Related
I want to call .map() on an array of enums:
enum Foo {
Value(i32),
Nothing,
}
fn main() {
let bar = [1, 2, 3];
let foos = bar.iter().map(|x| Foo::Value(*x)).collect::<[Foo; 3]>();
}
but the compiler complains:
error[E0277]: the trait bound `[Foo; 3]: std::iter::FromIterator<Foo>` is not satisfied
--> src/main.rs:8:51
|
8 | let foos = bar.iter().map(|x| Foo::Value(*x)).collect::<[Foo; 3]>();
| ^^^^^^^ a collection of type `[Foo; 3]` cannot be built from an iterator over elements of type `Foo`
|
= help: the trait `std::iter::FromIterator<Foo>` is not implemented for `[Foo; 3]`
How do I do this?
The issue is actually in collect, not in map.
In order to be able to collect the results of an iteration into a container, this container should implement FromIterator.
[T; n] does not implement FromIterator because it cannot do so generally: to produce a [T; n] you need to provide n elements exactly, however when using FromIterator you make no guarantee about the number of elements that will be fed into your type.
There is also the difficulty that you would not know, without supplementary data, which index of the array you should be feeding now (and whether it's empty or full), etc... this could be addressed by using enumerate after map (essentially feeding the index), but then you would still have the issue of deciding what to do if not enough or too many elements are supplied.
Therefore, not only at the moment one cannot implement FromIterator on a fixed-size array; but even in the future it seems like a long shot.
So, now what to do? There are several possibilities:
inline the transformation at call site: [Value(1), Value(2), Value(3)], possibly with the help of a macro
collect into a different (growable) container, such as Vec<Foo>
...
Update
This can work:
let array: [T; N] = something_iterable.[into_]iter()
.collect::<Vec<T>>()
.try_into()
.unwrap()
In newer version of rust, try_into is included in prelude, so it is not necessary to use std::convert::TryInto. Further, starting from 1.48.0, array support directly convert from Vec type, signature from stdlib source:
fn try_from(mut vec: Vec<T, A>) -> Result<[T; N], Vec<T, A>> {
...
}
Original Answer
as of rustc 1.42.0, if your element impl Copy trait, for simplicity, this just works:
use std::convert::TryInto;
...
let array: [T; N] = something_iterable.[into_]iter()
.collect::<Vec<T>>()
.as_slice()
.try_into()
.unwrap()
collect as_slice try_into + unwrap()
Iterator<T> ------> Vec<T> -------> &[T] ------------------> [T]
But I would just call it a workaround.
You need to include std::convert::TryInto because the try_into method is defined in the TryInto trait.
Below is the signature checked when you call try_into as above, taken from the source. As you can see, that requires your type T implement Copy trait, so theoritically, it will copy all your elements once.
#[stable(feature = "try_from", since = "1.34.0")]
impl<T, const N: usize> TryFrom<&[T]> for [T; N]
where
T: Copy,
[T; N]: LengthAtMost32,
{
type Error = TryFromSliceError;
fn try_from(slice: &[T]) -> Result<[T; N], TryFromSliceError> {
<&Self>::try_from(slice).map(|r| *r)
}
}
While you cannot directly collect into an array for the reasons stated by the other answers, that doesn't mean that you can't collect into a data structure backed by an array, like an ArrayVec:
use arrayvec::ArrayVec; // 0.7.0
use std::array;
enum Foo {
Value(i32),
Nothing,
}
fn main() {
let bar = [1, 2, 3];
let foos: ArrayVec<_, 3> = array::IntoIter::new(bar).map(Foo::Value).collect();
let the_array = foos
.into_inner()
.unwrap_or_else(|_| panic!("Array was not completely filled"));
// use `.expect` instead if your type implements `Debug`
}
Pulling the array out of the ArrayVec returns a Result to deal with the case where there weren't enough items to fill it; the case that was discussed in the other answers.
For your specific problem, Rust 1.55.0 allows you to directly map an array:
enum Foo {
Value(i32),
Nothing,
}
fn main() {
let bar = [1, 2, 3];
let foos = bar.map(Foo::Value);
}
In this case you can use Vec<Foo>:
#[derive(Debug)]
enum Foo {
Value(i32),
Nothing,
}
fn main() {
let bar = [1, 2, 3];
let foos = bar.iter().map(|&x| Foo::Value(x)).collect::<Vec<Foo>>();
println!("{:?}", foos);
}
.collect() builds data structures that can have arbitrary length, because the iterator's item number is not limited in general. (Shepmaster's answer already provides plenty details there).
One possibility to get data into an array from a mapped chain without allocating a Vec or similar is to bring mutable references to the array into the chain. In your example, that'd look like this:
#[derive(Debug, Clone, Copy)]
enum Foo {
Value(i32),
Nothing,
}
fn main() {
let bar = [1, 2, 3];
let mut foos = [Foo::Nothing; 3];
bar.iter().map(|x| Foo::Value(*x))
.zip(foos.iter_mut()).for_each(|(b, df)| *df = b);
}
The .zip() makes the iteration run over both bar and foos in lockstep -- if foos were under-allocated, the higher bars would not be mapped at all, and if it were over-allocated, it'd keep its original initialization values. (Thus also the Clone and Copy, they are needed for the [Nothing; 3] initialization).
You can actually define a Iterator trait extension to do this!
use std::convert::AsMut;
use std::default::Default;
trait CastExt<T, U: Default + AsMut<[T]>>: Sized + Iterator<Item = T> {
fn cast(mut self) -> U {
let mut out: U = U::default();
let arr: &mut [T] = out.as_mut();
for i in 0..arr.len() {
match self.next() {
None => panic!("Array was not filled"),
Some(v) => arr[i] = v,
}
}
assert!(self.next().is_none(), "Array was overfilled");
out
}
}
impl<T, U: Iterator<Item = T>, V: Default + AsMut<[T]>> CastExt<T, V> for U { }
fn main () {
let a: [i32; 8] = (0..8).map(|i| i * 2).cast();
println!("{:?}", a); // -> [0, 2, 4, 6, 8, 10, 12, 14]
}
Here's a playground link.
This isn't possible because arrays do not implement any traits. You can only collect into types which implement the FromIterator trait (see the list at the bottom of its docs).
This is a language limitation, since it's currently impossible to be generic over the length of an array and the length is part of its type. But, even if it were possible, it's very unlikely that FromIterator would be implemented on arrays because it'd have to panic if the number of items yielded wasn't exactly the length of the array.
You may combine arrays map method with Iterator::next.
Example:
fn iter_to_array<Element, const N: usize>(mut iter: impl Iterator<Item = Element>) -> [Element; N] {
// Here I use `()` to make array zero-sized -> no real use in runtime.
// `map` creates new array, which we fill by values of iterator.
let res = [(); N].map(|_| iter.next().unwrap());
// Ensure that iterator finished
assert!(matches!(iter.next(), None));
res
}
I ran into this problem myself — here's a workaround.
You can't use FromIterator, but you can iterate over the contents of a fixed-size object, or, if things are more complicated, indices that slice anything that can be accessed. Either way, mutation is viable.
For example, the problem I had was with an array of type [[usize; 2]; 4]:
fn main() {
// Some input that could come from another function and thus not be mutable
let pairs: [[usize; 2]; 4] = [[0, 0], [0, 1], [1, 1], [1, 0]];
// Copy mutable
let mut foo_pairs = pairs.clone();
for pair in foo_pairs.iter_mut() {
// Do some operation or other on the fixed-size contents of each
pair[0] += 1;
pair[1] -= 1;
}
// Go forth and foo the foo_pairs
}
If this is happening inside a small function, it's okay in my book. Either way, you were going to end up with a transformed value of identical type as the same one, so copying the whole thing first and then mutating is about the same amount of effort as referencing a value in a closure and returning some function of it.
Note that this only works if you plan to compute something that is going to be the same type, up to and including size/length. But that's implied by your use of Rust arrays. (Specifically, you could Value() your Foos or Nothing them as you like, and still be within type parameters for your array.)
I have a program where I need to append two Vec<u8> before they are are serialized.
Just to be sure how to do it, I made this example program:
let a: Vec<u8> = vec![1, 2, 3, 4, 5, 6];
let b: Vec<u8> = vec![7, 8, 9];
let c = [a, b].concat();
println!("{:?}", c);
Which works perfectly. The issue is now when I have to implement it in my own project.
Here I need to write a function, the function takes a struct as input that looks like this:
pub struct Message2 {
pub ephemeral_key_r: Vec<u8>,
pub c_r: Vec<u8>,
pub ciphertext2: Vec<u8>,
}
and the serialalization function looks like this:
pub fn serialize_message_2(msg: &Message2) -> Result<Vec<u8>> {
let c_r_and_ciphertext = [msg.c_r, msg.ciphertext2].concat();
let encoded = (
Bytes::new(&msg.ephemeral_key_r),
Bytes::new(&c_r_and_ciphertext),
);
Ok(cbor::encode_sequence(encoded)?)
}
The first issue that arises here is that it complains that msg.ciphertext2 and msg.c_r are moved values. This makes sense, so I add an & in front of both of them.
However, when I do this, the call to concat() fails, with this type error:
util.rs(77, 59): method cannot be called on `[&std::vec::Vec<u8>; 2]` due to unsatisfied trait bounds
So, when I borrow the values, then the expression [&msg.c_r, &msg.ciphertext2] becomes an array of two vec's, which there is not a concat() defined for.
I also tried calling clone on both vectors:
let c_r_and_ciphertext = [msg.c_r.clone(), msg.ciphertext2.clone()].concat();
and this actually works out!
But now I'm just wondering, why does borrowing the values change the types?
and is there any things to think about when slapping on clone to values that are moved, and where I cannot borrow for some reason?
The reasons on why .concat() behaves as it does are a bit awkward.
To be able to call .concat(), the Concat trait must be implemented. It is implemented on slices of strings, and slices of V, where V can be Borrowed as slices of copyable T.
First, you're calling concat on an array, not a slice. However, auto-borrowing and unsize coercion are applied when calling a function with .. This turns the [V; 2] into a &[V] (where V = Vec<u8> in the working case and V = &Vec<u8> in the non-workin case). Try calling Concat::concat([a, b]) and you'll notice the difference.
So now is the question whether V can be borrowed as/into some &[T] (where T = u8 in your case). Two possibilities exist:
There is an impl<T> Borrow<[T]> for Vec<T>, so Vec<u8> can be turned into &[u8].
There is an impl<'_, T> Borrow<T> for &'_ T, so if you already have a &[u8], that can be used.
However, there is no impl<T> Borrow<[T]> for &'_ Vec<T>, so concatting [&Vec<_>] won't work.
So much for the theory, on the practical side: You can avoid the clones by using [&msg.c_r[..], &msg.ciphertext2[..]].concat(), because you'll be calling concat on &[&[u8]]. The &x[..] is a neat trick to turn the Vecs into slices (by slicing it, without slicing anything off…). You can also do that with .borrow(), but that's a bit more awkward, since you may need an extra type specification: [msg.c_r.borrow(), msg.ciphertext2.borrow()].concat::<u8>()
I tried to reproduce your error message, which this code does:
fn main() {
let a = vec![1, 2];
let b = vec![3, 4];
println!("{:?}", [&a, &b].concat())
}
gives:
error[E0599]: the method `concat` exists for array `[&Vec<{integer}>; 2]`, but its trait bounds were not satisfied
--> src/main.rs:4:31
|
4 | println!("{:?}", [&a, &b].concat())
| ^^^^^^ method cannot be called on `[&Vec<{integer}>; 2]` due to unsatisfied trait bounds
|
= note: the following trait bounds were not satisfied:
`[&Vec<{integer}>]: Concat<_>`
It is a simple matter of helping the compiler to see that &a works perfectly fine as a slice, by calling it &a[..]:
fn main() {
let a = vec![1, 2];
let b = vec![3, 4];
println!("{:?}", [&a[..], &b[..]].concat())
}
why does borrowing the values change the types?
Borrowing changes a type into a reference to that same type, so T to &T. These types are related, but are not the same.
is there any things to think about when slapping on clone to values that are moved, and where I cannot borrow for some reason?
Cloning is a good way to sacrifice performance to make the borrow checker happy. It (usually) involves copying the entire memory that is cloned, but if your code is not performance critical (which most code is not), then it may still be a good trade-off...
I am a Rust newbie, I tested following code and got a question. Is type of slice [T]?If so, [T] is unsized, but it passed when I compiled the code. Why is that?
#[test]
fn test_scilce(){
let v = vec!['a', 'b', 'v'];
let slice = (v[1..3]).into_iter();
// let s: String = slice.collect();
println!("{:?}", slice);
println!("{:?}", v);
}
Since [T]::into_iter(self) doesn't exist, but [T]::into_iter(&self) does, the compiler inserts the missing reference and treats (v[1..3]).into_iter() as (&v[1..3]).into_iter(). That is in turn the same as (&v[1..3]).iter() and gives out references to the elements of the vector. (There is even a clippy lint warning you of using into_iter() on slice or other references.)
The same auto-referencing mechanism is what allows you to write v.len() instead of the "correct" (&v).len(), despite Vec::len taking &self.
I'm confused about the proper type to use for an iterator yielding string slices.
fn print_strings<'a>(seq: impl IntoIterator<Item = &'a str>) {
for s in seq {
println!("- {}", s);
}
}
fn main() {
let arr: [&str; 3] = ["a", "b", "c"];
let vec: Vec<&str> = vec!["a", "b", "c"];
let it: std::str::Split<'_, char> = "a b c".split(' ');
print_strings(&arr);
print_strings(&vec);
print_strings(it);
}
Using <Item = &'a str>, the arr and vec calls don't compile. If, instead, I use <Item = &'a'a str>, they work, but the it call doesn't compile.
Of course, I can make the Item type generic too, and do
fn print_strings<'a, I: std::fmt::Display>(seq: impl IntoIterator<Item = I>)
but it's getting silly. Surely there must be a single canonical "iterator of string values" type?
The error you are seeing is expected because seq is &Vec<&str> and &Vec<T> implements IntoIterator with Item=&T, so with your code, you end up with Item=&&str where you are expecting it to be Item=&str in all cases.
The correct way to do this is to expand Item type so that is can handle both &str and &&str. You can do this by using more generics, e.g.
fn print_strings(seq: impl IntoIterator<Item = impl AsRef<str>>) {
for s in seq {
let s = s.as_ref();
println!("- {}", s);
}
}
This requires the Item to be something that you can retrieve a &str from, and then in your loop .as_ref() will return the &str you are looking for.
This also has the added bonus that your code will also work with Vec<String> and any other type that implements AsRef<str>.
TL;DR The signature you use is fine, it's the callers that are providing iterators with wrong Item - but can be easily fixed.
As explained in the other answer, print_string() doesn't accept &arr and &vec because IntoIterator for &[T; n] and &Vec<T> yield references to T. This is because &Vec, itself a reference, is not allowed to consume the Vec in order to move T values out of it. What it can do is hand out references to T items sitting inside the Vec, i.e. items of type &T. In the case of your callers that don't compile, the containers contain &str, so their iterators hand out &&str.
Other than making print_string() more generic, another way to fix the issue is to call it correctly to begin with. For example, these all compile:
print_strings(arr.iter().map(|sref| *sref));
print_strings(vec.iter().copied());
print_strings(it);
Playground
iter() is the method provided by slices (and therefore available on arrays and Vec) that iterates over references to elements, just like IntoIterator of &Vec. We call it explicitly to be able to call map() to convert &&str to &str the obvious way - by using the * operator to dereference the &&str. The copied() iterator adapter is another way of expressing the same, possibly a bit less cryptic than map(|x| *x). (There is also cloned(), equivalent to map(|x| x.clone()).)
It's also possible to call print_strings() if you have a container with String values:
let v = vec!["foo".to_owned(), "bar".to_owned()];
print_strings(v.iter().map(|s| s.as_str()));
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.
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();