Identity comparison (===) in Swift - Time & Space Complexity

Time Complexity: Identity comparison (===)

O(n)

Understanding Time Complexity

We want to understand how fast identity comparison works in Swift.

Specifically, how the cost changes when we compare objects using the identity operator ===.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

class Person {}

let people = [Person(), Person(), Person(), Person(), Person()]

for i in 0..

This code checks each person in the array to see if it is the same object as the first person.

Identify Repeating Operations

Look for repeated actions that take time.

• Primary operation: The identity comparison === inside the loop.
• How many times: It runs once for each person in the array.

How Execution Grows With Input

As the number of people grows, the number of comparisons grows the same way.

Input Size (n)	Approx. Operations
10	10 comparisons
100	100 comparisons
1000	1000 comparisons

Pattern observation: The number of comparisons grows directly with the number of items.

Final Time Complexity

Time Complexity: O(n)

This means the time to finish grows in a straight line as the list gets bigger.

Common Mistake

[X] Wrong: "Identity comparison === is instant and does not depend on the number of items."

[OK] Correct: While each comparison is quick, doing many comparisons adds up, so total time depends on how many times you compare.

Interview Connect

Understanding how identity checks scale helps you explain performance when working with object references in Swift.

Self-Check

What if we stopped the loop early when we find the first match? How would the time complexity change?

Practice

1. What does the === operator check in Swift?

easy

A. If two variables refer to the exact same object instance

B. If two variables have the same value

C. If two variables are of the same type

D. If two variables are both nil

Solution

1. Step 1: Understand the purpose of ===

   The === operator in Swift is used to check if two references point to the exact same object in memory.

2. Step 2: Differentiate from value equality

   Unlike ==, which checks if values are equal, === checks identity, meaning the same instance.

3. Final Answer:

   If two variables refer to the exact same object instance -> Option A

4. Quick Check:

   Identity comparison = If two variables refer to the exact same object instance [OK]

Hint: Remember: === means same object, not just equal value [OK]

Common Mistakes:

• Confusing === with == for value equality
• Thinking === checks type equality
• Assuming === works with structs or enums

2. Which of the following is the correct syntax to check if two class instances a and b are identical in Swift?

easy

A. if a === b { }

B. if a == b { }

C. if a = b { }

D. if a is b { }

Solution

1. Step 1: Recall identity operator syntax

   In Swift, the identity operator to check if two references point to the same object is ===.

2. Step 2: Identify correct usage

   The correct syntax is if a === b { }. The other options are either value equality, assignment, or invalid syntax.

3. Final Answer:

   if a === b { } -> Option A

4. Quick Check:

   Identity check syntax = if a === b { } [OK]

Hint: Use three equals signs (===) for identity check [OK]

Common Mistakes:

• Using == instead of === for identity
• Using single = which is assignment
• Typing invalid operators like ===

3. Consider the following Swift code:

class Person {}

let p1 = Person()
let p2 = p1
let p3 = Person()

print(p1 === p2)
print(p1 === p3)

What will be the output?

medium

A. false\nfalse

B. false\ntrue

C. true\ntrue

D. true\nfalse

Solution

1. Step 1: Analyze object references

   Variable p2 is assigned to p1, so both point to the same object. p3 is a new instance.

2. Step 2: Evaluate identity comparisons

   p1 === p2 is true because they are the same instance. p1 === p3 is false because they are different instances.

3. Final Answer:

   true\nfalse -> Option D

4. Quick Check:

   Same instance = true, different instance = false [OK]

Hint: Same variable means true; new instance means false [OK]

Common Mistakes:

• Assuming different variables always mean false
• Confusing === with == and expecting value equality
• Ignoring that p2 points to p1's instance

4. The following Swift code gives a compile error. What is the problem?

class Car {}

let car1 = Car()
let car2 = Car()

if car1 == car2 {
    print("Same car")
}

medium

A. Cannot create instances of class Car

B. Missing parentheses in if statement

C. Cannot use == to compare class instances without Equatable conformance

D. Using === instead of ==

Solution

1. Step 1: Understand == operator requirements

   By default, class instances cannot be compared with == unless the class conforms to Equatable.

2. Step 2: Identify the error cause

   The code tries to use == without Car conforming to Equatable, causing a compile error.

3. Final Answer:

   Cannot use == to compare class instances without Equatable conformance -> Option C

4. Quick Check:

   == needs Equatable for classes [OK]

Hint: Use === for identity or add Equatable for == [OK]

Common Mistakes:

• Thinking === causes error here
• Assuming == works automatically for classes
• Ignoring Equatable protocol requirement

5. You have a cache dictionary storing class instances by keys. You want to check if a newly created instance newObj is already stored in the cache by identity. Which Swift code correctly checks this?

class Data {}

var cache: [String: Data] = [:]
let newObj = Data()

// Check if newObj is already in cache by identity

hard

A. if cache.values.contains(newObj) { print("Found") }

B. if cache.values.contains(where: { $0 === newObj }) { print("Found") }

C. if cache.keys.contains(where: { $0 === newObj }) { print("Found") }

D. if cache.values.contains(where: { $0 == newObj }) { print("Found") }

Solution

1. Step 1: Understand identity check in collections

   To check if an instance exists by identity, use === inside a closure with contains(where:).

2. Step 2: Analyze options

   if cache.values.contains(where: { $0 === newObj }) { print("Found") } correctly uses cache.values.contains(where: { $0 === newObj }). if cache.values.contains(newObj) { print("Found") } uses contains without identity check, which requires Equatable. if cache.keys.contains(where: { $0 === newObj }) { print("Found") } incorrectly checks keys (strings) with identity. if cache.values.contains(where: { $0 == newObj }) { print("Found") } uses == which needs Equatable.

3. Final Answer:

   if cache.values.contains(where: { $0 === newObj }) { print("Found") } -> Option B

4. Quick Check:

   Use contains(where:) with === for identity [OK]

Hint: Use contains(where:) and === to check identity in collections [OK]

Common Mistakes:

• Using contains without where and identity check
• Checking keys instead of values
• Using == without Equatable conformance