Pure functions concept in Ruby - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a pure function that returns the square of a number.

Ruby

def square(num)
  return num [1] num
end

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

A/

B*

C-

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' instead of '*' will add the number to itself, not square it.

Using '/' or '-' will not produce the square.

The function multiplies the input by itself to return the square. This is a pure function because it always returns the same output for the same input without side effects.

2fill in blank

medium

Complete the code to define a pure function that returns the length of a string.

Ruby

def length_of_string(str)
  return str.[1]
end

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

Alength

Bsize

Ccount

Dchars

Attempts:

3 left

💡 Hint

Common Mistakes

Using chars returns an array of characters, not the length.

Using count requires an argument and is not the best choice here.

The method length returns the number of characters in the string. This function is pure because it depends only on the input string and has no side effects.

3fill in blank

hard

Fix the error in the pure function that returns the sum of two numbers.

Ruby

def add_numbers(a, b)
  return a [1] b
end

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

A/

B-

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' subtracts instead of adds.

Using '*' or '/' changes the operation entirely.

The function should add the two inputs using the '+' operator. This keeps the function pure by returning the sum without side effects.

4fill in blank

hard

Complete the code to create a pure function that returns a hash with word lengths for words longer than 3 characters.

Ruby

def word_lengths(words)
  lengths = {word=> word.length for word in words if word.length [1] 3}
  return lengths
end

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

A=>

B<=

C>

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<=' or '==' in the condition will select wrong words.

Using ':' instead of '=>' in the hash mapping is incorrect syntax.

The hash uses the '=>' symbol to map words to their lengths. The condition filters words longer than 3 characters using the '>' operator. This function is pure because it depends only on input and returns a new hash without side effects.

5fill in blank

hard

Fill all three blanks to create a pure function that returns a hash of uppercase words and their lengths for words longer than 4 characters.

Ruby

def uppercase_word_lengths(words)
  result = { [1] => [2] for word in words if word.length [3] 4 }
  return result
end

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

Aword.upcase

Bword.length

C>

Dword

Attempts:

3 left

💡 Hint

Common Mistakes

Using word instead of word.upcase will not uppercase the words.

Using '<' or '<=' instead of '>' will select wrong words.

The function maps uppercase words (word.upcase) to their lengths (word.length) for words longer than 4 characters (using >). This is a pure function because it returns the same output for the same input without side effects.

Practice

1. Which of the following best describes a pure function in Ruby?

easy

A. A function that reads user input and prints output directly.

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

C. A function that modifies global variables to keep track of state.

D. A function that changes the value of its arguments.

Solution

1. Step 1: Understand pure function definition

   A pure function always returns the same result when given the same input and does not affect anything outside itself.

2. Step 2: Compare options to definition

   A function that always returns the same output for the same input and has no side effects. matches this definition exactly. Options A, B, and D involve side effects or changing external state.

3. Final Answer:

   A function that always returns the same output for the same input and has no side effects. -> Option B

4. Quick Check:

   Pure function = consistent output + no side effects [OK]

Hint: Pure functions never change outside data and always return same output [OK]

Common Mistakes:

• Thinking functions that modify variables are pure
• Confusing input/output consistency with side effects
• Assuming printing output is allowed in pure functions

2. Which Ruby function below is a pure function?

easy

A. def add(a, b); a + b; end

B. def add(a, b); puts a + b; end

C. def add(a, b); @sum = a + b; end

D. def add(a, b); print a + b; end

Solution

1. Step 1: Check each function for side effects and output consistency

   def add(a, b); a + b; end returns sum without changing anything outside or printing. def add(a, b); puts a + b; end prints (side effect). def add(a, b); @sum = a + b; end changes instance variable (side effect). def add(a, b); print a + b; end prints (side effect).

2. Step 2: Identify pure function

   Only def add(a, b); a + b; end meets pure function rules: same output for same input, no side effects.

3. Final Answer:

   def add(a, b); a + b; end -> Option A

4. Quick Check:

   Pure function = no side effects + returns value [OK]

Hint: Pure functions return values without printing or changing variables [OK]

Common Mistakes:

• Confusing printing output with returning value
• Assuming modifying instance variables is allowed
• Assuming print doesn't cause side effects

3. What is the output of this Ruby code?

def multiply(x, y)
  x * y
end

result = multiply(3, 4)
puts result

medium

A. Error

B. nil

C. 12

D. 7

Solution

1. Step 1: Understand the multiply function

   The function returns the product of x and y, so multiply(3, 4) returns 12.

2. Step 2: Check the output statement

   puts prints the returned value, so it prints 12.

3. Final Answer:

   12 -> Option C

4. Quick Check:

   multiply(3,4) = 12 [OK]

Hint: Pure functions return predictable output; multiply(3,4) = 12 [OK]

Common Mistakes:

• Thinking function returns nil if no explicit return
• Confusing addition with multiplication
• Expecting error due to missing return keyword

4. Identify the error in this Ruby function that is supposed to be pure:

def increment(n)
  n += 1
  puts n
end

medium

A. It modifies the input variable n.

B. It uses invalid syntax for increment.

C. It does not return any value explicitly.

D. It prints output, causing a side effect.

Solution

1. Step 1: Check for side effects

   The function prints the value of n, which is a side effect because it affects outside world.

2. Step 2: Verify other aspects

   Modifying n locally is allowed; syntax is correct; implicit return is allowed but printing breaks purity.

3. Final Answer:

   It prints output, causing a side effect. -> Option D

4. Quick Check:

   Pure functions must not print or cause side effects [OK]

Hint: Printing inside function breaks purity due to side effects [OK]

Common Mistakes:

• Thinking local variable change breaks purity
• Assuming implicit return is an error
• Ignoring side effects caused by printing

5. You want to write a pure function in Ruby that takes a list of numbers and returns a new list with each number doubled. Which implementation is pure?

hard

A. def double_list(nums)
nums.map { |n| n * 2 }
end

B. def double_list(nums)
nums.collect! { |n| n *= 2 }
nums
end

C. def double_list(nums)
nums.map! { |n| n * 2 }
end

D. def double_list(nums)
nums.clear
nums.push(2, 4, 6)
end

Solution

1. Step 1: Understand pure function requirements

   A pure function returns new data without changing input or external state.

2. Step 2: Analyze each option

   def double_list(nums) nums.collect! { |n| n *= 2 } nums end uses collect! which modifies original array (side effect). def double_list(nums) nums.map { |n| n * 2 } end returns a new array with doubled values without changing original. def double_list(nums) nums.map! { |n| n * 2 } end uses map! which modifies original array (side effect). def double_list(nums) nums.clear nums.push(2, 4, 6) end clears and modifies original array (side effect).

3. Final Answer:

   def double_list(nums) nums.map { |n| n * 2 } end -> Option A

4. Quick Check:

   Pure function = no input change + returns new data [OK]

Hint: Use non-destructive methods like map, avoid modifying input [OK]

Common Mistakes:

• Using destructive methods like map! or clear
• Modifying input array instead of returning new one
• Confusing each with map for transformation