Chapter 02: Quest App: Reading Your Commands (Input)

published:

tags: [ #rust, #programming, #tutorial, #cli, #quest app, #education, #kids, #children, #young, #learners ]

Hello, Future Quest Master!

Now that you know how to get your Rust project and peek at its code, let's learn how to tell your quest app what to do! When you type commands into the terminal, you can also give your program extra information. These are called "arguments."

Imagine you want to create a new quest. You'd want to tell your quest app what the quest is, right? Like:

cargo run learn about Rust

In this command:

  • cargo run is how we tell Rust to build and run our program.
  • learn about Rust is the extra information, or "arguments", we're giving to our quest app. We want this to become our new quest!

How Rust Reads Your Words

Rust has a special way to listen for these arguments you type. We use something called std::env::args(). Don't worry too much about the big words right now, just know it's like a magical ear for your program!

Here's how we can make our main.rs file listen for your quest description and add it to our list of quests:

fn main() {
    // Collect all the arguments you typed into a list
    let args: Vec<String> = std::env::args().collect();

    // The first argument is always the name of our program.
    // If there are arguments after the program name, it means you're
    // trying to create a new quest!
    let mut new_quest_description: Option<String> = None;
    if args.len() > 1 {
        // We join all the quest words together with spaces to make our quest description.
        new_quest_description = Some(args[1..].join(" "));
    }

    // Start with some quests we already know about (just like in 01-displaying-output.md!)
    let mut quests: Vec<String> = Vec::new();
    quests.push(String::from("Explore the mysterious forest"));
    quests.push(String::from("Learn a new Rust command"));

    // If you provided a new quest, let's add it to our list!
    if let Some(quest_text) = new_quest_description {
        quests.push(quest_text);
        println!("Awesome! We added your new quest.");
    } else {
        println!("To add a new quest, type 'cargo run' followed by your quest, like:");
        println!("  cargo run find the golden apple");
    }

    // Now, let's make our program tell us what quests we have,
    // including any new ones!
    println!("");
    println!("--- Your Current Quests ---");

    for quest in quests {
        println!("- {}", quest);
    }

    println!("---------------------------");
}

What's Happening Here Now?

Let's look at the new magic we added:

  1. let mut new_quest_description: Option<String> = None;

    • We created a special variable new_quest_description. It uses Option<String>, which is Rust's way of saying "this might have some text (String), or it might have nothing (None)". We start it with None because we don't know yet if you've given us a new quest.

  2. if args.len() > 1 { ... }

    • This is how we check if you actually typed something after cargo run. Remember, args[0] is always the program name. So if args.len() (the number of items in args) is greater than 1, it means you gave us more words for a new quest!
    • new_quest_description = Some(args[1..].join(" ")); If you did give us words, we put them into our new_quest_description inside Some().

  3. let mut quests: Vec<String> = Vec::new();

    • Just like in 01-displaying-output.md, we start our list of quests. We even added the same two starting quests to it!

  4. if let Some(quest_text) = new_quest_description { ... }

    • This is a clever way to say: "If new_quest_description actually has some text inside (Some(quest_text)), then let's take that text (which we'll call quest_text for a moment) and do something with it."
    • quests.push(quest_text); If there was a new quest from your command, we add it to our quests list!
    • The else part tells you how to add a new quest if you didn't provide one this time.

  5. The final println! loop:

    • This is the same as in 01-displaying-output.md! It goes through our quests list (which now might include your new quest!) and prints each one out.

Now your Quest App can both read new quest ideas and show you all your quests! Fantastic progress!

Your Turn

Now for the fun part!

  1. Update your src/main.rs file with the new code from above.

  2. Run it without any extra words:

    cargo run

    You should see your two default quests and a message telling you how to add a new one.

  3. Run it with a new quest idea:

    cargo run find the golden apple

    You should see all three quests (the two old ones, plus your new "find the golden apple" quest)! Try it with different quest ideas!

You're doing great! You've just combined input (reading your command) and output (showing your quests)!