Merge and update methods in Ruby - Mini Project: Build & Apply

Merge and Update Methods

📖 Scenario: You are managing a small online store's inventory. You have a list of products with their current stock counts. Occasionally, you receive new shipments that update the stock counts or add new products.

🎯 Goal: Build a Ruby program that merges two hashes representing product stocks. The program will update existing products' stock counts and add new products from the shipment.

📋 What You'll Learn

Create a hash called current_stock with exact product names and stock counts

Create a hash called new_shipment with exact product names and stock counts

Use the merge! method with a block to update current_stock by adding stock counts from new_shipment

Print the updated current_stock hash

💡 Why This Matters

🌍 Real World

Managing inventory updates in small business software or online stores often requires merging stock data from different sources.

💼 Career

Understanding how to merge and update hashes is useful for backend developers working with data aggregation, inventory systems, or configuration management.

1

Create the current stock hash

Create a hash called current_stock with these exact entries: 'apple' => 30, 'banana' => 15, 'orange' => 20

Ruby

# Create the current_stock hash with given products and counts
# Your code here

Hint

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

2

Create the new shipment hash

Create a hash called new_shipment with these exact entries: 'banana' => 10, 'orange' => 5, 'grape' => 12

Ruby

current_stock = { 'apple' => 30, 'banana' => 15, 'orange' => 20 }
# Create the new_shipment hash with given products and counts
# Your code here

Hint

Use the same syntax as the first hash to create new_shipment.

3

Merge and update the stock counts

Use the merge! method on current_stock with new_shipment and a block that adds the old and new stock counts for matching products

Ruby

current_stock = { 'apple' => 30, 'banana' => 15, 'orange' => 20 }
new_shipment = { 'banana' => 10, 'orange' => 5, 'grape' => 12 }
# Merge new_shipment into current_stock, adding counts for duplicates
# Your code here

Hint

The block receives the key, old value, and new value. Return the sum to update the stock count.

4

Print the updated stock

Print the current_stock hash to display the updated stock counts

Ruby

current_stock = { 'apple' => 30, 'banana' => 15, 'orange' => 20 }
new_shipment = { 'banana' => 10, 'orange' => 5, 'grape' => 12 }
current_stock.merge!(new_shipment) { |product, old_count, new_count| old_count + new_count }
# Print the updated current_stock hash
# Your code here

Hint

Use puts to print the hash. The output shows the updated stock counts.

Practice

1.

Which Ruby method returns a new hash by combining two hashes without changing the original?

easy

A. merge

B. merge!

C. update

D. replace

Solution

1. Step 1: Understand merge behavior

   merge returns a new hash combining two hashes without modifying the original.

2. Step 2: Compare with merge! and update

   Both merge! and update modify the original hash, so they don't return a new one.

3. Final Answer:

   merge -> Option A

4. Quick Check:

   merge returns new hash [OK]

Hint: Remember: merge returns new hash, merge! updates original [OK]

Common Mistakes:

• Confusing merge and merge! methods
• Thinking update returns a new hash
• Assuming replace merges hashes

2.

Which of the following is the correct syntax to update hash a with hash b in Ruby?

a = {x: 1}

b = {y: 2}

easy

A. a + b

B. a.merge!b

C. a.merge b

D. a.update(b)

Solution

1. Step 1: Check syntax for updating hash

   update is a method that modifies the original hash and takes another hash as argument with parentheses.

2. Step 2: Compare with other options

   merge! also updates but usually called with ! and parentheses are optional but update(b) is the most explicit and correct syntax here.

3. Final Answer:

   a.update(b) -> Option D

4. Quick Check:

   Use update() to modify original hash [OK]

Hint: Use update(hash) to change original hash [OK]

Common Mistakes:

• Using + operator to combine hashes (not valid)
• Forgetting parentheses in method call
• Confusing merge and update syntax

3.

What is the output of this Ruby code?

a = {a: 1, b: 2}

b = {b: 3, c: 4}

c = a.merge(b)

puts c

medium

A. {:a=>1, :b=>2, :c=>4}

B. {:a=>1, :b=>3}

C. {:a=>1, :b=>3, :c=>4}

D. {:b=>3, :c=>4}

Solution

1. Step 1: Understand merge behavior with overlapping keys

   merge returns a new hash combining keys; overlapping keys get values from the second hash.

2. Step 2: Apply to given hashes

   Key :b is in both hashes; value from b (3) replaces value from a (2). Keys :a and :c are included as is.

3. Final Answer:

   {:a=>1, :b=>3, :c=>4} -> Option C

4. Quick Check:

   Overlapping keys replaced by second hash [OK]

Hint: Overlapping keys take second hash's value in merge [OK]

Common Mistakes:

• Assuming original hash is changed by merge
• Not replacing overlapping key values
• Missing keys from either hash

4.

Identify the error in this Ruby code snippet:

a = {x: 10}

b = {y: 20}

a.merge! b

puts a

medium

A. Output will be empty hash

B. No error; code works correctly

C. Runtime error because merge! cannot be used with variables

D. Syntax error due to missing parentheses in merge!

Solution

1. Step 1: Check syntax of merge! method call

   merge! can be called without parentheses in Ruby, so a.merge! b is valid syntax.

2. Step 2: Verify runtime behavior

   merge! updates a with keys and values from b, so a becomes {x: 10, y: 20}. No errors occur.

3. Final Answer:

   No error; code works correctly -> Option B

4. Quick Check:

   merge! updates original hash without error [OK]

Hint: merge! can be called without parentheses [OK]

Common Mistakes:

• Thinking parentheses are mandatory
• Assuming merge! returns a new hash
• Expecting runtime error with variable argument

5.

You have two hashes:

h1 = {a: 1, b: 2, c: 3}

h2 = {b: 20, d: 4}

You want to update h1 with h2 but only keep keys from h2 that have values greater than 10. Which code correctly does this?

hard

A. h1.update(h2.select { |k, v| v > 10 })

B. h1.update(h2.reject { |k, v| v > 10 })

C. h1.merge(h2.select { |k, v| v > 10 })

D. h1.replace(h2.select { |k, v| v > 10 })

Solution

1. Step 1: Filter h2 keys with values > 10

   Use select { |k, v| v > 10 } to keep only keys with values greater than 10.

2. Step 2: Update h1 with filtered h2

   Use update to modify h1 in place with the filtered hash.

3. Step 3: Check other options

   merge! also updates but update is clearer; merge returns new hash and does not modify h1.

4. Final Answer:

   h1.update(h2.select { |k, v| v > 10 }) -> Option A

5. Quick Check:

   Filter then update original hash [OK]

Hint: Filter with select, then update original hash [OK]

Common Mistakes:

• Using merge instead of update (does not modify original)
• Not filtering keys before update
• Using reject incorrectly