Quiz — First homework: 0x check

Question

Your first Rust program. Check whether a given string is an Ethereum address starting with 0x. Combines &str.starts_with + .len() + if/else + && — the basics in one function.

Principle (minimum model)

• Function signature. fn is_valid_address(addr: &str) -> bool. Borrow &str; return bool.
• addr.starts_with("0x"). The standard &str method. has_prefix does not exist; contains is substring, not prefix.
• Length check. addr.len() == 42. Ethereum address = 40 hex digits + 0x prefix = 42 chars total.
• Idiomatic. addr.starts_with("0x") && addr.len() == 42 && addr[2..].chars().all(|c| c.is_ascii_hexdigit()) — three conditions ANDed.
• In Alloy. addr.parse::<Address>() delegates all of this to Alloy. Production code does this.
• Rust Playground. https://play.rust-lang.org/ — try Rust without Alloy. URL-share for snippets.

Worked example + steps

First homework: 0x check

The simplest possible task:

Given a string that's supposed to be an Ethereum address, check whether it starts with 0x and print a message. As a stretch: also verify the length (42 characters total).

What you'll need

This is your first Rust program in the EVM stack tradition. Three primitives appear in every Rust program you'll ever write:

1. Variables — let and let mut (we just covered these)
2. Methods — Rust strings have built-in methods. There's one that checks whether a string begins with another string. Find it in the std docs: &str documentation
3. Conditionals — if / else

You also need to know the length of an Ethereum address. Look it up if unsure (hint: 40 hex chars + the 0x prefix).

Try it yourself

Open Rust Playground and write a function with this signature:

fn is_valid_address(addr: &str) -> bool {
    // your code
}

Test it against:

fn main() {
    println!("{}", is_valid_address("0x1234567890abcdef1234567890abcdef12345678")); // true
    println!("{}", is_valid_address("1234567890abcdef1234567890abcdef12345678"));   // false
}

A few hints if you get stuck:

• addr.-something — there's a method on &str whose name is exactly what you want
• addr.len() returns the character byte length
• Two boolean conditions are joined with &&

Don't peek at the answer until you've tried writing it. The whole point of Rust is the compiler teaches you — let it.

Quiz

Once you've written and run your version, take the quiz below. Each question targets a specific Rust idiom — you should be able to answer them all from the experience of having written this function.

Summary (3 lines)

• Function is_valid_address(addr: &str) -> bool = starts_with("0x") && len == 42 && all hex digits. Three conditions AND.
• Alloy production: addr.parse::<Address>() delegates everything. Rust Playground for Alloy-free experimentation.
• Five quiz questions cover this homework and the basic syntax. Next: final Beginner quiz.