Get Data From Structs With String Key, And Struct to Array

Question

Is it possible to get a value from a struct using a string key? Like this:

struct Messages {
    greetings: String,
    goodbyes: String
}

impl Structure {
    fn new() -> Structure{
        Messages {
            greetings: String::from("Hello world"),
            goodbyes: String::from("Bye!")
    }
}

fn main() {
    let messages = Messages::new();
    // now how do I print it out with a string key?
    println!("{}",messages["greetings"]); // prints "Hello world"

    // and can I even turn a struct into an array? Function name is made up
    let arr = Struct::to_array(messages);
}

Pls help thz

loop through a structure to print it out in a tic tac toe format (I'm still learning rust) I'm doing a 9x9 tictactoe board so it is gonna be painful to type everything out (I stored data in 0,1 and 2) — Siriusmart
– Siriusmart, Commented Oct 12, 2021 at 16:57
What you are asking is possible but I doubt that a new rust learner would require such a workaround. For your question, you can see Index trait and you can automate its generation by macros. — Mihir Luthra
– Mihir Luthra, Commented Oct 12, 2021 at 17:02
@Siriusmart maybe I am not understanding but would you mind elaborating your tic-tac-toe issue a little more? — Mihir Luthra
– Mihir Luthra, Commented Oct 12, 2021 at 17:03
@Siriusmart, just adding to this, from personal experience I feel rust-lang discord channel will be more helpful while new to Rust :) — Mihir Luthra
– Mihir Luthra, Commented Oct 12, 2021 at 19:32

cameron1024 · Accepted Answer · 2021-10-13 00:56:26Z

2

In short, no, this is not possible. The names of your fields aren't necessarily available at runtime to perform such a check. However, there are other ways to achieve similar results:

use a HashMap<String, String>
write a function to do it:

impl MyStruct {
  pub fn get_field(&self, key: String) -> &str {
    if key == 'field1' {
      self.field1
    } else if ...
  }
}

Write a derive macro to generate the above function automatically (https://crates.io/crates/field_names might be a useful starting point for how you might go about writing a derive macro)

Or solve it a different way

This pattern is not well supported in Rust, especially compared to more dynamic languages like JavaScript. A problem many new Rust learners face is solving problems "the Rust way".

It's not 100% clear from your comment, but it sounds like your struct is representing a tic-tac-toe board and looks something like this:

struct Board {
  top_right: String,
  top_middle: String,
  top_left: String,
  // etc...
}

While this works, there are much better ways of doing this. For example, you could represent each tile with an enum instead of a String, and you could also use an Vec (similar to arrays/lists from other languages) to store that data:

enum Tile {
  Empty,
  Cross,
  Circle,
}

struct Board {
  tiles: Vec<Tile>,
}

impl Board {
  pub fn print(&self) {
    for tile in self.tiles {
      println!("{}", match tile {
        Tile::Empty => " ",        
        Tile::Cross => "X"
        Tile::Circle => "O",
      });  
    }
  }
}

answered Oct 13, 2021 at 0:56

cameron1024

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2 Comments

Jmb Over a year ago

Unless the size of the board is dynamic, it would probably be easier to represent it as a 2D array: tiles: [ [ Tile; 3]; 3]

cameron1024 Over a year ago

100% agree. In this answer I felt like using a Vec made more sense, since it's likely a more familiar type to someone asking this sort of question, and I wanted to keep the potential "new concepts" to a minimum

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