Dictionary methods and default values in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the updateValue(_:forKey:) method do in a Swift dictionary?

It updates the value for the given key if it exists, or adds a new key-value pair if the key is not present. It returns the old value if it was replaced, or nil if the key was new.

beginner

How can you safely access a value from a Swift dictionary with a default value if the key is missing?

Use the subscript with default value syntax: dict[key, default: defaultValue]. This returns the value for the key if it exists, or the default value otherwise.

intermediate

What is the difference between dict[key] and dict[key, default: defaultValue] in Swift?

dict[key] returns an optional value that is nil if the key is missing. dict[key, default: defaultValue] returns a non-optional value, using the default if the key is missing.

beginner

How does the removeValue(forKey:) method work in Swift dictionaries?

It removes the key-value pair for the given key if it exists and returns the removed value. If the key is not found, it returns nil.

beginner

What happens when you assign a value to a dictionary key using subscript syntax in Swift?

If the key exists, the value is updated. If the key does not exist, a new key-value pair is added to the dictionary.

What does dict.updateValue(10, forKey: "a") return if the key "a" was not in the dictionary?

A 10

B nil

C An error

D The old value for "a"

It returns nil because there was no old value for the key "a" before the update.

How do you provide a default value when accessing a dictionary key in Swift?

A dict.defaultValue(forKey: key)

B dict.get(key, defaultValue)

C dict[key] ?? defaultValue

D dict[key, default: defaultValue]

The correct syntax is dict[key, default: defaultValue]. Option C is valid Swift syntax but uses optional coalescing, not dictionary default subscript.

What type does dict[key] return in Swift?

A Optional value

B Always nil

C Non-optional value

D Array

dict[key] returns an optional because the key might not exist in the dictionary.

Which method removes a key-value pair from a Swift dictionary?

A remove(key:)

B delete(key:)

C removeValue(forKey:)

D pop(key:)

The correct method to remove a key-value pair is removeValue(forKey:).

What happens if you assign a value to a new key in a Swift dictionary using subscript syntax?

A The key-value pair is added

B The dictionary throws an error

C Nothing happens

D The dictionary clears all entries

Assigning a value to a new key adds that key-value pair to the dictionary.

Explain how to update a value in a Swift dictionary and how to handle the old value.

Think about the method that returns the previous value when you change a dictionary entry.

Describe how to access a dictionary value with a default fallback in Swift.

It's a special subscript syntax that avoids optionals.