Add elements of data type to array (or similar)

[st_sc]

I'm using data types (PubKey and PrivKey) from a library and create them in a loop with constant indices 1..NUM_INSTANCES.

const NUM_INSTANCES: usize = 32;

Those types don't have any default initialization.

I create elements of those types and wanna add those to an array or ArrayVec.

1. via Array

let mut private_keys: [PrivKey<Sha256_256>; NUM_INSTANCES];

I can add thee elements inside the loop into the array, e.g.:

priv_keys[idx-1] = priv_key;

But if I try to use those elements later in a loop the following way as an argument to a function:

&priv_keys[idx-1].as_slice()

then I get the following (of course correct) message:

used binding priv_keys isn't initialized

I don't think I can initialize an array properly. I could create fake keys and try to add them in a loop to initialize the array...

2. via ArrayVec

Another approach was to use an ArrayVec:

let mut priv_keys: ArrayVec<[PrivKey<Sha256_256>; NUM_INSTANCES]>;

and push() the created elements after creating:

priv_keys.push(priv_key);

but that yielded the following message:

the method `push` exists for struct `ArrayVec<[PrivKey<Sha256_256>; 32]>`, but its trait bounds were not satisfied
the following trait bounds were not satisfied:
`[my_lib::PrivKey<my_lib::Sha256_256>; 32]: Array`

I don't really understand this message...I can't find anything regarding a trait Array:

• Bounds - Rust By Example
• array - Rust

My questions

• I'd like to understand the message with the "trait bounds"
• Any ideas what's a good way to achieve what I wanna do, assuming that I can't change the types PrivKey and PubKey?

[manpacket]

@st_sc

let mut priv_keys: ArrayVec<[PrivKey<Sha256_256>; NUM_INSTANCES]>;

I think proper invocation should be

let mut priv_keys: ArrayVec<PrivKey<Sha256_256>, NUM_INSTANCES>;

ArrayVec in arrayvec - Rust

[kpreid]

@st_sc

I don't think I can initialize an array properly.

You do have to initialize an array all at once. But perhaps you can replace your loop with std::array::from_fn()?

Another approach was to use an ArrayVec … I can't find anything regarding a trait Array

There are several libraries containing an ArrayVec type, and probably that library also defines an Array trait. Which one do you mean?

[st_sc]

Generally the way I'm declaring it also works (copy & paste from existing code), but I'll probably change it then to the documented approach.

[st_sc]

You do have to initialize an array all at once. But perhaps you can replace your loop with std::array::from_fn()?

Thanks -- I'll have to look into that.

There are several libraries containing an ArrayVec type, and probably that library also defines an Array trait. Which one do you mean?

Seems you're right, I didn't think of looking there:

ArrayVec in tinyvec - Rust

[manpacket]

I see. tinyvec and arrayvec are two different crates implementing similar but slightly different structures.

[kpreid]

tinyvec's main documentation:

The limitation is that the element type of a vec from this crate must support the Default trait. This means that this crate isn’t suitable for all situations, but a very surprising number of types do support Default.

So, you can't use tinyvec::ArrayVec without Default. You could use arrayvec::ArrayVec instead.

[st_sc]

@st_sc

You do have to initialize an array all at once. But perhaps you can replace your loop with std::array::from_fn()?

Nice, that seems to do the trick!

I'm having another problem with my code, but I assume that's nothing to do with the arrays that are getting constructed with this function.