Reduce/inject for accumulation in Ruby - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to sum all numbers in the array using inject.

Ruby

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

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

Ainject

Breduce

Cmap

Dselect

Attempts:

3 left

💡 Hint

Common Mistakes

Using map instead of inject, which returns an array instead of a single value.

Using select, which filters elements but does not accumulate.

The inject method is used to accumulate a value by applying the block to each element. Here, it sums all numbers starting from 0.

2fill in blank

medium

Complete the code to multiply all numbers in the array using reduce.

Ruby

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

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

Ainject

Bmap

Creduce

Deach

Attempts:

3 left

💡 Hint

Common Mistakes

Using map which returns an array instead of a single value.

Using each which returns the original array.

The reduce method multiplies all numbers starting from 1, accumulating the product.

3fill in blank

hard

Fix the error in the code to correctly sum the lengths of all words.

Ruby

words = ["apple", "banana", "cherry"]
total_length = words.inject(0) { |sum, word| sum + word.[1] }
puts total_length

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

Alength

Bcount

Csize

Dlength()

Attempts:

3 left

💡 Hint

Common Mistakes

Using length() with parentheses which is valid but less idiomatic.

Using count which counts elements matching a condition, not length.

The length method returns the number of characters in a string. It is used here to sum all word lengths.

4fill in blank

hard

Fill both blanks to create a hash mapping words longer than 5 characters to their lengths.

Ruby

words = ["apple", "banana", "cherry", "date"]
lengths = words.inject({}) { |h, word| (word.[1] [2] 5) ? h.update(word => word.[1]) : h }
puts lengths

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

Alength

Bsize

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' which selects shorter words.

Using size instead of length, which is acceptable but less common.

The code creates a hash with words as keys and their lengths as values, but only for words longer than 5 characters.

5fill in blank

hard

Fill all three blanks to create a hash with uppercase words as keys and original words as values if length greater than 4.

Ruby

words = ["pear", "peach", "plum", "pineapple"]
result = words.inject({}) { |h, word| (word.length [3] 4) ? h.update([1] => [2]) : h }
puts result

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

Aword.upcase

Bword

C>

Dword.length

Attempts:

3 left

💡 Hint

Common Mistakes

Using word.length as key instead of value.

Using '<' instead of '>' in the condition.

The hash keys are uppercase words, values are original words, filtered by length greater than 4.

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