Tasks in memory: add, rename, complete, remove

Prerequisites: lessons 21, 24, 27, 31–33 and 36. Review lesson 21 if mutable borrowing is unclear.

Familiar image and recall map

Picture task cards laid out in a row. You can add a card, rewrite its title, mark it complete, or take it away. Vec<Task> is such a row in program memory. These cards still disappear when the program closes; files come later. The image has a limit: a place in the row is not a permanent card number.

Check → change Vec<Task> → show result; close the program → in-memory data disappear. Task groups title: String and done: bool. The done field becomes true after completion. &mut Vec<Task> lends a function exclusive access to change the list; &[Task] lends read-only access to a slice of it. get_mut(index) returns Option<&mut Task> because a position may be absent. tasks.remove(index) removes an element and shifts later ones left; we check the boundary first because remove itself would stop the program on a bad index. usize is the list-position type from lesson 15. Result<(), String> means a successful action returns no value (()), while an error carries a reason. for task in tasks reads the cards in order.

Four actions

Save and replace the previous src/main.rs. Run cargo run in organizer. No new dependencies are needed. Predict which card remains after position 0 is removed. [ ] marks a pending task and [done] a completed one. In this teaching main, .expect(...) extracts a successful Ok but stops the program on Err; use it only for fixed, known-good data, never raw user input.

struct Task {
    title: String,
    done: bool,
}

fn add(tasks: &mut Vec<Task>, title: &str) -> Result<(), String> {
    if title.trim().is_empty() {
        return Err(String::from("Title is empty"));
    }
    tasks.push(Task {
        title: String::from(title),
        done: false,
    });
    Ok(())
}

fn rename(tasks: &mut Vec<Task>, index: usize, title: &str) -> Result<(), String> {
    match tasks.get_mut(index) {
        Some(task) => {
            task.title = String::from(title);
            Ok(())
        }
        None => Err(String::from("No such position")),
    }
}

fn complete(tasks: &mut Vec<Task>, index: usize) -> Result<(), String> {
    match tasks.get_mut(index) {
        Some(task) => {
            task.done = true;
            Ok(())
        }
        None => Err(String::from("No such position")),
    }
}

fn remove(tasks: &mut Vec<Task>, index: usize) -> Result<(), String> {
    if index >= tasks.len() {
        return Err(String::from("No such position"));
    }
    tasks.remove(index);
    Ok(())
}

fn show(tasks: &[Task]) {
    for task in tasks {
        if task.done {
            println!("[done] {}", task.title);
        } else {
            println!("[ ] {}", task.title);
        }
    }
}

fn main() {
    let mut tasks = Vec::new();
    add(&mut tasks, "Buy bread").expect("valid title");
    add(&mut tasks, "Read a chapter").expect("valid title");
    rename(&mut tasks, 1, "Read two chapters").expect("existing position");
    complete(&mut tasks, 0).expect("existing position");
    remove(&mut tasks, 0).expect("existing position");
    show(&tasks);
}

[ ] Read two chapters

The teaching main uses .expect(...) only because its positions and nonempty titles are fixed in advance. This Result method extracts Ok but stops the program on Err; it is unsuitable for real user input. The exercise handles errors explicitly. After removal, the second element moves to position 0. That motivates a stable ID in the next lesson.

Check your understanding

1. Why does get_mut return Option rather than &mut Task directly?
2. What happens to the second card position when the first is removed?
3. Will these tasks survive another run?

The position may be absent; the second card moves to position 0; no, there is no saving yet. Rebuild the recall map.

Exercise

Required. Extend rename to reject a title made only of spaces without changing the old title. After successful actions, try renaming position 0 to " " and completing position 9. Print both errors, then the list; the old title must remain.

Hint

Put title.trim().is_empty() before get_mut. Print each Result error with match, as in lesson 32.

Reference after attempting

struct Task {
    title: String,
    done: bool,
}

fn add(tasks: &mut Vec<Task>, title: &str) -> Result<(), String> {
    if title.trim().is_empty() {
        return Err(String::from("Title is empty"));
    }
    tasks.push(Task {
        title: String::from(title),
        done: false,
    });
    Ok(())
}

fn rename(tasks: &mut Vec<Task>, index: usize, title: &str) -> Result<(), String> {
    if title.trim().is_empty() {
        return Err(String::from("Title is empty"));
    }
    match tasks.get_mut(index) {
        Some(task) => {
            task.title = String::from(title);
            Ok(())
        }
        None => Err(String::from("No such position")),
    }
}

fn complete(tasks: &mut Vec<Task>, index: usize) -> Result<(), String> {
    match tasks.get_mut(index) {
        Some(task) => {
            task.done = true;
            Ok(())
        }
        None => Err(String::from("No such position")),
    }
}

fn remove(tasks: &mut Vec<Task>, index: usize) -> Result<(), String> {
    if index >= tasks.len() {
        return Err(String::from("No such position"));
    }
    tasks.remove(index);
    Ok(())
}

fn show(tasks: &[Task]) {
    for task in tasks {
        if task.done {
            println!("[done] {}", task.title);
        } else {
            println!("[ ] {}", task.title);
        }
    }
}

fn main() {
    let mut tasks = Vec::new();
    add(&mut tasks, "Buy bread").expect("valid title");
    add(&mut tasks, "Read a chapter").expect("valid title");
    rename(&mut tasks, 1, "Read two chapters").expect("existing position");
    complete(&mut tasks, 0).expect("existing position");
    remove(&mut tasks, 0).expect("existing position");
    for result in [rename(&mut tasks, 0, "   "), complete(&mut tasks, 9)] {
        if let Err(message) = result {
            println!("Error: {message}");
        }
    }
    show(&tasks);
}

Error: Title is empty
Error: No such position
[ ] Read two chapters

Documentation for Vec::get_mut and Vec::remove.

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