Conditional Statements

# CHAPTER 7

Conditional Statements #

1. Chapter Introduction #

2. Learning Objectives #

• Use if, else if, and else blocks.

• Understand that conditions must be strict booleans.

• Use if as an Expression to assign values.

• Use the match statement for powerful control flow.

3. The if Statement #

fn main() {
    let number = 7;

    if number < 5 {
        println!("The number is small.");
    } else if number == 7 {
        println!("The number is exactly 7.");
    } else {
        println!("The number is big.");
    }
}

Strict Booleans Only! In languages like JavaScript or Python, you can write if (number). If the number is not 0, it is treated as "truthy". Rust does NOT do this. The condition MUST evaluate to a strict bool (true or false).

let number = 3;

// ERROR: expected `bool`, found integer
// if number { println!("Number exists"); } 

// CORRECT:
if number != 0 {
    println!("Number exists");
}

4. Using if in a let Statement (Expression) #

fn main() {
    let is_vip = true;
    
    // The if expression returns a value directly into 'price'
    let price = if is_vip {
        50 // Notice: no semicolon!
    } else {
        100 // Notice: no semicolon!
    };

    println!("Your ticket price is ${}", price);
}

*Rule: Both the if block and the else block MUST return the exact same data type. You cannot return an integer in one and a string in the other.*

5. The Power of match #

match compares a value against a series of patterns and executes code based on which pattern matches.

fn main() {
    let error_code = 404;

    match error_code {
        200 => println!("OK: Success!"),
        403 => println!("Forbidden: You do not have access."),
        404 => println!("Not Found: The page is missing."),
        500 => println!("Server Error: Something broke."),
        _ => println!("Unknown error code."), // The '_' is a catch-all (default)
    }
}

Why match is awesome:

1. 1. Exhaustive: The compiler forces you to handle *every possible outcome*. If errorcode is an integer, you must provide a catch-all to handle numbers like 999. If you forget, the code will not compile!

1. 2. Returns Values: Just like if, match is an expression and can return values to variables!

let day_num = 3;
let day_name = match day_num {
    1 => "Monday",
    2 => "Tuesday",
    3 => "Wednesday",
    _ => "Weekend",
};
println!("Today is {}", day_name);

6. Mini Project: Grade Calculator #

use std::io;

fn main() {
    println!("Enter your test score (0-100):");
    let mut input = String::new();
    io::stdin().read_line(&mut input).expect("Failed");
    
    let score: u32 = input.trim().parse().expect("Not a number");

    let grade = match score {
        90..=100 => "A", // ..= means an inclusive range from 90 to 100!
        80..=89 => "B",
        70..=79 => "C",
        0..=69 => "F",
        _ => "Invalid Score",
    };

    println!("Your grade is: {}", grade);
}

7. Common Mistakes #

• Missing the catch-all in match: If you write a match statement for an integer and don't include =>, the compiler will throw an error saying "pattern not covered".

11. Interview Questions #

• Q: Explain why Rust does not have truthy/falsy values for non-boolean types.

• Q: Describe how "Exhaustiveness Checking" in a match statement prevents bugs in production.

12. FAQs #

• Can I match on strings? Yes! match mystring.asstr() { "yes" => ..., "no" => ..., _ => ...} works perfectly.

13. Summary #

14. Next Chapter Recommendation #