If–else expression in Rust - Time & Space Complexity

Time Complexity: If-else expression

O(1)

Understanding Time Complexity

We want to see how the time it takes to run an if-else expression changes as the input changes.

Specifically, does choosing one path or the other affect how long the program runs?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

fn check_number(n: i32) -> &'static str {
    if n > 0 {
        "Positive"
    } else {
        "Non-positive"
    }
}
    

This code checks if a number is positive or not and returns a message accordingly.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: A single if-else check.
• How many times: Exactly once per function call.

How Execution Grows With Input

The program does one simple check no matter what the input number is.

Input Size (n)	Approx. Operations
10	1 check
100	1 check
1000	1 check

Pattern observation: The number of operations stays the same no matter how big the input number is.

Final Time Complexity

Time Complexity: O(1)

This means the time to run the if-else expression does not grow with the input size; it stays constant.

Common Mistake

[X] Wrong: "If-else expressions take longer when the input number is bigger."

[OK] Correct: The if-else only checks one condition once, so the input size does not affect how long it takes.

Interview Connect

Understanding that simple decisions like if-else checks run in constant time helps you explain how your code behaves clearly and confidently.

Self-Check

"What if we added a loop inside the if or else block? How would the time complexity change?"

Practice

1. What does an if-else expression in Rust do?

easy

A. Declares a variable without value

B. Repeats a block of code multiple times

C. Defines a new function

D. Chooses between two options and returns a value

Solution

1. Step 1: Understand the purpose of if-else

   An if-else expression lets the program pick one of two paths based on a condition.

2. Step 2: Recognize that it returns a value

   In Rust, if-else expressions return a value that can be stored in a variable.

3. Final Answer:

   Chooses between two options and returns a value -> Option D

4. Quick Check:

   If-else expression = choice with value [OK]

Hint: If-else picks one path and returns a value [OK]

Common Mistakes:

• Thinking if-else repeats code like a loop
• Confusing if-else with function definition
• Ignoring that if-else returns a value

2. Which of the following is the correct syntax for an if-else expression in Rust?

easy

A. let x = if condition { 5 } else { 10 };

B. if (condition) then { 5 } else { 10 };

C. let x = if condition: 5 else: 10;

D. let x = if condition { 5 } elseif { 10 };

Solution

1. Step 1: Check Rust if-else syntax

   Rust uses if condition { value1 } else { value2 } with braces and no parentheses.

2. Step 2: Identify correct option

   let x = if condition { 5 } else { 10 }; matches Rust syntax exactly with variable assignment and braces.

3. Final Answer:

   let x = if condition { 5 } else { 10 }; -> Option A

4. Quick Check:

   Rust if-else syntax = let x = if condition { 5 } else { 10 }; [OK]

Hint: Use braces and no parentheses in Rust if-else [OK]

Common Mistakes:

• Using parentheses around condition
• Writing elseif instead of else if
• Using colons instead of braces

3. What is the output of this Rust code?

let number = 7;
let result = if number % 2 == 0 { "even" } else { "odd" };
println!("{}", result);

medium

A. odd

B. even

C. 7

D. Syntax error

Solution

1. Step 1: Check the condition number % 2 == 0

   7 % 2 equals 1, which is not 0, so condition is false.

2. Step 2: Determine which branch runs

   Since condition is false, the else branch runs, returning "odd".

3. Final Answer:

   odd -> Option A

4. Quick Check:

   7 is odd, so output = odd [OK]

Hint: Check condition result to pick if or else branch [OK]

Common Mistakes:

• Assuming 7 is even
• Confusing modulo operator
• Expecting number to print instead of result

4. Identify the error in this Rust code:

let x = 10;
let y = if x > 5 { 1 };
println!("{}", y);

medium

A. Variable y is not declared

B. Incorrect comparison operator

C. Missing else branch in if expression

D. println! macro syntax error

Solution

1. Step 1: Check if-else expression completeness

   When assigning with if, Rust requires both if and else branches to return a value.

2. Step 2: Identify missing else branch

   The code has no else branch, so y's value is undefined, causing a compile error.

3. Final Answer:

   Missing else branch in if expression -> Option C

4. Quick Check:

   if-else must have else when used as expression [OK]

Hint: Always include else when assigning from if [OK]

Common Mistakes:

• Forgetting else branch when assigning
• Thinking if alone returns a value
• Misreading error as println! issue

5. You want to assign a variable grade based on a score: if score is 90 or above, grade is 'A', else 'F'. Which code correctly uses if-else expression?

hard

A. let grade = if score > 90 { 'A' } else if score < 90 { 'F' };

B. let grade = if score >= 90 { 'A' } else { 'F' };

C. let grade = if score >= 90 { 'A' }; else { 'F' };

D. let grade = if score >= 90 then { 'A' } else { 'F' };

Solution

1. Step 1: Check correct if-else expression syntax

   Rust uses if condition { value } else { value } without 'then' or semicolons inside.

2. Step 2: Validate condition and branches

   let grade = if score >= 90 { 'A' } else { 'F' }; correctly checks >= 90 and returns 'A' or 'F' in else branch.

3. Final Answer:

   let grade = if score >= 90 { 'A' } else { 'F' }; -> Option B

4. Quick Check:

   Correct syntax and logic = let grade = if score >= 90 { 'A' } else { 'F' }; [OK]

Hint: Use braces and else for complete if-else expression [OK]

Common Mistakes:

• Using 'then' keyword
• Missing else branch
• Incorrect semicolon placement