How Ruby interprets code at runtime - Practice Exercises

Challenge - 5 Problems

❓ Predict Output

intermediate

Ruby method call and variable scope

What is the output of this Ruby code when run?

Ruby

def greet
  name = "Alice"
  puts "Hello, #{name}!"
end

greet
puts name

A Hello, Alice!\nNameError: undefined local variable or method `name' for main:Object

B Hello, Alice!\nAlice

C NameError: undefined local variable or method `name' for main:Object\nHello, Alice!

D Hello, Alice!\nnil

💡 Hint

Think about where variables defined inside methods are accessible.

The variable name is local to the greet method. It is not accessible outside the method, so puts name raises a NameError.

❓ Predict Output

intermediate

Ruby block variable behavior

What will this Ruby code output?

Ruby

x = 10
[1, 2, 3].each do |x|
  puts x
end
puts x

A 1\n2\n3\n10

B 10\n10\n10\n10

C 1\n2\n3\n3

D 1\n2\n3\nNameError

💡 Hint

Consider if the block variable x affects the outer x.

The block variable x is a new variable local to the block and does not overwrite the outer x. So the last puts x prints 10.

🔧 Debug

advanced

Identify the runtime error in Ruby code

What error does this Ruby code raise when run?

Ruby

def divide(a, b)
  a / b
end

puts divide(10, 0)

A NoMethodError: undefined method `/`

B ArgumentError: wrong number of arguments

C TypeError: String can't be coerced into Integer

D ZeroDivisionError: divided by 0

💡 Hint

What happens when you divide by zero in Ruby?

Dividing by zero in Ruby raises a ZeroDivisionError.

🧠 Conceptual

advanced

Ruby constant lookup at runtime

Given this Ruby code, what will be printed when run?

Ruby

module A
  X = 100
  class B
    X = 200
    def self.show
      puts X
    end
  end
end

A::B.show

A NameError: uninitialized constant X

B 200

C 100

D nil

💡 Hint

Constants inside classes override outer module constants with the same name.

The constant X inside class B is 200, which is used by show. The outer A::X is not used here.

❓ Predict Output

expert

Ruby dynamic method definition and runtime behavior

What is the output of this Ruby code?

Ruby

class C
  define_method(:hello) do |name|
    "Hello, #{name}!"
  end
end

obj = C.new
puts obj.hello("Bob")

A ArgumentError: wrong number of arguments (given 1, expected 0)

B NoMethodError: undefined method `hello' for #<C:0x0000>

C Hello, Bob!

D SyntaxError: unexpected `do'

💡 Hint

Look at how define_method creates methods at runtime.

define_method dynamically defines an instance method hello that takes one argument and returns a greeting string.

Practice

1. What does it mean that Ruby interprets code at runtime?

easy

A. Ruby only checks code syntax but does not run it.

B. Ruby converts all code to machine language before running.

C. Ruby runs code only after saving it to a file.

D. Ruby reads and runs code line by line as the program runs.

Solution

1. Step 1: Understand interpretation

   Interpreting means reading and executing code step by step during program execution.

2. Step 2: Compare options

   The option "Ruby reads and runs code line by line as the program runs." describes this process correctly; others describe compiling or partial actions.

3. Final Answer:

   Ruby reads and runs code line by line as the program runs. -> Option D

4. Quick Check:

   Interpretation = line-by-line execution [OK]

Hint: Interpret means run code step-by-step as you go [OK]

Common Mistakes:

• Confusing interpretation with compilation
• Thinking Ruby only checks syntax
• Believing Ruby runs code after saving only

2. Which of the following is the correct way to print "Hello" in Ruby?

easy

A. echo "Hello"

B. console.log("Hello")

C. print("Hello")

D. printf("Hello %d")

Solution

1. Step 1: Identify Ruby print syntax

   Ruby uses the method print or puts to output text; print("Hello") is valid.

2. Step 2: Check other options

   echo is PHP, console.log is JavaScript, printf("Hello %d") is C-style but errors due to missing argument for %d.

3. Final Answer:

   print("Hello") -> Option C

4. Quick Check:

   Ruby print method = print("Hello") [OK]

Hint: Use print or puts to show text in Ruby [OK]

Common Mistakes:

• Using JavaScript or PHP print commands
• Missing parentheses in print
• Confusing printf with print

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

count = 0
3.times do
  count += 1
end
print count

medium

A. 3

B. 0

C. 1

D. Error

Solution

1. Step 1: Understand the times loop

   The times method runs the block 3 times, increasing count by 1 each time.

2. Step 2: Calculate final count

   Starting from 0, after 3 increments, count becomes 3.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   0 + 1 + 1 + 1 = 3 [OK]

Hint: times runs block that many times, increment count each time [OK]

Common Mistakes:

• Thinking count stays 0
• Confusing times with for loop syntax
• Expecting error due to block

4. Find the error in this Ruby code:

def greet
  puts "Hello"
end

greet(1)

medium

A. ArgumentError: wrong number of arguments

B. No error, prints Hello

C. SyntaxError due to missing parentheses

D. NameError: greet not defined

Solution

1. Step 1: Check method definition

   greet is defined without parameters, so it expects zero arguments.

2. Step 2: Check method call

   Calling greet(1) passes one argument, causing ArgumentError.

3. Final Answer:

   ArgumentError: wrong number of arguments -> Option A

4. Quick Check:

   Method params mismatch = ArgumentError [OK]

Hint: Method defined with no params, calling with one causes error [OK]

Common Mistakes:

• Thinking Ruby ignores extra arguments
• Confusing syntax error with argument error
• Assuming greet is undefined

5. Given this Ruby code:

def add(x, y)
  x + y
end

result = add(2, 3) + add(4, 5)
print result

How does Ruby interpret and calculate the final output?

hard

A. It prints the string 'add(2, 3)add(4, 5)'

B. It adds 2 and 3, then adds 4 and 5, sums both results to print 14

C. It throws an error because add is called twice

D. It adds 2 and 3, ignores second add call, prints 5

Solution

1. Step 1: Evaluate first add call

   add(2, 3) returns 5 by adding 2 + 3.

2. Step 2: Evaluate second add call and sum

   add(4, 5) returns 9; sum 5 + 9 = 14.

3. Step 3: Print the result

   print outputs 14 as the final result.

4. Final Answer:

   14 -> Option B

5. Quick Check:

   5 + 9 = 14 [OK]

Hint: Each method call runs fully, then sums results [OK]

Common Mistakes:

• Thinking only first add runs
• Expecting string concatenation instead of addition
• Assuming error on multiple calls