

Using * or - will cause errors or unexpected results.

The << operator appends the right string to the left string, modifying it in place.

3fill in blank

hard

Fix the error in the code to correctly concatenate strings using <<.

Ruby

message = "Hi"
message [1]= " there!"

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

A+

B<<

C+=

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using += works but creates a new string instead of modifying the original.

Using - causes syntax errors.

The << operator can be used with = to append and modify the original string.

4fill in blank

hard

Complete the code to create a hash where keys are words and values are their lengths using string concatenation.

Ruby

words = ["cat", "dog", "bird"]
lengths = {word: for word in words if word[1] 3}

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

A:

B>

C<

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using == instead of > will select only words exactly 3 letters long.

Using < will select shorter words, not longer.

The colon : separates keys and values in a hash. The > operator filters words longer than 3 letters.

5fill in blank

hard

Fill all three blanks to build a hash with uppercase keys and values greater than 3.

Ruby

words = ["apple", "bat", "carrot"]
result = {word[1] word.length for word in words if word.length [2] 3 and word[3] "a"}

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

A.upcase

B>

C.include?

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using < instead of > will select shorter words.

Using == instead of .include? will cause errors.

.upcase converts the word to uppercase for the key. > filters words longer than 3. .include? checks if the word contains 'a'.

Practice

1. What is the main difference between + and << when used with strings in Ruby?

easy

A. + creates a new string, << modifies the original string.

B. + modifies the original string, << creates a new string.

C. Both + and << create new strings without modifying the original.

D. Both + and << modify the original string in place.

Solution

1. Step 1: Understand + behavior

   The + operator joins two strings but returns a new string without changing the original.

2. Step 2: Understand << behavior

   The << operator appends text directly to the original string, modifying it in place.

3. Final Answer:

   + creates a new string, << modifies the original string. -> Option A

4. Quick Check:

   + new string, << modifies original [OK]

Hint: Remember: + new string, << changes original [OK]

Common Mistakes:

• Thinking + changes the original string
• Confusing << with creating a new string
• Assuming both operators behave the same

2. Which of the following is the correct syntax to append " world" to the string variable greeting using << in Ruby?

easy

A. greeting.add(" world")

B. greeting + " world"

C. greeting.append(" world")

D. greeting << " world"

Solution

1. Step 1: Identify the correct operator for appending

   In Ruby, << is used to append text to an existing string variable.

2. Step 2: Check syntax correctness

   greeting << " world" correctly appends " world" to greeting. Other options are either invalid methods or use + which does not modify in place.

3. Final Answer:

   greeting << " world" -> Option D

4. Quick Check:

   Use << to append text in place [OK]

Hint: Use << to add text directly to string [OK]

Common Mistakes:

• Using + which creates a new string
• Trying to use non-existent methods like add
• Confusing method names like append which is not standard

3. What will be the output of the following Ruby code?

str = "Hi"
str << " there"
puts str

medium

A. Hi there

B. Hi

C. ereht iH

D. Error: undefined method << for String

Solution

1. Step 1: Analyze the initial string and operation

   The variable str starts as "Hi". The << operator appends " there" to it, modifying str to "Hi there".

2. Step 2: Understand the output of puts

   puts str prints the string with a newline, but the output shown is the string content without the newline character visible.

3. Final Answer:

   Hi there -> Option A

4. Quick Check:

   Appending with << changes string to "Hi there" [OK]

Hint: Remember << adds text inside original string [OK]

Common Mistakes:

• Expecting str to remain "Hi"
• Confusing puts output with return value
• Thinking << causes an error

4. Identify the error in the following Ruby code snippet:

text = "Hello"
text +<< " World"
puts text

medium

A. No error; output is "Hello World"

B. SyntaxError due to incorrect use of +<<

C. RuntimeError because text is immutable

D. Output is "Hello" because + does not modify original

Solution

1. Step 1: Check the operator usage

   The code uses +<< together, which is not a valid Ruby operator or syntax.

2. Step 2: Understand Ruby syntax rules

   Ruby does not allow chaining + and << without spaces or parentheses; this causes a syntax error.

3. Final Answer:

   SyntaxError due to incorrect use of +<< -> Option B

4. Quick Check:

   Invalid operator +<< causes syntax error [OK]

Hint: Operators + and << cannot be combined directly [OK]

Common Mistakes:

• Assuming +<< is valid syntax
• Expecting + to modify original string
• Ignoring syntax errors from invalid operators

5. You want to build a sentence step by step by appending words stored in an array words = ["I", "love", "Ruby"]. Which code snippet correctly uses << to create the sentence with spaces between words?

hard

A. sentence = ""; words.each { |w| sentence + w + " " }; sentence.strip!

B. sentence = ""; words.each { |w| sentence = w + " " }; sentence.strip!

C. sentence = ""; words.each { |w| sentence << w << " " }; sentence.strip!

D. sentence = ""; words.each { |w| sentence + w << " " }; sentence.strip!

Solution

1. Step 1: Understand how to append words with spaces

   We want to add each word and a space to the same string sentence using << which modifies in place.

2. Step 2: Analyze each option

   sentence = ""; words.each { |w| sentence + w + " " }; sentence.strip! computes a new concatenated string but doesn't assign it back.sentence = ""; words.each { |w| sentence = w + " " }; sentence.strip! overwrites sentence with the current word + space each time.sentence = ""; words.each { |w| sentence << w << " " }; sentence.strip! correctly chains << to append the word then the space in place.sentence = ""; words.each { |w| sentence + w << " " }; sentence.strip! creates and modifies a temporary string but discards the result.

3. Final Answer:

   sentence = ""; words.each { |w| sentence << w << " " }; sentence.strip! -> Option C

4. Quick Check:

   Use chained << to append multiple strings [OK]

Hint: Chain << to append multiple strings efficiently [OK]

Common Mistakes:

• Using + which creates new strings inside loop
• Mixing + and << incorrectly
• Forgetting to remove trailing space with strip!