If expression in Rust - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Rust code using an if expression?

Consider this Rust code snippet. What will it print?

Rust

fn main() {
    let x = 7;
    let y = if x > 5 { "big" } else { "small" };
    println!("{}", y);
}

A big

B small

C true

D 7

💡 Hint

Remember, the if expression returns a value based on the condition.

The variable y is assigned the string "big" because x is 7, which is greater than 5.

🧠 Conceptual

intermediate

What type must both branches of an if expression have in Rust?

In Rust, when using an if expression to assign a value, what must be true about the types of the values in the if and else branches?

A They must be the same type.

B They can be any types, Rust will convert automatically.

C They must both be integers.

D They must both be strings.

💡 Hint

Think about how Rust enforces type safety.

Rust requires both branches of an if expression to have the same type so the compiler knows the type of the whole expression.

❓ Predict Output

advanced

What does this Rust code print?

Analyze the following Rust code and select the output it produces.

Rust

fn main() {
    let a = 10;
    let b = 20;
    let max = if a > b { a } else { b };
    println!("{}", max);
}

A 10

B 20

C a

D b

💡 Hint

The if expression chooses the bigger number.

Since b (20) is greater than a (10), the else branch runs and max is 20.

🔧 Debug

advanced

What error does this Rust code produce?

Look at this Rust code snippet. What error will the compiler show?

Rust

fn main() {
    let x = 5;
    let y = if x > 3 { 10 } else { "small" };
    println!("{}", y);
}

A no error, prints 10

B unused variable warning

C mismatched types error

D missing semicolon error

💡 Hint

Check the types returned by each branch of the if expression.

The if branch returns an integer (10), but the else branch returns a string ("small"). Rust requires both branches to have the same type, so it raises a type mismatch error.

🚀 Application

expert

How many items does this Rust vector contain after running?

Consider this Rust code using an if expression inside a vector initialization. How many elements does the vector v have?

Rust

fn main() {
    let condition = true;
    let v = vec![1, 2, if condition { 3 } else { 4 }];
    println!("{}", v.len());
}

A 2

B 4

C 1

D 3

💡 Hint

Count all elements added to the vector.

The vector v contains three elements: 1, 2, and either 3 or 4 depending on the condition. Since the condition is true, the third element is 3, so the length is 3.

Practice

1. What does an if expression do in Rust?

easy

A. It chooses between code blocks based on a condition.

B. It repeats code multiple times.

C. It defines a new function.

D. It declares a variable.

Solution

1. Step 1: Understand the purpose of if

   The if expression checks a condition to decide which code block to run.

2. Step 2: Compare with other options

   Repeating code is done by loops, functions define behavior, and variables store data, so these are incorrect.

3. Final Answer:

   It chooses between code blocks based on a condition. -> Option A

4. Quick Check:

   If expression = choose code [OK]

Hint: If checks true/false to pick code [OK]

Common Mistakes:

• Confusing if with loops
• Thinking if declares variables
• Mixing if with function definitions

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

easy

A. if condition: { /* code */ } else: { /* code */ }

B. if condition { /* code */ } else { /* code */ }

C. if (condition) then { /* code */ } else { /* code */ }

D. if condition -> { /* code */ } else -> { /* code */ }

Solution

1. Step 1: Recall Rust if syntax

   Rust uses if condition { ... } else { ... } without parentheses or arrows.

2. Step 2: Check each option

   if condition { /* code */ } else { /* code */ } matches Rust syntax; others use invalid keywords or symbols.

3. Final Answer:

   if condition { /* code */ } else { /* code */ } -> Option B

4. Quick Check:

   Rust if syntax = braces, no parentheses [OK]

Hint: Rust if uses braces, no parentheses [OK]

Common Mistakes:

• Adding parentheses like other languages
• Using 'then' keyword
• Using colons or arrows incorrectly

3. What is the output of this Rust code?

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

medium

A. Compilation error

B. small

C. big

D. 10

Solution

1. Step 1: Evaluate the condition x > 5

   Since x is 10, which is greater than 5, the condition is true.

2. Step 2: Determine which branch runs

   The if expression returns "big" because the condition is true, so y is assigned "big".

3. Final Answer:

   big -> Option C

4. Quick Check:

   10 > 5 is true, so output = big [OK]

Hint: Check condition true or false to pick output [OK]

Common Mistakes:

• Confusing output with variable x
• Thinking else branch runs
• Expecting numeric output

4. Identify the error in this Rust code snippet:

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

medium

A. No error, code runs correctly

B. Mismatched types in if and else branches

C. Missing semicolon after if expression

D. Using % operator incorrectly

Solution

1. Step 1: Check syntax and types

   The if expression returns string slices "even" or "odd", both same type, so no type mismatch.

2. Step 2: Verify semicolons and operators

   The semicolon after the if expression is present, and the modulo operator is used correctly.

3. Final Answer:

   No error, code runs correctly -> Option A

4. Quick Check:

   Correct syntax and types = no error [OK]

Hint: Check types and semicolons carefully [OK]

Common Mistakes:

• Thinking semicolon is missing
• Confusing types in branches
• Misusing modulo operator

5. How can you use an if expression to assign a value to a variable based on multiple conditions in Rust? Choose the correct code snippet.

hard

A. let grade = if score >= 90 -> 'A' else if score >= 80 -> 'B' else -> 'C';

B. let grade = if (score >= 90) then 'A' else if (score >= 80) then 'B' else 'C';

C. let grade = if score >= 90: 'A' elif score >= 80: 'B' else: 'C';

D. let grade = if score >= 90 { 'A' } else if score >= 80 { 'B' } else { 'C' };

Solution

1. Step 1: Understand Rust's if-else if syntax

   Rust uses if condition { value } and else if condition { value } blocks with braces, no keywords like then or arrows.

2. Step 2: Check each option for syntax correctness

   let grade = if score >= 90 { 'A' } else if score >= 80 { 'B' } else { 'C' }; uses correct Rust syntax; others use invalid keywords or punctuation.

3. Final Answer:

   let grade = if score >= 90 { 'A' } else if score >= 80 { 'B' } else { 'C' }; -> Option D

4. Quick Check:

   Rust if-else if uses braces, no then [OK]

Hint: Use braces and else if without extra keywords [OK]

Common Mistakes:

• Adding 'then' keyword
• Using colons or arrows
• Incorrect else if syntax