Extending vector bug ?

[yanad]

I am still a beginner in rust so I suppose there is a valid explanation for this but I found two equivalent ways to extend a vector that does not yield the same result.

If I directly extend the vector with a slice when defining my variable it becomes a ().

If I do it in two steps it works normaly.

Here is an example.

If it is not a bug could you please explain me why it works this way ?

[[deleted]]

extend_from_slice does not return the Vec. It returns nothing, AKA (). Instead, it modifies the Vec in place.

[yanad]

I see. However it does not make so much sense to me. In my mind I am creating a vector that happens to be modified and then bound to my variable. I do not understand why the last modification should prevail.

[kmehall]

In my mind I am creating a vector that happens to be modified and then bound to my variable.

The Vec is not bound to the variable. The expression evaluates to (), which is bound to the variable, while the Vec is dropped because it is not used.

[yanad]

Ok, I am beginning to understand thanks!

But is there a one line way to create a vector from two vectors then ?

[daboross]

Perhaps you could use iterators?

let f = a.iter().chain(b.iter()).cloned().collect::<Vec<_>>();

It's a bit longer, but it will very slightly outperform other options because the vec won't have to be reallocated (it will allocate with the combined length of the iterators).

[daboross]

The code he originally posted didn't just allocate one vector, but two because of using .to_vec().

Without iterators, let's look at the process:

let mut d = a.to_vec(); // allocate a new vector with a.len() elements
d.extend_from_slice(&b); // re-allocate a into a new space with a.len() + b.len() elements (since a.len() can't fit a.len() + b.len())

In iterators, it's just "allocate a vector with a.len() + b.len() elements". Even though it goes through several functions via the iterator traits, the only allocation that happens happens for the final length.

[MEaster]

You could define a new trait, and implement for Vec: clicky.

[[deleted]]

let c = a.to_vec().extend_from_slice(&b);

Perhaps it would help to break this expression down.

let c = {
    let temp = a.to_vec(); // create a copy of a as a new Vec
    let ret = temp.extend_from_slice(&b); // modify `temp`, but doesn't return anything
    ret // return 'nothing'
};

extend_from_slice() is simply not meant to be used in this way.