Control Flow

if Expressions

In Rust, if is an expression, not just a statement. This means it returns a value.

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fn main() {
    let number = 3;

    if number < 5 {
        println!("condition was true");
    } else {
        println!("condition was false");
    }
}

Using if in a let Statement

Since if is an expression, we can use it on the right side of a let statement:

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let condition = true;
let number = if condition { 5 } else { 6 };

Repetition with Loops

Rust has three kinds of loops: loop, while, and for.

1. The loop

loop tells Rust to execute a block of code over and over again forever or until you explicitly tell it to stop.

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fn main() {
    let mut count = 0;
    loop {
        count += 1;
        if count == 3 {
            break; // Stop the loop
        }
    }
}

Returning values from loops: You can add a value after break to return it from the loop:

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let result = loop {
    counter += 1;
    if counter == 10 {
        break counter * 2;
    }
};

2. The while Loop

Runs as long as a condition is true.

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let mut number = 3;
while number != 0 {
    println!("{}!", number);
    number -= 1;
}

3. The for Loop

The most common loop in Rust. It’s used to iterate over a collection or a range.

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fn main() {
    let a = [10, 20, 30, 40, 50];

    for element in a {
        println!("the value is: {}", element);
    }

    // Using a range (1 to 3, excluding 4)
    for number in 1..4 {
        println!("{}!", number);
    }
}

Summary

Today you learned:

• if is an expression and can be used to assign values.
• loop is for infinite repetition (can return values via break).
• while is for conditional repetition.
• for is the safest and most common way to iterate through collections and ranges.

Practice Exercise

1. Write a program that prints numbers from 1 to 20, but for multiples of 3 print “Fizz” and for multiples of 5 print “Buzz” (FizzBuzz!).
2. Use a for loop to reverse an array.
3. Use loop to find the first power of 2 greater than 1000.

Next Steps

Tomorrow, we’ll cover the most important concept in Rust: Ownership! This is what makes Rust different from every other language.