Proc composition in Ruby - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a Proc that doubles a number.

Ruby

double = Proc.new { |x| x [1] 2 }

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 2 instead of doubling.

Using - or / will not double the number.

The * operator multiplies the number by 2, doubling it.

2fill in blank

medium

Complete the code to compose two Procs: one that doubles, then one that adds 3.

Ruby

double = Proc.new { |x| x * 2 }
add_three = Proc.new { |x| x [1] 3 }
composed = Proc.new { |x| add_three.call(double.call(x)) }

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

A+

B*

C-

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using * will multiply instead of adding.

Using - or / will subtract or divide instead of adding.

The + operator adds 3 to the doubled number.

3fill in blank

hard

Fix the error in the Proc composition to correctly chain two Procs.

Ruby

square = Proc.new { |x| x * x }
double = Proc.new { |x| x * 2 }
composed = Proc.new { |x| [1].call([2].call(x)) }

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

Asquare

Bcomposed

Cdouble

Dx

Attempts:

3 left

💡 Hint

Common Mistakes

Calling the same Proc twice causes errors.

Using the wrong Proc name in the call.

To double then square, call square.call(double.call(x)). The outer blank should be square (A) and the inner blank double (C).

4fill in blank

hard

Fill both blanks to create a Proc that adds 1 then multiplies by 5.

Ruby

add_one = Proc.new { |x| x [1] 1 }
multiply_five = Proc.new { |x| x [2] 5 }
composed = Proc.new { |x| multiply_five.call(add_one.call(x)) }

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

A+

B-

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction instead of addition.

Using division instead of multiplication.

The first Proc adds 1 using +, the second multiplies by 5 using *.

5fill in blank

hard

Fill all three blanks to create a Proc that squares, subtracts 4, then divides by 2.

Ruby

square = Proc.new { |x| x [1] x }
subtract_four = Proc.new { |x| x [2] 4 }
divide_two = Proc.new { |x| x [3] 2 }
composed = Proc.new { |x| divide_two.call(subtract_four.call(square.call(x))) }

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

A*

B-

C/

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up operators like using + instead of -.

Using multiplication instead of division at the end.

The first Proc squares the number using *, the second subtracts 4 using -, and the third divides by 2 using /.

Practice

1. What does the >> operator do when used with two Procs in Ruby?

easy

A. It merges two Procs into one that runs them both in parallel.

B. It creates a new Proc that runs the first Proc, then passes its result to the second Proc.

C. It swaps the order of execution of two Procs.

D. It deletes the second Proc and keeps only the first.

Solution

1. Step 1: Understand Proc composition operator

   The >> operator chains two Procs so the output of the first becomes input of the second.

2. Step 2: Explain the effect of chaining

   Calling the combined Proc runs the first, then passes its result to the second, producing a final result.

3. Final Answer:

   It creates a new Proc that runs the first Proc, then passes its result to the second Proc. -> Option B

4. Quick Check:

   Proc composition = chaining with >> [OK]

Hint: Remember >> chains Procs passing output as input [OK]

Common Mistakes:

• Thinking >> runs Procs in parallel
• Assuming >> swaps execution order
• Believing >> deletes one Proc

2. Which of the following is the correct syntax to compose two Procs p1 and p2 in Ruby?

easy

A. combined = p1 >> p2

B. combined = p1 >>> p2

C. combined = p1 << p2

D. combined = p1 >> p2.call

Solution

1. Step 1: Recall correct Proc composition operator

   The operator to compose Procs is >>, which is written as >> but in Ruby code it is >> (two greater-than signs).

2. Step 2: Check each option

   combined = p1 >> p2 uses >> correctly. combined = p1 >>> p2 uses three > which is invalid. combined = p1 << p2 uses << which is not for composition. combined = p1 >> p2.call calls p2 immediately, which is wrong.

3. Final Answer:

   combined = p1 >> p2 -> Option A

4. Quick Check:

   Proc composition syntax = p1 >> p2 [OK]

Hint: Use exactly two > signs to compose Procs [OK]

Common Mistakes:

• Using >>> instead of >>
• Using << instead of >>
• Calling Proc instead of composing

3. What is the output of this Ruby code?

p1 = Proc.new { |x| x + 2 }
p2 = Proc.new { |y| y * 3 }
combined = p1 >> p2
puts combined.call(4)

medium

A. 18

B. 14

C. 20

D. 12

Solution

1. Step 1: Evaluate first Proc with input 4

   p1 adds 2 to input: 4 + 2 = 6.

2. Step 2: Pass result 6 to second Proc

   p2 multiplies input by 3: 6 * 3 = 18.

3. Final Answer:

   18 -> Option A

4. Quick Check:

   (4 + 2) * 3 = 18 [OK]

Hint: Apply first Proc, then second on result [OK]

Common Mistakes:

• Adding instead of multiplying in second Proc
• Multiplying first, then adding
• Confusing order of Procs

4. Identify the error in this Ruby code that tries to compose two Procs:

p1 = Proc.new { |x| x * 2 }
p2 = Proc.new { |y| y + 5 }
combined = p1 >> p2.call
puts combined.call(3)

medium

A. Syntax error due to missing parentheses

B. Using wrong operator for composition

C. Missing block parameters in p1

D. Calling p2 immediately instead of composing

Solution

1. Step 1: Check how p2 is used in composition

   The code uses p2.call which calls p2 immediately, returning a value, not a Proc.

2. Step 2: Understand correct composition usage

   To compose, use p1 >> p2 without calling p2. Calling p2 breaks composition.

3. Final Answer:

   Calling p2 immediately instead of composing -> Option D

4. Quick Check:

   Compose Procs without calling them [OK]

Hint: Do not call Procs when composing, just use >> [OK]

Common Mistakes:

• Calling Proc instead of composing
• Using wrong operator like <<
• Forgetting block parameters

5. Given these Procs:

p1 = Proc.new { |x| x.to_s }
p2 = Proc.new { |s| s + "!" }
p3 = Proc.new { |str| str.reverse }

How do you compose them so that calling the combined Proc with 5 returns "5!"?

hard

A. combined = p3 >> p2 >> p1

B. combined = p1 >> p2 >> p3

C. combined = p1 >> p3 >> p2

D. combined = p2 >> p1 >> p3

Solution

1. Step 1: Understand each Proc's effect

   p1 converts number to string, p2 adds "!", p3 reverses string.

2. Step 2: Find composition order to get "5!"

   "5".reverse = "5", so p1 >> p3 >> p2 gives: "5" -> "5" -> "5!". But p1 >> p2 >> p3: "5" -> "5!" -> "!5". Only A produces "5!".

3. Final Answer:

   combined = p1 >> p3 >> p2 -> Option C

4. Quick Check:

   to_s, reverse, then add ! = "5!" [OK]

Hint: Order Procs by desired transformations stepwise [OK]

Common Mistakes:

• Putting p2 before p3 changes output to "!5"
• Reversing order causes errors on non-string
• Ignoring effect of reverse on "5!"