String concatenation and << in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code?

Consider the following Ruby code snippet. What will be printed when it runs?

Ruby

str = "Hello"
str << " World"
puts str

A HelloWorld

B Hello World\n

C Hello World

D Hello << World

💡 Hint

The << operator appends the string on the right to the string on the left with a space if included.

The << operator appends the exact string on the right to the string on the left without adding extra spaces. Since " World" includes a space at the start, the output is "Hello World".

❓ Predict Output

intermediate

What does this code print?

What will be the output of this Ruby code?

Ruby

a = "foo"
b = a
b << "bar"
puts a

A foobar

B foo

C bar

D foobarbar

💡 Hint

Remember that << modifies the original string in place.

Both a and b point to the same string object. Using << on b modifies that object, so a also reflects the change.

🔧 Debug

advanced

Why does this code raise an error?

This Ruby code raises an error. What is the cause?

Ruby

str = nil
str << "test"

A NoMethodError because nil does not have << method

B RuntimeError because string is empty

C TypeError because << expects a number

D SyntaxError due to missing quotes

💡 Hint

Check what methods nil supports.

nil in Ruby does not have the << method, so calling str << "test" when str is nil raises NoMethodError.

🧠 Conceptual

advanced

What is the difference between + and << for strings in Ruby?

Choose the correct statement about the difference between + and << when used with strings in Ruby.

A + and << both modify the original string in place

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

C + and << both create new string objects

D + creates a new string object, << modifies the original string in place

💡 Hint

Think about whether the original string changes after using + or <<.

The + operator returns a new string without changing the original, while << appends to the original string modifying it.

❓ Predict Output

expert

What is the final output of this Ruby code?

Analyze the following Ruby code and determine what it prints.

Ruby

str1 = "abc"
str2 = str1
str1 += "def"
str2 << "xyz"
puts str1
puts str2

A abcdef\nabcdef

B abcdef\nabcxyz

C abcxyz\nabcxyz

D abcdefxyz\nabcdefxyz

💡 Hint

Remember that += creates a new string object, while << modifies the original.

str1 += "def" creates a new string object for str1, so str2 still points to the original "abc". Then str2 << "xyz" modifies that original string. So str1 is "abcdef" and str2 is "abcxyz".

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!