Enum backed values in PHP - Time & Space Complexity

Time Complexity: Enum backed values

O(n)

Understanding Time Complexity

When using enum backed values in PHP, it's important to understand how quickly operations like searching or accessing values happen as the number of enum cases grows.

We want to know how the time to find or use a backed value changes when the enum has more cases.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

    enum Status: int {
        case Pending = 1;
        case Active = 2;
        case Completed = 3;
        case Cancelled = 4;
    }

    function getStatusName(int $value): ?string {
        foreach (Status::cases() as $case) {
            if ($case->value === $value) {
                return $case->name;
            }
        }
        return null;
    }
    

This code looks through all enum cases to find the name matching a given backed value.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through all enum cases with foreach.
• How many times: Once for each enum case, until a match is found or all are checked.

How Execution Grows With Input

As the number of enum cases increases, the time to find a matching value grows roughly in direct proportion.

Input Size (n)	Approx. Operations
10	Up to 10 checks
100	Up to 100 checks
1000	Up to 1000 checks

Pattern observation: The number of checks grows linearly with the number of enum cases.

Final Time Complexity

Time Complexity: O(n)

This means the time to find a backed value grows directly with the number of enum cases.

Common Mistake

[X] Wrong: "Looking up a backed value in an enum is instant no matter how many cases there are."

[OK] Correct: Because the code checks each case one by one, the time depends on how many cases exist, so it is not instant for large enums.

Interview Connect

Understanding how enum backed value lookups scale helps you explain how your code behaves with more data, showing you think about efficiency in real situations.

Self-Check

"What if we stored the enum cases in a hash map keyed by value? How would the time complexity change?"

Practice

1.

What is the main purpose of using backed enums in PHP?

easy

A. To allow enums to inherit from classes

B. To link enum cases to fixed strings or numbers

C. To enable enums to have methods

D. To make enums mutable

Solution

1. Step 1: Understand enum backed values

   Backed enums associate each enum case with a fixed string or integer value.

2. Step 2: Identify the purpose

   This linking helps in storing, comparing, or displaying enum data easily.

3. Final Answer:

   To link enum cases to fixed strings or numbers -> Option B

4. Quick Check:

   Backed enums link cases to values = A [OK]

Hint: Backed enums always connect cases to fixed values [OK]

Common Mistakes:

• Thinking enums can inherit classes
• Confusing backed enums with mutable enums
• Assuming enums cannot have methods

2.

Which of the following is the correct syntax to declare a backed enum with string values in PHP?

enum Status: string {
    case ACTIVE = 'active';
    case INACTIVE = 'inactive';
}

easy

A. enum Status: string { case ACTIVE = 'active'; case INACTIVE = 'inactive'; }

B. enum Status { case ACTIVE = 'active'; case INACTIVE = 'inactive'; }

C. enum Status: int { case ACTIVE = 'active'; case INACTIVE = 'inactive'; }

D. enum Status: string { ACTIVE = 'active'; INACTIVE = 'inactive'; }

Solution

1. Step 1: Check enum declaration syntax

   Backed enums require specifying the type after the enum name, e.g., : string.

2. Step 2: Verify case declarations

   Each case must use the case keyword and assign a value with =.

3. Final Answer:

   enum Status: string { case ACTIVE = 'active'; case INACTIVE = 'inactive'; } -> Option A

4. Quick Check:

   Backed enum syntax with type and cases = B [OK]

Hint: Backed enums need type after enum name and case keyword [OK]

Common Mistakes:

• Omitting the type after enum name
• Not using 'case' keyword before cases
• Using wrong type like int for string values

3.

What will be the output of the following PHP code?

enum Color: int {
    case RED = 1;
    case GREEN = 2;
    case BLUE = 3;
}

echo Color::from(2)->name;

medium

A. Color::GREEN

B. 2

C. GREEN

D. Error: No case for value 2

Solution

1. Step 1: Understand Color::from(2)

   The from() method returns the enum case matching the value 2, which is GREEN.

2. Step 2: Access the name property

   Using ->name returns the case name as a string, here "GREEN".

3. Final Answer:

   GREEN -> Option C

4. Quick Check:

   from(2) returns GREEN name = GREEN [OK]

Hint: from(value) returns enum case; ->name gives case name [OK]

Common Mistakes:

• Printing the value instead of the name
• Confusing from() with tryFrom()
• Expecting full enum class name output

4.

Identify the error in the following PHP enum backed values code:

enum Status: string {
    case PENDING = 'pending';
    case COMPLETED = 'completed';
}

$status = Status::from('done');

medium

A. No error, code runs fine

B. Compile error: Enum must use int type

C. Syntax error: Missing semicolon after enum cases

D. Runtime error: Value 'done' not found in enum cases

Solution

1. Step 1: Check the from() method usage

   The from() method throws an error if the value does not match any enum case.

2. Step 2: Analyze the value 'done'

   Since 'done' is not a defined case value, a runtime error occurs.

3. Final Answer:

   Runtime error: Value 'done' not found in enum cases -> Option D

4. Quick Check:

   from() with invalid value causes runtime error = D [OK]

Hint: from() fails if value not in enum cases [OK]

Common Mistakes:

• Assuming from() returns null on failure
• Thinking syntax error due to semicolons
• Believing enum must be int type only

5.

Given the following enum, which code snippet correctly converts a string value to an enum case safely without throwing an error?

enum Direction: string {
    case NORTH = 'north';
    case SOUTH = 'south';
    case EAST = 'east';
    case WEST = 'west';
}

Choose the correct usage:

hard

A. $dir = Direction::tryFrom('east');

B. $dir = Direction::from('southwest');

C. $dir = Direction::from('up');

D. $dir = Direction::tryFrom('down');

Solution

1. Step 1: Understand from() vs tryFrom()

   from() throws error if value not found; tryFrom() returns null instead.

2. Step 2: Check each option's value

   Only tryFrom('east') matches a valid case and safely returns the enum case.

3. Final Answer:

   $dir = Direction::tryFrom('east'); -> Option A

4. Quick Check:

   tryFrom() safely converts valid value = C [OK]

Hint: Use tryFrom() to avoid errors on invalid values [OK]

Common Mistakes:

• Using from() with invalid values causing errors
• Expecting tryFrom() to throw errors
• Confusing enum case names with values