Accessing and setting values in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

How do you access the value of a hash by its key in Ruby?

You use the key inside square brackets after the hash name, like hash[key]. This returns the value stored for that key.

beginner

How do you set or change a value in a Ruby hash?

Assign a new value to a key using hash[key] = value. If the key exists, it updates the value; if not, it adds a new key-value pair.

beginner

What happens if you try to access a key that does not exist in a Ruby hash?

Ruby returns nil if the key is not found in the hash.

intermediate

How can you safely access nested hash values without errors?

You can use the safe navigation operator &. or check if keys exist before accessing nested values to avoid errors.

intermediate

What is the difference between accessing values with hash[key] and hash.fetch(key)?

hash[key] returns nil if the key is missing, while hash.fetch(key) raises an error unless you provide a default value.

How do you get the value for the key :name from a hash called person?

A person.name

B person[:name]

C person->name

D person.get(:name)

In Ruby, you access hash values using square brackets with the key, like person[:name].

What does Ruby return if you access a hash with a key that does not exist?

A nil

B 0

C An error

D false

Ruby returns nil when a key is not found in a hash.

How do you add or update a key :age with value 30 in a hash called person?

A person[:age] = 30

B person.add(:age, 30)

C person.set(:age, 30)

D person.age = 30

You assign the value directly with person[:age] = 30 to add or update a key.

Which method raises an error if the key is missing in a hash?

A hash.get(key)

B hash[key]

C hash.fetch(key)

D hash.find(key)

hash.fetch(key) raises a KeyError if the key is missing, unlike hash[key] which returns nil.

What is a safe way to access nested hash values?

A Use hash.getNested(key1, key2).

B Use dot notation like hash.key.

C Use hash[key][key2] without checks.

D Use safe navigation operator &.

Using the safe navigation operator &. helps avoid errors when accessing nested hashes.

Explain how to access and set values in a Ruby hash with examples.

Think about how you get and change values in a dictionary or map.

What happens when you try to access a key that does not exist in a Ruby hash? How can you handle this safely?

Consider what Ruby returns by default and how to avoid errors.

Practice

1. Which of the following correctly accesses the first element of an array arr in Ruby using indexing?

easy

A. arr[1]

B. arr.first()

C. arr[0]

D. arr.get(0)

Solution

1. Step 1: Understand array indexing in Ruby

   Ruby arrays start at index 0, so the first element is at arr[0].

2. Step 2: Check each option

   arr[0] correctly accesses the first element. arr[1] accesses the second element. arr.first() is a method call that also returns the first element but is not using square brackets. arr.get(0) is invalid syntax in Ruby.

3. Final Answer:

   arr[0] -> Option C

4. Quick Check:

   First element index = 0 [OK]

Hint: Remember: array indexes start at zero in Ruby [OK]

Common Mistakes:

• Using index 1 for first element
• Using method calls instead of brackets
• Trying to use non-existent methods like get()

2. Which syntax correctly sets the value of key :name to "Alice" in a Ruby hash person?

easy

A. person[:name] == "Alice"

B. person.name = "Alice"

C. person["name"] = Alice

D. person[:name] = "Alice"

Solution

1. Step 1: Understand hash key assignment syntax

   To set a value in a hash, use square brackets with the key and assign with =.

2. Step 2: Analyze each option

   person[:name] = "Alice" correctly sets the key :name. person.name = "Alice" is invalid for hashes. person["name"] = Alice misses quotes around Alice, causing error. person[:name] == "Alice" is a comparison, not assignment.

3. Final Answer:

   person[:name] = "Alice" -> Option D

4. Quick Check:

   Use brackets and = for hash assignment [OK]

Hint: Use brackets and = to set hash values [OK]

Common Mistakes:

• Using dot notation for hashes
• Forgetting quotes around string values
• Using == instead of = for assignment

3. What is the output of the following Ruby code?

arr = [10, 20, 30]
arr[1] = 50
puts arr.inspect

medium

A. [10, 50, 30]

B. [10, 20, 30]

C. [50, 20, 30]

D. Error

Solution

1. Step 1: Understand array element assignment

   Assigning arr[1] = 50 changes the element at index 1 (second element) to 50.

2. Step 2: Check the array after assignment

   Original array is [10, 20, 30]. After assignment, it becomes [10, 50, 30].

3. Final Answer:

   [10, 50, 30] -> Option A

4. Quick Check:

   Index 1 value changed to 50 [OK]

Hint: Index 1 means second element in array [OK]

Common Mistakes:

• Thinking index 1 is first element
• Expecting original array unchanged
• Confusing assignment with comparison

4. Identify the error in this Ruby code that tries to set a hash value:

person = {name: "Bob"}
person.name = "Charlie"

medium

A. Missing quotes around "Charlie"

B. Using dot notation to set hash value causes error

C. Hash keys must be strings, not symbols

D. person variable is not defined

Solution

1. Step 1: Understand hash value setting syntax

   Hashes use square brackets with keys to access or set values, not dot notation.

2. Step 2: Analyze the code

   person.name = "Charlie" tries to use dot notation, which is invalid for hashes and causes an error.

3. Final Answer:

   Using dot notation to set hash value causes error -> Option B

4. Quick Check:

   Use brackets, not dot, for hash assignment [OK]

Hint: Use brackets, not dot, to set hash values [OK]

Common Mistakes:

• Using dot notation for hashes
• Confusing symbols and strings for keys
• Assuming variable is undefined

5. Given the nested hash data = {user: {name: "Eve", age: 28}}, how do you change the user's age to 30?

hard

A. data[:user][:age] = 30

B. data[:user.age] = 30

C. data.user[:age] = 30

D. data[:user][age] = 30

Solution

1. Step 1: Access nested hash keys with brackets

   To access nested values, use square brackets for each key: data[:user][:age].

2. Step 2: Assign new value to nested key

   Set data[:user][:age] = 30 to update the age.

3. Final Answer:

   data[:user][:age] = 30 -> Option A

4. Quick Check:

   Use chained brackets for nested hashes [OK]

Hint: Chain brackets for nested hash keys [OK]

Common Mistakes:

• Using dot notation for hashes
• Missing quotes or symbols for keys
• Using undefined variables as keys