5
Reading Club: The Book - Ownership Inventory Quiz no 1 [PROJECT] (rust-book.cs.brown.edu)
submitted 2 years ago* (last edited 2 years ago) by maegul@lemmy.ml to c/learningrustandlemmy@lemmy.ml
6 comments fedilink hide all child comments

After Chs 5 and 6 (see the reading club post here), we get a capstone quiz that covers ownership along with struts and enums.

So, lets do the quiz together! If you've done it already, revisiting might still be very instructive! I certainly thought these questions were useful "revision".

I'll post a comment for each question with the answer, along with my own personal notes (and quotes from The Book if helpful), behind spoiler tags.

Feel free to try to answer in a comment before checking (if you dare). But the main point is to understand the point the question is making, so share any confusions/difficulties too, and of course any corrections of my comments/notes!.

you are viewing a single comment's thread
view the rest of the comments
[-] maegul@lemmy.ml 1 points 2 years ago

Q3: How fix

Of the following fixes (highlighted in yellow), which fix best satisfies these three criteria:

  • The fixed function passes the Rust compiler,
  • The fixed function preserves the intention of the original code, and
  • The fixed function does not introduce unnecessary inefficiencies

1:

fn make_separator(user_str: &str) -> &str {
    if user_str == "" {
        let default = "=".repeat(10);
        &default
    } else {
        &user_str
    }
}

2:

fn make_separator(user_str: String) -> String {
    if user_str == "" {
        let default = "=".repeat(10);
        default
    } else {
        user_str
    }
}

3:

fn make_separator(user_str: &str) -> String {
    if user_str == "" {
        let default = "=".repeat(10);
        default
    } else {
        user_str.to_string()        
    }
}

Answer

3

  • Return owned default
  • Convert user_str to a String to keep a consistent return type
  • Change return type to String
  • 2 is too restrictive in requiring use_str to be a String
  • 1 doesn't solve the problem
fn make_separator(user_str: &str) -> String {
    if user_str == "" {
        let default = "=".repeat(10);
        default
    } else {
        user_str.to_string()        
    }
}

Context: There is no valid way to return a pointer to a stack-allocated variable. The simple solution is therefore to change the return type to String and copy the input user_str into an owned string. However, requiring user_str to be a String would reduce the flexibility of the API, e.g. a caller could not call make_separator on a substring of a bigger string. It would also require callers to heap-allocate strings, e.g. they could not use a string literal like make_separator("Rust").

The most idiomatic solution to this problem uses a construct you haven't seen yet: Cow - Clone-on-write. The clone-on-write smart pointer would enable this function to return either an owned string or a string reference without a type error.

  • Interesting!!

this post was submitted on 21 Jun 2024
5 points (100.0% liked)

Learning Rust and Lemmy

460 readers
7 users here now

Welcome

A collaborative space for people to work together on learning Rust, learning about the Lemmy code base, discussing whatever confusions or difficulties we're having in these endeavours, and solving problems, including, hopefully, some contributions back to the Lemmy code base.

Rules TL;DR: Be nice, constructive, and focus on learning and working together on understanding Rust and Lemmy.

Running Projects

Policies and Purposes

  1. This is a place to learn and work together.
  2. Questions and curiosity is welcome and encouraged.
  3. This isn't a technical support community. Those with technical knowledge and experienced aren't obliged to help, though such is very welcome. This is closer to a library of study groups than stackoverflow. Though, forming a repository of useful information would be a good side effect.
  4. This isn't an issue tracker for Lemmy (or Rust) or a place for suggestions. Instead, it's where the nature of an issue, what possible solutions might exist and how they could be or were implemented can be discussed, or, where the means by which a particular suggestion could be implemented is discussed.

See also:

Rules

  1. Lemmy.ml rule 2 applies strongly: "Be respectful, even when disagreeing. Everyone should feel welcome" (see Dessalines's post). This is a constructive space.
  2. Don't demean, intimidate or do anything that isn't constructive and encouraging to anyone trying to learn or understand. People should feel free to ask questions, be curious, and fill their gaps knowledge and understanding.
  3. Posts and comments should be (more or less) within scope (on which see Policies and Purposes above).
  4. See the Lemmy Code of Conduct
  5. Where applicable, rules should be interpreted in light of the Policies and Purposes.

Relevant links and Related Communities

Thumbnail and banner generated by ChatGPT.

founded 2 years ago
MODERATORS
maegul@lemmy.ml
sorrybookbroke@sh.itjust.works
Jayjader@jlai.lu