How to create an array within a constructor in Rust

Question

I want to migrate some code from C to Rust for learning purposes and to make my learning library a bit more multi-lingual.

The problem is that I know there's a way to integrate C libraries into Rust. That way I could use calloc in Rust to allow creating my array with a range specified at runtime.

But I don't want to use calloc here - I'd like to see the Rust way. But I really don't want to use vec! either; I had some stupid issues with it before so I don't want to use it just yet.

Here is the code:

pub struct Canvas {
    width: usize,
    height: usize,
    array: [char], // I want to declare its type but not its size yet
}

impl Canvas{
    pub fn new (&self, width: usize, height: usize) -> Canvas {
        Canvas {
            width: width,
            height: height,
            array: calloc(width, height), // alternative to calloc ?            }
    }
}

I hope my question is still idiomatic to the Rust way of code.

qthree · Accepted Answer · 2016-03-16 07:57:36Z

5

i want to access that array with coordinates style

Something like this?

pub struct Canvas {
    width: usize,
    height: usize,
    data: Vec<u8>,
}

impl Canvas {
    pub fn new(width: usize, height: usize) -> Canvas {
        Canvas {
            width: width,
            height: height,
            data: vec![0; width*height],
        }
    }

    fn coords_to_index(&self, x: usize, y: usize) -> Result<usize, &'static str> {
        if x<self.width && y<self.height {
            Ok(x+y*self.width)
        } else {
            Err("Coordinates are out of bounds")
        }
    }

    pub fn get(&self, x: usize, y: usize) -> Result<u8, &'static str> {
        self.coords_to_index(x, y).map(|index| self.data[index])
    }

    pub fn set(&mut self, x: usize, y: usize, new_value: u8) -> Result<(), &'static str>{
        self.coords_to_index(x, y).map(|index| {self.data[index]=new_value;})
    }
}

fn main() {
    let mut canvas = Canvas::new(100, 100);
    println!("{:?}", canvas.get(50, 50)); // Ok(0)
    println!("{:?}", canvas.get(101, 50)); // Err("Coordinates are out of bounds")
    println!("{:?}", canvas.set(50, 50, 128)); // Ok(())
    println!("{:?}", canvas.set(101, 50, 128)); // Err("Coordinates are out of bounds")
    println!("{:?}", canvas.get(50, 50)); // Ok(128)
}

edited Mar 16, 2016 at 7:57

answered Mar 16, 2016 at 7:46

qthree

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2 Comments

xetra11 Over a year ago

Exactly what I was looking for !!

Matthieu M. Over a year ago

For a "built-in" feel, note the presence of the Index and IndexMut which could implement for (usize, usize). They cannot report errors, so they would come in supplement to the methods you wrote here, and then you could use canvas[(50,50)] = 128.

Matthieu M. · Accepted Answer · 2016-03-16 13:06:50Z

5

First of all, I deeply suspect you don't want char; I'm assuming you want an "array of bytes", in which case you want u8.

Secondly, you can't really use [u8] like this. I'm not going to get into the why because that would just derail the answer. For now: if you see a [T] that isn't behind a reference or pointer of some kind, it's probably a mistake.

Finally, this is what Vec is for; use it. You say you don't want to use it, but don't specify why. Vec is how you allocate a dynamically sized array in Rust. If you're trying to allocate a structure that's compatible with the exact same structure in C, then the question changes quite a bit, and you should make that clear.

Assuming what you wanted was the "Rust equivalent" of doing it in C:

pub struct Canvas {
    width: usize,
    height: usize,
    array: Vec<u8>,
}

impl Canvas {
    pub fn new(width: usize, height: usize) -> Canvas {
        Canvas {
            width: width,
            height: height,
            array: vec![0; width*height],
        }
    }
}

edited Mar 16, 2016 at 13:06

Matthieu M.

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answered Mar 16, 2016 at 6:24

DK.

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3 Comments

xetra11 Over a year ago

OK then I'll use vec! macro. But I don't come along with them very well. When I construct my canvas with a width of 10 and height of 10 the "array" binding will hold a vec with a length of 100 (index range 0-99). After using this API I want to access the 99th index of the array which I could to with array[99] - no problem so far. But when i want to access that array with coordinates style for a mutli-dimensional array I had to do array[9][9] which is not possible and array[9*9] will result in array[81] and not 9 in width and 9 in height...thats the issue I have here tbh

Vladimir Matveev Over a year ago

@xetra11 viewing a flat array as a 2d array can be done with trivial arithmetics (x + y*width, for example, in row-major order (that's how it is called I think)).

qthree Over a year ago

fix pub fn new(&self, width: usize, height: usize) to pub fn new(width: usize, height: usize)

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