Initialize vector using vec! macro and fill it with values from existing array

Question

How can I initialize new Vector using the vec! macro and automatically fill it up with values from an existing array? Here's the code example:

let a = [10, 20, 30, 40]; // a plain array
let v = vec![??];       // TODO: declare your vector here with the macro for vectors

What can I fill in (syntax wise) instead of the ??? characters?

Platy

Vec implements From<[T; N]>. – Strawboard

@ネロク thanks for your comment, but since I am very new to the language, I have no idea what it means. Could you please explain it a little bit? – Platy

You don't need the macro. let v = a.to_vec(); – Irma

@AdamComer thanks, however the example in rustlings implies that you should use the vec! macro (and I would like to learn it as well) – Platy

According to the stdlib vec docs, the vec![] can't create a new Vec from a Slice. – Irma

@ChayimFriedman I'm asking how to use it. I would like to initilize a new vector using this macro. This new vector should copy into itself the same values as the a array. (I'm coming from c# background where there are no macros, so there is a possibility that I have poor understanding of macros) – Platy

Answer 1

Since Vec<T> impls From<[T; N]>, it can be created from an array by using the From::from() method or the Into::into() method:

let v = Vec::from(a);
// Or
let v: Vec<_> = a.into(); // Sometimes you can get rid of the type annoation if the compiler can infer it

The vec![] macro is not intended for that; it is intended for creating Vecs from scratch, like array literals. Instead of creating an array and converting it, you could use the vec![] macro:

let v = vec![10, 20, 30, 40];

Obola

Answer 2

You can also use the to_vec method:

let a = [10, 20, 30, 40]; // a plain array
let v = a.to_vec(); 

Playground

A per the comments, notice that it clones the elements, so the vector items should implement Clone.

Multiplicate

While it works, it clones the element, meaning redundant copies + a T: Clone requirement. This is because this methods operates on slices, and not arrays. – Obola

Nice point, let me add that information as part of the answer though. – Multiplicate

What is the difference between the a.to_vec() and Vec::from(a)? To be exact, why does it matter that the a.to_vec clones the element? – Platy

@TDiblik, it do matters, if instead of creating a copy (as here) of the items you just transform (take ownership) of the underlaying memmory from the original array, usually is more efficient (Imagine a huuuge array, it is always easier not to spend resources on copying it). – Multiplicate

Oh, ok, and if I take ownership of the underlying data, will I be able to use the a variable again (for example return it from function and push some new data to it)? – Platy

@TDiblik, no, you will not be able to use a again. From/Into takes ownership of it (self). If you need to have both is better to use to_vec. – Multiplicate

Ok, thanks, that's exactlly what I needed to hear! You guys have been so helpfull btw <3 – Platy

Answer 3

I'm new to rust.

I think the question comes from rustlings / exercises / vecs that help understand basic syntax for rust.

fn array_and_vec() -> ([i32; 4], Vec<i32>) {
    let a = [10, 20, 30, 40]; // a plain array
    let v = // TODO: declare your vector here with the macro for vectors

    (a, v)
}

When I try to solve the problem, I searched whether there is a way to enumerate all element in the existing array and use it to construct vector by using vec! macro. It is easy to use Vec::from. However, it's not macro but a function. Or the exercise does not make sense?

Fracture

I had the same issue and found this post. I think there's a mistake in the rustlings instruction for the exercise. Or maybe it was a tricky way to make one look up for the documentation and search a bit... Anyway, the other answers to this post cover the case very well. – Finch