Type inference by the compiler in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Swift code using type inference?

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

Swift

let number = 42
let message = "The answer is \(number)"
print(message)

A 42

B The answer is number

C The answer is 42

D Error: Cannot infer type

💡 Hint

Look at how Swift infers the type of number and uses string interpolation.

Swift infers number as an Int. The string interpolation inserts the value 42 into the string, so the output is "The answer is 42".

❓ Predict Output

intermediate

What type does the compiler infer for this variable?

Given this Swift code, what type does the compiler infer for value?

Swift

let value = 3.14

A String

B Float

C Int

D Double

💡 Hint

Swift defaults to a certain floating-point type when no explicit type is given.

Swift infers value as a Double by default for floating-point literals.

🔧 Debug

advanced

Why does this Swift code cause a compile error?

Examine the code below. Why does the compiler produce an error?

Swift

let x = nil
print(x)

A print requires a non-optional argument

B Cannot infer type for nil without explicit type annotation

C nil is not allowed in Swift

D x is immutable and cannot be printed

💡 Hint

Think about what type nil represents and how Swift infers types.

Swift cannot infer the type of x because nil alone does not specify a type. You must provide a type annotation.

❓ Predict Output

advanced

What is the inferred type and output of this Swift closure?

Consider this Swift code snippet. What is the inferred type of increment and what will be printed?

Swift

let increment = { (number: Int) in number + 1 }
print(increment(5))

A Type: (Int) -> Int, Output: 6

B Type: (Int) -> Int, Output: 5

C Type: (Int) -> Void, Output: 6

D Type: () -> Int, Output: 6

💡 Hint

Look at the closure parameter and return expression.

The closure takes an Int and returns an Int by adding 1. Calling increment(5) returns 6.

🧠 Conceptual

expert

How does Swift's type inference handle mixed numeric literals in an expression?

What is the inferred type of result in the following Swift code?

let result = 5 + 3.0

A Double

B Int

C Float

D Error: Cannot mix Int and Double without casting

💡 Hint

Swift promotes numeric types to the more general type in mixed expressions.

Swift promotes the Int 5 to Double to match 3.0, so the result is inferred as Double.