If–else expression in Rust

Introduction

If-else expressions help your program choose between two paths based on a condition. This lets your program make decisions like a person would.

When you want to check if a number is positive or negative and do something different for each.

When you want to decide what message to show based on a user's age.

When you want to set a value depending on whether a condition is true or false.

Syntax

Rust

let variable = if condition {
    value_if_true
} else {
    value_if_false
};

The if and else parts are expressions that return values.

You must include both if and else to assign a value to a variable.

Examples

This checks if number is greater than zero and prints if it is positive or not.

Rust

let number = 5;
let result = if number > 0 {
    "positive"
} else {
    "non-positive"
};
println!("{} is {}", number, result);

This sets can_vote to true if age is 18 or more, otherwise false.

Rust

let age = 18;
let can_vote = if age >= 18 { true } else { false };
println!("Can vote: {}", can_vote);

Sample Program

This program checks the temperature and prints if the weather is hot or cool.

Rust

fn main() {
    let temperature = 30;
    let weather = if temperature > 25 {
        "hot"
    } else {
        "cool"
    };
    println!("The weather is {}.", weather);
}

Important Notes

Remember, if and else in Rust are expressions, so they return values.

Every if must have an else if you want to assign the result to a variable.

Summary

If-else expressions let your program choose between two options.

They return values, so you can store the result in a variable.

Always include else when assigning the result to a variable.

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