Pattern matching in switch in C Sharp (C#) - Time & Space Complexity

Time Complexity: Pattern matching in switch

O(n)

Understanding Time Complexity

We want to understand how the time it takes to run a switch statement with pattern matching changes as the input changes.

Specifically, how does the number of checks grow when we add more patterns?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

object obj = GetInput();
switch (obj)
{
    case int i:
        Console.WriteLine($"Integer: {i}");
        break;
    case string s when s.Length > 5:
        Console.WriteLine($"Long string: {s}");
        break;
    case null:
        Console.WriteLine("Null value");
        break;
    default:
        Console.WriteLine("Other type");
        break;
}
    

This code checks the input against several patterns to decide what to print.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The switch statement checks each pattern one by one until it finds a match.
• How many times: At most, it checks all patterns once each in order.

How Execution Grows With Input

Each new pattern adds one more check to do in the worst case.

Input Size (number of patterns)	Approx. Operations (pattern checks)
4	Up to 4 checks
10	Up to 10 checks
100	Up to 100 checks

Pattern observation: The number of checks grows directly with the number of patterns.

Final Time Complexity

Time Complexity: O(n)

This means the time to find a matching pattern grows linearly with the number of patterns to check.

Common Mistake

[X] Wrong: "The switch with pattern matching checks all patterns instantly or in constant time."

[OK] Correct: The switch checks patterns one after another until it finds a match, so more patterns mean more checks.

Interview Connect

Understanding how pattern matching scales helps you explain code efficiency clearly and shows you know how language features work under the hood.

Self-Check

What if the switch used a hash-based lookup for patterns? How would the time complexity change?

Practice

1. What does pattern matching in a switch statement primarily help you do in C#?

easy

A. Check an object's type and properties in a clear way

B. Loop through collections more efficiently

C. Declare variables with less code

D. Handle exceptions automatically

Solution

1. Step 1: Understand pattern matching purpose

   Pattern matching in switch lets you test an object's type and properties clearly.

2. Step 2: Compare with other options

   Looping, variable declaration, and exception handling are unrelated to pattern matching in switch.

3. Final Answer:

   Check an object's type and properties in a clear way -> Option A

4. Quick Check:

   Pattern matching = type and property checks [OK]

Hint: Pattern matching checks types and conditions clearly [OK]

Common Mistakes:

• Confusing pattern matching with loops
• Thinking it handles exceptions
• Believing it declares variables automatically

2. Which of the following is the correct syntax to match an integer variable num equal to 10 using pattern matching in a switch expression?

easy

A. case int n when n == 10:

B. case int n == 10:

C. case int n => 10:

D. case int n is 10:

Solution

1. Step 1: Recall correct pattern matching syntax

   To match a value with a condition, use case Type variable when condition:.

2. Step 2: Check each option

   case int n when n == 10: uses when n == 10, which is correct. Others use invalid syntax.

3. Final Answer:

   case int n when n == 10: -> Option A

4. Quick Check:

   Use 'when' for conditions in pattern matching [OK]

Hint: Use 'when' keyword for conditions in switch patterns [OK]

Common Mistakes:

• Using '==' directly after case without 'when'
• Using '=>' or 'is' incorrectly in case
• Omitting the variable name

3. What will be the output of this C# code?

object obj = 42;
switch (obj)
{
    case int n when n > 40:
        Console.WriteLine("Greater than 40");
        break;
    case int n:
        Console.WriteLine("Integer");
        break;
    default:
        Console.WriteLine("Other");
        break;
}

medium

A. Integer

B. Other

C. Greater than 40

D. No output (compile error)

Solution

1. Step 1: Identify the object type and value

   obj is an integer with value 42.

2. Step 2: Evaluate switch cases in order

   First case matches int n when n > 40, which is true for 42, so prints "Greater than 40" and breaks.

3. Final Answer:

   Greater than 40 -> Option C

4. Quick Check:

   Pattern with 'when' matched first [OK]

Hint: Check cases top to bottom; first match runs [OK]

Common Mistakes:

• Ignoring the 'when' condition
• Assuming second case runs first
• Thinking default runs for int

4. Identify the error in this switch statement using pattern matching:

object val = "hello";
switch (val)
{
    case string s when s.Length > 3
        Console.WriteLine("Long string");
        break;
    default:
        Console.WriteLine("Other");
        break;
}

medium

A. Cannot use 'when' with string type

B. Missing colon ':' after case pattern

C. Missing parentheses around condition

D. Incorrect variable name in case

Solution

1. Step 1: Check switch case syntax

   Each case pattern must end with a colon ':' before the statements.

2. Step 2: Locate missing colon

   The case line lacks ':' after when s.Length > 3, causing syntax error.

3. Final Answer:

   Missing colon ':' after case pattern -> Option B

4. Quick Check:

   Case patterns need ':' after condition [OK]

Hint: Always put ':' after case pattern line [OK]

Common Mistakes:

• Forgetting ':' after case pattern
• Adding parentheses around 'when' condition
• Thinking 'when' can't be used with strings

5. Given a list of objects items containing integers and strings, how would you use pattern matching in a switch expression to return a new list of strings where:
- Integers greater than 10 become "Big number"
- Strings longer than 5 characters become "Long text"
- All others become "Other"?

Which code snippet correctly implements this?

hard

A. items.Select(item => item switch {
int n when n > 10: "Big number",
string s when s.Length > 5: "Long text",
_ => "Other"
}).ToList();

B. items.Select(item => switch (item) {
case int n if n > 10: "Big number";
case string s if s.Length > 5: "Long text";
default: "Other";
}).ToList();

C. items.Select(item => item switch {
int n if n > 10: "Big number",
string s if s.Length > 5: "Long text",
_ => "Other"
}).ToList();

D. items.Select(item => item switch {
int n when n > 10 => "Big number",
string s when s.Length > 5 => "Long text",
_ => "Other"
}).ToList();

Solution

1. Step 1: Understand switch expression syntax

   Switch expressions use '=>' for cases, and 'when' for conditions.

2. Step 2: Check each option's syntax

   items.Select(item => item switch { int n when n > 10 => "Big number", string s when s.Length > 5 => "Long text", _ => "Other" }).ToList(); uses correct 'item switch { case => result }' with 'when' and '=>'. Other options use invalid syntax ('switch' inside lambda or 'if' instead of 'when'). items.Select(item => item switch { int n when n > 10: "Big number", string s when s.Length > 5: "Long text", _ => "Other" }).ToList(); uses ':' instead of '=>', which is invalid in switch expressions.

3. Final Answer:

   items.Select(item => item switch { int n when n > 10 => "Big number", string s when s.Length > 5 => "Long text", _ => "Other" }).ToList(); -> Option D

4. Quick Check:

   Switch expressions use '=>' and 'when' [OK]

Hint: Use '=>' and 'when' in switch expressions for conditions [OK]

Common Mistakes:

• Using ':' instead of '=>' in switch expressions
• Using 'if' instead of 'when' for conditions
• Trying to put 'switch' keyword inside lambda