Merge and update methods in Ruby

Introduction

Merge and update methods help you combine two sets of information into one. They make it easy to add or change data in a hash (a collection of key-value pairs).

You want to combine two lists of settings into one.

You need to add new information to an existing collection without losing old data.

You want to change some values in a collection based on new input.

You want to create a new combined collection without changing the original.

You want to update the original collection directly with new data.

Syntax

Ruby

hash1.merge(hash2)  # returns a new hash with combined keys and values
hash1.merge!(hash2) # updates hash1 with hash2's keys and values

hash1.update(hash2) # same as merge! - updates hash1 with hash2

merge returns a new hash and does not change the original.

merge! and update change the original hash directly.

Examples

This creates a new hash combining h1 and h2. If keys overlap, h2's values are used.

Ruby

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

This changes h1 by adding h2's keys and values. Overlapping keys get updated.

Ruby

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

update works like merge! and changes h1 directly.

Ruby

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

Sample Program

This program shows how merge creates a new combined hash without changing the original, while update changes the original hash directly.

Ruby

settings = {volume: 5, brightness: 7}
new_settings = {brightness: 10, contrast: 3}

# Using merge to create a new hash
combined = settings.merge(new_settings)
puts "Combined settings: #{combined}"

# Original settings stay the same
puts "Original settings: #{settings}"

# Using update to change original
settings.update(new_settings)
puts "Updated original settings: #{settings}"

Important Notes

If keys overlap, the values from the second hash replace those in the first.

Use merge when you want to keep the original hash unchanged.

Use merge! or update when you want to change the original hash.

Summary

merge returns a new hash combining two hashes.

merge! and update change the original hash.

Overlapping keys get their values replaced by the second hash.

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