Merge and update methods in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code using merge?

Consider the following Ruby code snippet. What will be printed?

Ruby

h1 = {a: 1, b: 2}
h2 = {b: 3, c: 4}
result = h1.merge(h2)
puts result

A {:a=>1, :b=>2}

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

C {:a=>1, :b=>3}

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

💡 Hint

Remember that merge returns a new hash with keys from both hashes, and values from the second hash overwrite the first.

The merge method combines two hashes. When keys overlap, the value from the second hash replaces the first. Here, :b is overwritten from 2 to 3.

❓ Predict Output

intermediate

What does update do to the original hash?

Given this Ruby code, what will be the value of h1 after update?

Ruby

h1 = {x: 10, y: 20}
h2 = {y: 30, z: 40}
h1.update(h2)
puts h1

A {:x=>10, :y=>30}

B {:x=>10, :y=>20, :z=>40}

C {:x=>10, :y=>20}

D {:x=>10, :y=>30, :z=>40}

💡 Hint

Update modifies the original hash by adding or replacing keys from the argument hash.

The update method changes h1 in place, adding :z and replacing :y with 30.

🔧 Debug

advanced

Why does this merge code raise an error?

This Ruby code raises an error. What is the cause?

Ruby

h1 = {a: 1, b: 2}
h2 = nil
h1.merge(h2)

A NoMethodError because nil does not have to_hash method

B ArgumentError because merge expects a hash argument

C TypeError because nil cannot be merged

D SyntaxError due to invalid syntax

💡 Hint

Check what merge expects as an argument and what nil is.

merge expects a hash or something convertible to a hash. nil does not respond to to_hash, causing NoMethodError.

❓ Predict Output

advanced

What is the output when using merge with a block?

What will this Ruby code print?

Ruby

h1 = {a: 100, b: 200}
h2 = {b: 300, c: 400}
result = h1.merge(h2) { |key, old_val, new_val| old_val + new_val }
puts result

A {:a=>100, :b=>500, :c=>400}

B {:a=>100, :b=>300, :c=>400}

C {:a=>100, :b=>200, :c=>400}

D {:a=>100, :b=>200, :c=>300}

💡 Hint

The block tells merge how to combine values for duplicate keys.

For key :b, the block adds old and new values: 200 + 300 = 500. Other keys are merged normally.

🧠 Conceptual

expert

How many keys are in the hash after this update?

Given this Ruby code, how many keys does h1 have after update?

Ruby

h1 = {a: 1, b: 2, c: 3}
h2 = {b: 4, d: 5, e: 6}
h1.update(h2)
h1.delete(:c)
h1[:f] = 7
puts h1.keys.size

A 6

B 4

C 5

D 3

💡 Hint

Count keys after update, delete, and adding a new key.

After update, h1 has keys :a, :b, :c, :d, :e. Then :c is deleted, leaving 4 keys. Adding :f makes 5 keys total.

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