Reduce/inject for accumulation in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the reduce (or inject) method do in Ruby?

It combines all elements of a collection by applying a block or operation, accumulating a single result.

beginner

How do you start an accumulation with a specific initial value using reduce?

Pass the initial value as an argument to reduce, like array.reduce(0) { |sum, n| sum + n }.

beginner

What is the difference between reduce and inject in Ruby?

There is no difference; inject is an alias for reduce.

beginner

Example: What does [1, 2, 3].reduce(:+) return?

It returns 6, the sum of all elements.

intermediate

Can reduce be used to accumulate non-numeric results? Give an example.

Yes. For example, ['a', 'b', 'c'].reduce('') { |acc, s| acc + s } returns 'abc' by joining strings.

What is the purpose of the reduce method in Ruby?

A To create a new array with transformed elements

B To combine all elements of a collection into a single value

C To filter elements based on a condition

D To sort elements in ascending order

reduce combines elements to produce one result, like a sum or product.

How do you specify an initial value for accumulation with reduce?

A By calling <code>initialize</code> method

B By setting a global variable

C By passing it as an argument to <code>reduce</code>

D By using <code>map</code> before <code>reduce</code>

You pass the initial value directly, e.g., reduce(0).

What will [2, 3, 4].reduce(1) { |product, n| product * n } return?

A 9

B None of the above

C 10

D 24

It multiplies all elements starting from 1: 1*2*3*4 = 24.

Which method is an alias for reduce in Ruby?

A inject

B map

C select

D each

inject is exactly the same as reduce.

Can reduce be used to concatenate strings?

A Yes, by accumulating strings

B No, it only works with numbers

C Only if you convert strings to numbers first

D Only with arrays of integers

You can accumulate strings by joining them inside the block.

Explain how reduce works for accumulating values in a Ruby array.

Think about how you add or multiply all elements step by step.

Give an example of using reduce to join an array of strings into one string.

Start with an empty string and add each element.

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