Switch expression (modern C#) - Time & Space Complexity

Time Complexity: Switch expression (modern C#)

O(1)

Understanding Time Complexity

We want to understand how the time it takes to run a switch expression changes as the input grows.

Specifically, does adding more cases make the program slower?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

string GetDayType(int day) => day switch
{
    1 => "Monday",
    2 => "Tuesday",
    3 => "Wednesday",
    4 => "Thursday",
    5 => "Friday",
    6 => "Saturday",
    7 => "Sunday",
    _ => "Invalid"
};
    

This code returns the name of the day based on a number using a modern switch expression.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: None repeating. The switch is a single control structure.
• How many times: Evaluated once per call in constant time.

How Execution Grows With Input

The C# compiler optimizes switch expressions on integers to a jump table (array lookup), which is constant time regardless of the number of cases.

Input Size (number of cases)	Approx. Operations
5	1 lookup
10	1 lookup
100	1 lookup

Pattern observation: Constant time O(1), independent of the number of cases.

Final Time Complexity

Time Complexity: O(1)

This means the execution time is constant, even as the number of cases grows, thanks to compiler optimizations like jump tables.

Common Mistake

[X] Wrong: "Switch expressions check cases one by one, so O(n) where n is number of cases."

[OK] Correct: For integral types like int with known values, the compiler generates a jump table for O(1) lookup, not linear scanning.

Interview Connect

Understanding compiler optimizations for switch shows deep knowledge of how high-level code translates to efficient machine instructions.

Self-Check

"What if the switch expression used a dictionary lookup instead? How would the time complexity change?"

(Hint: Dictionary lookup is amortized O(1), similar to switch jump table.)

Practice

1. What is the purpose of the _ symbol in a modern C# switch expression?

easy

A. It declares a new variable inside the switch.

B. It marks the end of the switch expression.

C. It acts as a default case to catch all unmatched values.

D. It is used to separate multiple cases.

Solution

1. Step 1: Understand the role of _ in switch expressions

   The underscore _ is used as a discard pattern that matches any value not matched by previous cases.

2. Step 2: Recognize it as the default case

   This means it acts like a default or fallback case to handle all other inputs.

3. Final Answer:

   It acts as a default case to catch all unmatched values. -> Option C

4. Quick Check:

   _ = default case [OK]

Hint: Remember '_' means 'anything else' or default [OK]

Common Mistakes:

• Thinking '_' declares a variable
• Confusing '_' with case separator
• Assuming '_' ends the switch

2. Which of the following is the correct syntax for a simple switch expression in C# that returns "Even" for 2 and 4, and "Odd" for 1 and 3?

easy

A. var result = switch(number) { 2: "Even", 4: "Even", 1 => "Odd", 3 => "Odd" };

B. var result = number switch { 2 => "Even", 4 => "Even", 1 => "Odd", 3 => "Odd" };

C. var result = number switch (2 => "Even", 4 => "Even", 1 => "Odd", 3 => "Odd");

D. var result = number switch { case 2: "Even"; case 4: "Even"; case 1: "Odd"; case 3: "Odd"; };

Solution

1. Step 1: Identify correct switch expression syntax

   In modern C#, switch expressions use the syntax: variable switch { pattern => result, ... }.

2. Step 2: Compare options to syntax

   var result = number switch { 2 => "Even", 4 => "Even", 1 => "Odd", 3 => "Odd" }; matches this syntax exactly. Options B, C, and D use incorrect syntax like parentheses or keywords not used in switch expressions.

3. Final Answer:

   var result = number switch { 2 => "Even", 4 => "Even", 1 => "Odd", 3 => "Odd" }; -> Option B

4. Quick Check:

   Correct switch expression syntax = var result = number switch { 2 => "Even", 4 => "Even", 1 => "Odd", 3 => "Odd" }; [OK]

Hint: Switch expressions use curly braces and => for cases [OK]

Common Mistakes:

• Using parentheses instead of braces
• Using 'case' keyword inside switch expression
• Using colons instead of =>

3. What is the output of the following C# code?

int day = 3;
string dayName = day switch
{
  1 => "Monday",
  2 => "Tuesday",
  3 => "Wednesday",
  _ => "Unknown"
};
Console.WriteLine(dayName);

medium

A. Wednesday

B. Monday

C. Unknown

D. Tuesday

Solution

1. Step 1: Match the value of day in the switch expression

   The variable day is 3, so the switch expression looks for the case matching 3.

2. Step 2: Find the corresponding string for case 3

   The case 3 returns "Wednesday". Since it matches, the default case _ is ignored.

3. Final Answer:

   Wednesday -> Option A

4. Quick Check:

   day=3 matches 3 => "Wednesday" [OK]

Hint: Match the variable value to the case pattern [OK]

Common Mistakes:

• Choosing default case instead of matching case
• Confusing case numbers with strings
• Ignoring the switch expression syntax

4. Identify the error in this switch expression code snippet:

string result = value switch
{
  1 => "One",
  2 => "Two",
  3 => "Three",
  default => "Other"
};

medium

A. Using 'default' keyword instead of '_' for default case.

B. Switch expression must use parentheses, not braces.

C. Missing comma after case 3.

D. The variable 'value' is not declared.

Solution

1. Step 1: Check syntax for default case in switch expressions

   Modern C# switch expressions use _ as the default case, not the keyword default.

2. Step 2: Identify the incorrect use of 'default'

   The code uses default => "Other", which is invalid syntax in switch expressions.

3. Final Answer:

   Using 'default' keyword instead of '_' for default case. -> Option A

4. Quick Check:

   Default case = '_' not 'default' [OK]

Hint: Use '_' for default, not 'default' keyword [OK]

Common Mistakes:

• Using 'default' instead of '_'
• Forgetting commas between cases
• Confusing switch statement syntax with switch expression

5. You want to write a switch expression that returns "Small" for numbers less than 10, "Medium" for numbers between 10 and 20 inclusive, and "Large" for numbers greater than 20. Which of the following switch expressions correctly implements this?

hard

A. var size = number switch { < 10 => "Small", 10 <= number <= 20 => "Medium", _ => "Large" };

B. var size = number switch { < 10 => "Small", > 10 => "Medium", _ => "Large" };

C. var size = number switch { < 10 => "Small", > 10 and < 20 => "Medium", _ => "Large" };

D. var size = number switch { < 10 => "Small", >= 10 and <= 20 => "Medium", _ => "Large" };

Solution

1. Step 1: Understand pattern matching syntax in switch expressions

   Modern C# supports relational patterns like < 10 and logical patterns like and to combine conditions.

2. Step 2: Check each option for correct range coverage

   var size = number switch { < 10 => "Small", >= 10 and <= 20 => "Medium", _ => "Large" }; uses < 10 for "Small", and >= 10 and <= 20 for "Medium", which correctly covers 10 to 20 inclusive. The default _ covers "Large".

3. Final Answer:

   var size = number switch { < 10 => "Small", >= 10 and <= 20 => "Medium", _ => "Large" }; -> Option D

4. Quick Check:

   Use relational and logical patterns for ranges [OK]

Hint: Use 'and' to combine conditions in switch patterns [OK]

Common Mistakes:

• Using invalid syntax like '10 <= number <= 20'
• Missing inclusive range boundaries
• Using overlapping or incorrect ranges