Chain Vector and IntoIterator element in Rust

Question

I want to write a function in Rust that will return the vector composed of start integer, then all intermediate integers and then end integer. The assertion it should hold is this:

assert_eq!(intervals(0, 4, 1..4), vec![0, 1, 2, 3, 4]);

The hint is to use chain method for iterators. The function declaration is predefined, I implemented it in one way, which is the following code:

pub fn intervals< I>(start: u32, end: u32, intermediate: I) -> Vec<u32>
where
    I: IntoIterator<Item = u32>,
{
    let mut a1 = vec![];
    a1.push(start);

    let inter: Vec<u32> = intermediate.into_iter().collect();
    let mut iter : Vec<u32> = a1.iter().chain(inter.iter()).map(|x| *x).collect();

    iter.push(end);
    return iter;
}

But I am quite convinced this is not really optimal way to do this. I am sure I am doing lots of unnecessary things in the middle two lines. I tried to use intermediate directly like this:

let mut iter: Vec<u32> = a1.iter().chain(intermediate).map(|x| *x).collect();

But I am getting this error for chain method and I don't know how to solve it:

type mismatch resolving <I as std::iter::IntoIterator>::Item==&u32, 
expected u32, found &u32

I am super new in Rust so any advice would be helpful to understand what's the right way to use intermediate parameter here.

harmic · Accepted Answer · 2019-10-14 22:55:20Z

8

Here are a few hints:

You have created three separate vectors (one explicitly, two using collect) when in fact you only need one.
You can use the std::iter::once iterator to produce iterators for the start and end integers
No need to collect the intermediate range. The intermediate argument implements IntoIterator, so you can feed it directly to chain. So, you can chain together the start, intermediate and end.
No need to use the 'return' keyword at the end of a function - the result of a function is the value of the last expression in it (as long as there is no semicolon on the end).

Applying those tips your function would look like this:

use std::iter::once;

pub fn intervals< I>(start: u32, end: u32, intermediate: I) -> Vec<u32>
where
    I: IntoIterator<Item = u32>,
{
    once(start).chain(intermediate).chain(once(end)).collect()
}

One additional thing to note, to answer your question from the comments:

why trying this: a1.iter().chain(intermediate) gives an error with chain method

Calling Vec::iter() returns an iterator that returns references to the values in the vector. This makes sense: calling iter() does not consume the vector, and its contents remain intact: you could iterate over it multiple times if you wanted.

On the other hand, invoking into_iter() from the IntoIterator trait returns an iterator that returns the values. This also makes sense: into_iter() does consume the object you are calling it on, so the iterator then takes ownership of the items that were previously owned by the object.

Trying to chain together two such iterators does not work because they are each iterating different types. One resolution would be to consume a1 as well, like this:

let mut iter : Vec<u32> = a1.into_iter().chain(intermediate).collect();

edited Oct 14, 2019 at 22:55

answered Oct 13, 2019 at 22:42

harmic

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3 Comments

Ana Khorguani Over a year ago

Thanks a lot, I got you points and your suggested solution seems way optimal than my code. Would it be also possible to explain why trying this: a1.iter().chain(intermediate) gives an error with chain method: type mismatch resolving ... ? because I think this should be giving iterator as well as once(start) does, not?

harmic Over a year ago

@AnaKhorguani answer has been updated to address your question.

Ana Khorguani Over a year ago

Ah, I see now. Thank you very much :)

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