If–else expression in Rust - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple if–else expression

What is the output of this Rust code snippet?

Rust

fn main() {
    let x = 10;
    let y = if x > 5 { "big" } else { "small" };
    println!("{}", y);
}

A small

B Compilation error

C 10

D big

💡 Hint

Check the condition x > 5 and what the if–else expression returns.

The variable y is assigned the string "big" because x is 10, which is greater than 5.

❓ Predict Output

intermediate

If–else expression with numeric result

What value does variable result hold after running this code?

Rust

fn main() {
    let a = 3;
    let result = if a % 2 == 0 { a * 2 } else { a + 1 };
    println!("{}", result);
}

A 4

B 3

C 6

D Compilation error

💡 Hint

Check if a is even or odd and what the if–else expression returns.

Since 3 is odd, the else branch runs, so result is 3 + 1 = 4.

❓ Predict Output

advanced

Nested if–else expression output

What is printed by this Rust program?

Rust

fn main() {
    let n = 7;
    let description = if n < 5 {
        "small"
    } else if n < 10 {
        "medium"
    } else {
        "large"
    };
    println!("{}", description);
}

A large

B small

C medium

D Compilation error

💡 Hint

Check the conditions in order and see which branch matches n = 7.

7 is not less than 5, but it is less than 10, so the second branch runs.

❓ Predict Output

advanced

If–else expression with mismatched types

What error does this Rust code produce?

Rust

fn main() {
    let flag = true;
    let value = if flag { 10 } else { "ten" };
    println!("{:?}", value);
}

A Runtime panic

B Type mismatch error

C Compilation succeeds and prints 10

D Compilation succeeds and prints "ten"

💡 Hint

Check the types returned by the if and else branches.

The if branch returns an integer, the else branch returns a string slice, causing a type mismatch error.

🧠 Conceptual

expert

Why must if–else expressions have matching types?

In Rust, why must the if and else branches of an if–else expression return the same type?

A Because Rust uses static typing and the expression must have a single, known type at compile time.

B Because Rust allows dynamic typing only inside if–else expressions.

C Because the else branch is optional and can be omitted.

D Because Rust automatically converts all types to strings in if–else expressions.

💡 Hint

Think about how Rust checks types before running the program.

Rust is statically typed, so every expression must have a single, known type at compile time. If branches return different types, the compiler cannot determine the expression's type.

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