Reduce/inject for accumulation in Ruby - Mini Project: Build & Apply

Sum of Prices Using Reduce/Inject

📖 Scenario: You are managing a small store's inventory. Each product has a price, and you want to find out the total value of all products combined.

🎯 Goal: Build a Ruby program that uses inject to calculate the total price of all products in a dictionary.

📋 What You'll Learn

Create a hash called products with exact product-price pairs

Create a variable called total_price to hold the sum

Use inject on products.values to calculate the total

Print the total_price value

💡 Why This Matters

🌍 Real World

Calculating totals is common in shopping carts, budgets, and reports where you add many numbers together.

💼 Career

Knowing how to use <code>inject</code> helps you write clean and efficient Ruby code for data processing tasks.

1

Create the products hash

Create a hash called products with these exact entries: 'apple' => 100, 'banana' => 50, 'orange' => 75

Ruby

# Create the products hash with exact entries
# Your code here

Hint

Use curly braces {} to create a hash with keys and values separated by =>.

2

Create the total_price variable

Create a variable called total_price and set it to 0

Ruby

products = { 'apple' => 100, 'banana' => 50, 'orange' => 75 }
# Create total_price variable and set to 0
# Your code here

Hint

Just write total_price = 0 to start your sum at zero.

3

Use inject to sum the prices

Use inject on products.values to add all prices and assign the result to total_price

Ruby

products = { 'apple' => 100, 'banana' => 50, 'orange' => 75 }
total_price = 0
# Use inject on products.values to sum prices
# Your code here

Hint

Use inject(0) { |sum, price| sum + price } to add all values starting from zero.

4

Print the total price

Write puts total_price to display the total price

Ruby

products = { 'apple' => 100, 'banana' => 50, 'orange' => 75 }
total_price = products.values.inject(0) { |sum, price| sum + price }
# Print the total_price
# Your code here

Hint

Use puts total_price to print the number on the screen.

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