Is operator for type checking in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Swift code using 'is' operator?

Consider the following Swift code. What will be printed when it runs?

Swift

class Animal {}
class Dog: Animal {}

let pet: Animal = Dog()

if pet is Dog {
    print("It's a dog")
} else {
    print("It's not a dog")
}

A Compile-time error

B It's not a dog

C It's a dog

D Runtime error

💡 Hint

The 'is' operator checks if an instance is of a certain type or inherits from it.

The variable 'pet' is declared as type Animal but holds an instance of Dog. The 'is' operator returns true if the instance is of the specified type or a subclass. So 'pet is Dog' is true, printing "It's a dog".

❓ Predict Output

intermediate

What does this Swift code print when checking type with 'is'?

Analyze the code below and select the output:

Swift

let value: Any = 42

if value is String {
    print("String detected")
} else {
    print("Not a String")
}

A String detected

B Not a String

C Compile-time error

D Runtime error

💡 Hint

The 'is' operator checks if the value is of the specified type.

The variable 'value' holds an Int (42), not a String. So 'value is String' is false, printing "Not a String".

❓ Predict Output

advanced

What is the output of this Swift code using 'is' with protocol conformance?

Given the code below, what will be printed?

Swift

protocol Vehicle {}
class Car: Vehicle {}
class Bicycle {}

let myRide: Any = Car()

if myRide is Vehicle {
    print("It's a vehicle")
} else {
    print("It's not a vehicle")
}

A It's a vehicle

B It's not a vehicle

C Compile-time error

D Runtime error

💡 Hint

The 'is' operator works with protocol conformance as well as class inheritance.

The instance is of class Car which conforms to Vehicle protocol. So 'myRide is Vehicle' is true, printing "It's a vehicle".

❓ Predict Output

advanced

What error does this Swift code raise when using 'is' incorrectly?

Examine the code and select the error it produces when compiled or run:

Swift

let number = 10

if number is Int? {
    print("Optional Int")
} else {
    print("Not Optional Int")
}

A Prints "Not Optional Int"

B Prints "Optional Int"

C Compile-time error: 'is' cannot check optional type like this

D Runtime error: unexpected nil

💡 Hint

Check how 'is' works with optional types and non-optional values.

The variable 'number' is Int, not Int?. The 'is' operator returns false because 'number' is not an Optional Int, so it prints "Not Optional Int". No error occurs.

🧠 Conceptual

expert

Which option correctly explains the behavior of 'is' operator in Swift?

Select the statement that best describes how the 'is' operator works in Swift.

A The 'is' operator performs a runtime cast and changes the instance type if the check passes.

B The 'is' operator checks if an instance is exactly of the specified type, excluding subclasses or protocol conformances.

C The 'is' operator can only be used with classes, not with structs or enums.

D The 'is' operator checks if an instance is of the specified type or any subclass, including protocol conformance, returning true if so.

💡 Hint

Think about inheritance and protocol conformance in Swift type checking.

The 'is' operator returns true if the instance is of the specified type or any subclass, or if it conforms to a protocol. It does not change the instance type and works with classes, structs, enums, and protocols.