Reduce/inject for accumulation in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code using inject?

Consider the following Ruby code that uses inject to accumulate values. What will be printed?

Ruby

numbers = [1, 2, 3, 4]
result = numbers.inject(0) { |sum, n| sum + n * 2 }
puts result

A 24

B 10

C 20

D 16

💡 Hint

Remember that inject starts with the initial value and adds each element multiplied by 2.

The inject starts at 0. Then it adds 1*2=2, then 2*2=4, then 3*2=6, then 4*2=8. Sum is 2+4+6+8=20.

❓ Predict Output

intermediate

What does this inject code return?

What is the value of result after running this Ruby code?

Ruby

words = ['a', 'b', 'c']
result = words.inject('') { |acc, w| acc + w.upcase }
puts result

A abc

B Error

C aBc

D ABC

💡 Hint

Each word is converted to uppercase before adding to the accumulator.

The accumulator starts as an empty string. Each word is upcased and concatenated: '' + 'A' + 'B' + 'C' = 'ABC'.

❓ Predict Output

advanced

What is the output of this inject with a hash accumulator?

Given this Ruby code, what will be printed?

Ruby

arr = [1, 2, 3, 4]
result = arr.inject(Hash.new(0)) do |hash, n|
  hash[n % 2] += 1
  hash
end
puts result

A {0=>2, 1=>2}

B {0=>1, 1=>3}

C {0=>3, 1=>1}

D TypeError

💡 Hint

Count how many numbers are even (key 0) and odd (key 1).

Numbers 2 and 4 are even (key 0), count 2. Numbers 1 and 3 are odd (key 1), count 2.

❓ Predict Output

advanced

What does this inject code output when accumulating arrays?

What will be the output of this Ruby code?

Ruby

arrays = [[1, 2], [3, 4], [5]]
result = arrays.inject([]) { |acc, arr| acc + arr }
puts result.inspect

A [1, 2, 3, 4, 5]

B [[1, 2, 3, 4, 5]]

C [[1, 2], [3, 4], [5]]

D TypeError

💡 Hint

The + operator concatenates arrays in Ruby.

Starting with empty array, each sub-array is concatenated: [] + [1,2] = [1,2], then + [3,4] = [1,2,3,4], then + [5] = [1,2,3,4,5].

❓ Predict Output

expert

What is the final value of result after this complex inject?

Analyze this Ruby code using inject with a nested block and determine the final value of result.

Ruby

data = [2, 3, 4]
result = data.inject({}) do |hash, n|
  hash[n] = (1..n).inject(1) { |prod, x| prod * x }
  hash
end
puts result.inspect

A NoMethodError

B {2=>2, 3=>6, 4=>24}

C {2=>2, 3=>3, 4=>4}

D {2=>1, 3=>6, 4=>24}

💡 Hint

The inner inject calculates factorial for each number.

For each n, inner inject calculates factorial: 2! = 2, 3! = 6, 4! = 24. These are stored as values in the hash with keys n.

Practice

1. What does the Ruby method inject do when used on an array?

easy

A. It combines all elements into a single value using a block.

B. It sorts the array in ascending order.

C. It removes duplicate elements from the array.

D. It returns the first element of the array.

Solution

1. Step 1: Understand the purpose of inject

   The inject method takes all elements of an array and combines them into one value using the block provided.

2. Step 2: Compare with other options

   Sorting, removing duplicates, or returning the first element are different methods like sort, uniq, or first.

3. Final Answer:

   It combines all elements into a single value using a block. -> Option A

4. Quick Check:

   inject combines elements = C [OK]

Hint: Remember: inject reduces many to one value [OK]

Common Mistakes:

• Confusing inject with sorting methods
• Thinking inject returns an array
• Assuming inject removes duplicates

2. Which of the following is the correct syntax to sum all elements in an array arr using inject?

easy

A. arr.inject { |sum, n| sum / n }

B. arr.inject(0) { |sum, n| sum + n }

C. arr.inject(1) { |sum, n| sum * n }

D. arr.inject { |sum, n| sum - n }

Solution

1. Step 1: Identify the correct initial value and operation

   To sum elements, start with 0 and add each element: inject(0) { |sum, n| sum + n }.

2. Step 2: Check other options

   arr.inject { |sum, n| sum - n } subtracts, C multiplies starting at 1, A divides. Only D sums correctly.

3. Final Answer:

   arr.inject(0) { |sum, n| sum + n } -> Option B

4. Quick Check:

   Sum with inject = D [OK]

Hint: Sum needs 0 start and + inside block [OK]

Common Mistakes:

• Starting sum with 1 instead of 0
• Using wrong operation like subtraction
• Omitting initial value causing errors

3. What is the output of this Ruby code?

numbers = [2, 3, 4]
result = numbers.inject(1) { |product, n| product * n }
puts result

medium

A. Error

B. 9

C. 10

D. 24

Solution

1. Step 1: Understand the initial value and operation

   The initial value is 1, and each element multiplies the accumulator: 1*2=2, 2*3=6, 6*4=24.

2. Step 2: Calculate the final product

   The final result after all multiplications is 24.

3. Final Answer:

   24 -> Option D

4. Quick Check:

   2*3*4 = 24 [OK]

Hint: Multiply with inject starts at 1 [OK]

Common Mistakes:

• Adding instead of multiplying
• Starting with 0 causing zero result
• Confusing output with sum

4. Identify the error in this code snippet:

arr = [1, 2, 3]
sum = arr.inject { |total, n| total + n }
puts sum

medium

A. No error; code works fine.

B. Missing initial value causes incorrect sum.

C. Block parameters are reversed.

D. inject cannot be used without a symbol.

Solution

1. Step 1: Check if initial value is required

   When no initial value is given, inject uses the first element as the start and iterates from the second element.

2. Step 2: Verify the sum calculation

   It correctly sums 1 + 2 + 3 = 6 without error.

3. Final Answer:

   No error; code works fine. -> Option A

4. Quick Check:

   inject without initial value sums correctly = A [OK]

Hint: inject uses first element as start if no initial value [OK]

Common Mistakes:

• Assuming initial value is always required
• Thinking block parameters order is wrong
• Believing inject needs symbol argument

5. Given an array of hashes representing products with keys :name and :price, how can you use inject to find the total price of all products?

products = [
  { name: "Book", price: 12 },
  { name: "Pen", price: 3 },
  { name: "Notebook", price: 7 }
]

hard

A. products.inject { |total, product| total + product.name }

B. products.inject(1) { |total, product| total * product[:price] }

C. products.inject(0) { |total, product| total + product[:price] }

D. products.inject(0) { |total, product| total + product.price }

Solution

1. Step 1: Access the price key correctly

   Each product is a hash, so access price with product[:price].

2. Step 2: Sum prices starting at 0

   Start with 0 and add each price to get total price.

3. Step 3: Check other options

   products.inject { |total, product| total + product.name } uses dot notation which is invalid for hashes, C multiplies prices, D uses dot notation again.

4. Final Answer:

   products.inject(0) { |total, product| total + product[:price] } -> Option C

5. Quick Check:

   Sum prices with inject and hash keys = B [OK]

Hint: Use hash keys with [:key] inside inject block [OK]

Common Mistakes:

• Using dot notation for hash keys
• Multiplying prices instead of summing
• Starting sum at 1 instead of 0