Functions and closures in iOS Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a function in Swift?

A function is a reusable block of code that performs a specific task. It can take inputs, called parameters, and can return a value.

beginner

What is a closure in Swift?

A closure is a self-contained block of code that can be passed around and used in your code. It can capture and store references to variables and constants from the surrounding context.

beginner

How do you declare a simple function that adds two numbers in Swift?

func add(a: Int, b: Int) -> Int { return a + b }

intermediate

What does it mean that closures can "capture" values?

Closures can remember and use constants and variables from the place where they were created, even if that place no longer exists when the closure is called.

intermediate

How can you write a closure that multiplies two numbers and assign it to a variable?

let multiply = { (a: Int, b: Int) -> Int in
  return a * b
}

Which keyword is used to define a function in Swift?

A def

B function

C method

D func

In Swift, the keyword to define a function is func.

What is a closure in Swift?

A A block of code that can be passed around and used later

B A type of variable

C A class method

D A Swift keyword

A closure is a block of code that can be passed around and used later.

How do closures capture values?

A By copying variables from outside their scope

B By referencing variables from their surrounding context

C Closures cannot capture values

D By passing variables as parameters only

Closures capture values by referencing variables from their surrounding context.

Which of the following is the correct syntax for a closure that adds two Ints?

A let add = { (a: Int, b: Int) -> Int in return a + b }

B closure add(a: Int, b: Int) { return a + b }

C func add(a: Int, b: Int) -> Int { return a + b }

D let add = (a: Int, b: Int) -> Int { return a + b }

The correct closure syntax uses curly braces and the 'in' keyword: let add = { (a: Int, b: Int) -> Int in return a + b }.

What keyword is used inside a closure to separate parameters from the body?

A then

B do

C in

D where

The keyword in separates the closure's parameters and return type from its body.

Explain in your own words what a closure is and how it differs from a function in Swift.

Think about how closures can remember variables from where they were created.

Write a simple Swift function and a closure that both add two numbers. Describe their syntax differences.

Remember functions use 'func' keyword; closures use curly braces and 'in'.

Practice

1. What is a closure in Swift?
func greet() { print("Hello") }

easy

A. An unnamed block of code that can be stored and passed around

B. A named function that cannot be reused

C. A variable that holds a number

D. A type of loop

Solution

1. Step 1: Understand closure definition

   A closure is a block of code without a name that can be saved and used later.

2. Step 2: Compare with function

   Functions have names, closures do not but both can be reused.

3. Final Answer:

   An unnamed block of code that can be stored and passed around -> Option A

4. Quick Check:

   Closure = unnamed reusable code block [OK]

Hint: Closures are like functions without names [OK]

Common Mistakes:

• Confusing closures with variables
• Thinking closures must have names
• Mixing closures with loops

2. Which of the following is the correct syntax to declare a closure that takes no parameters and returns an Int?

easy

A. let closure = (Int) -> { return 5 }

B. let closure = { () -> Int in return 5 }

C. let closure = func() -> Int { return 5 }

D. func closure() -> Int { return 5 }

Solution

1. Step 1: Identify closure syntax

   Closures use curly braces with parameters and return type before 'in'.

2. Step 2: Check each option

   'let closure = { () -> Int in return 5 }' matches closure syntax. 'func closure() -> Int { return 5 }' declares a named function. 'let closure = func() -> Int { return 5 }' uses invalid syntax. 'let closure = (Int) -> { return 5 }' lacks 'in' keyword and has wrong parameter placement.

3. Final Answer:

   let closure = { () -> Int in return 5 } -> Option B

4. Quick Check:

   Closure syntax = curly braces + 'in' [OK]

Hint: Closures use braces and 'in' keyword [OK]

Common Mistakes:

• Using func keyword inside closure
• Missing 'in' keyword
• Wrong parameter syntax

3. What will be printed by this Swift code?

let multiply = { (a: Int, b: Int) -> Int in
  return a * b
}
print(multiply(3, 4))

medium

A. 34

B. 7

C. 12

D. Error

Solution

1. Step 1: Understand closure parameters and return

   The closure takes two Ints and returns their product.

2. Step 2: Calculate multiply(3, 4)

   3 * 4 equals 12, so print outputs 12.

3. Final Answer:

   12 -> Option C

4. Quick Check:

   3 times 4 = 12 [OK]

Hint: Closures return calculated values like functions [OK]

Common Mistakes:

• Adding instead of multiplying
• Concatenating numbers as strings
• Syntax errors in closure call

4. Identify the error in this closure declaration:

let add = { (a: Int, b: Int) Int in
  return a + b
}

medium

A. Missing '->' before return type

B. Missing 'in' keyword

C. Parameters should not have types

D. Closure cannot return Int

Solution

1. Step 1: Check closure signature syntax

   Return type must be preceded by '->' in closure signature.

2. Step 2: Identify missing arrow

   Code has '(a: Int, b: Int) Int' but should be '(a: Int, b: Int) -> Int'.

3. Final Answer:

   Missing '->' before return type -> Option A

4. Quick Check:

   Closure return type needs '->' [OK]

Hint: Return type needs '->' before it in closures [OK]

Common Mistakes:

• Omitting '->' in closure signature
• Placing 'in' incorrectly
• Removing parameter types

5. You want to sort an array of names alphabetically using a closure. Which code correctly uses a closure with the sorted(by:) method?

hard

A. let sortedNames = names.sorted(by: { s1, s2 -> Bool { return s1 > s2 } })

B. let sortedNames = names.sorted(by: { s1, s2 in s1 + s2 })

C. let sortedNames = names.sorted(by: (s1, s2) -> s1 < s2)

D. let sortedNames = names.sorted(by: { s1, s2 in s1 < s2 })

Solution

1. Step 1: Understand sorted(by:) closure

   It expects a closure that returns true if first element should come before second.

2. Step 2: Check each option

   'let sortedNames = names.sorted(by: { s1, s2 in s1 < s2 })' returns true when s1 < s2 (correct for ascending alphabetical order). The version with s1 > s2 sorts descending. The version with (s1, s2) -> s1 < s2 uses invalid syntax without braces or 'in'. The version with s1 + s2 returns String, not Bool.

3. Final Answer:

   let sortedNames = names.sorted(by: { s1, s2 in s1 < s2 }) -> Option D

4. Quick Check:

   sorted(by:) needs Bool-returning closure [OK]

Hint: sorted(by:) needs closure returning true/false [OK]

Common Mistakes:

• Returning wrong type from closure
• Using wrong comparison operator
• Incorrect closure syntax