Pure functions concept in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a pure function call

What is the output of this Ruby code that uses a pure function?

Ruby

def multiply_by_two(x)
  x * 2
end

result = multiply_by_two(5)
puts result

A 10

B 5

C nil

D Error

💡 Hint

Think about what the function does with the input number.

The function multiply_by_two returns the input number multiplied by 2 without changing anything else. So 5 * 2 equals 10.

🧠 Conceptual

intermediate

Identifying pure functions

Which of the following Ruby methods is a pure function?

A

def add_random(x)
  x + rand(10)
end

B

def square(x)
  x * x
end

C

def increment_counter(x)
  $counter += 1
  x + $counter
end

D

def print_value(x)
  puts x
  x
end

💡 Hint

A pure function always returns the same output for the same input and has no side effects.

Only the square method returns the same output for the same input and does not change anything outside itself.

🔧 Debug

advanced

Why is this function not pure?

This Ruby function looks simple but is not pure. What causes it to be impure?

Ruby

def add_to_list(item, list = [])
  list << item
  list
end

result1 = add_to_list(1)
result2 = add_to_list(2)
puts result2.inspect

A The function returns a list which is mutable, so it is impure.

B The function uses puts which is a side effect.

C The default list argument is shared between calls, causing side effects.

D The function does not return a value explicitly.

💡 Hint

Think about what happens to the default list argument when the function is called multiple times.

In Ruby, default arguments are evaluated once. The same list is used in every call, so the function changes external state making it impure.

📝 Syntax

advanced

Which function definition is a pure function?

Select the Ruby function that is syntactically correct and pure.

A

def add(x, y)
  x + y
  puts "Sum calculated"
end

B

def add(x, y)
  return x + y if x > 0
end

C

def add(x, y)
  @sum = x + y
end

D

def add(x, y)
  x + y
end

💡 Hint

A pure function must not have side effects and must return a value consistently.

Option D returns the sum without side effects. Option D prints output, C changes instance variable, D returns nil if x <= 0.

🚀 Application

expert

Predict the output of a function with external state

What will be printed when this Ruby code runs?

Ruby

def counter
  @count ||= 0
  @count += 1
end

puts counter
puts counter
puts counter

A

1
2
3

B

0
1
2

C

1
1
1

D Error: undefined method or variable

💡 Hint

Look at how the instance variable @count changes each time the function is called.

The @count variable starts at 0 and increments by 1 each call, so outputs are 1, 2, and 3.