Why does the following code work?
use std::rc::Rc;
fn main () {
let c = vec![1, 2, 3, 4, 5];
let r = Rc::new(c);
println!("{:?}", (**r)[0]);
}
I can understand it working with single deference (println!("{:?}", (*r)[0]);). But not able to understand it working with double-dereference too.
Both, Rc and Vec implements Deref, whichs deref-method is called with the *.
let c = vec![1, 2, 3, 4, 5];
creates a Vec with the given elements with the vec!-macro.
let r = Rc::new(c);
creates a Reference counted Object from the Vector. The Vector is moved into the RC.
println!("{:?}", (**r)[0]);
This one is a bit more tricky: *r dereferences the Rc, so we get the underlying Vector. *rc dereferences the Vector as a slice. slice[0] indexes the first element of the slice, which results in the first element 1. println! finally prints the result.
It might be easier to understand what happens once we build a function prototype around the expression (**r)[0]:
fn foo<T, U>(r: T) -> i32
where
T: Deref<Target=U>,
U: Deref<Target=[i32]>,
{
(**r)[0]
}
Playground
Rc<T>, as is typical for most smart containers in Rust, implements Deref so that it can be used as an ordinary reference to the underlying value. In turn, Vec<T> implements Deref so that it can be used as a slice (Target = [T]). The explicit dereferencing *, when performed twice, applies the two conversions in sequence.
Of course, usually you wouldn't need to do this, because Vec also implements the Index operator.
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...
As the title reads, how would I go about doing this?
fn foo(array: &[u32; 10]) -> &[u32; 5] {
&array[0..5]
}
Compiler error
error[E0308]: mismatched types
--> src/main.rs:2:5
|
2 | &array[0..5]
| ^^^^^^^^^^^^ expected array of 5 elements, found slice
|
= note: expected type `&[u32; 5]`
= note: found type `&[u32]`
arrayref implements a safe interface for doing this operation, using macros (and compile-time constant slicing bounds, of course).
Their readme explains
The goal of arrayref is to enable the effective use of APIs that involve array references rather than slices, for situations where parameters must have a given size.
and
let addr: &[u8; 16] = ...;
let mut segments = [0u16; 8];
// array-based API with arrayref
for i in 0 .. 8 {
segments[i] = read_u16_array(array_ref![addr,2*i,2]);
}
Here the array_ref![addr,2*i,2] macro allows us to take an array reference to a slice consisting of two bytes starting at 2*i. Apart from the syntax (less nice than slicing), it is essentially the same as the slice approach. However, this code makes explicit the need for precisely two bytes both in the caller, and in the function signature.
Stable Rust
It's not possible to do this using only safe Rust. To understand why, it's important to understand how these types are implemented. An array is guaranteed to have N initialized elements. It cannot get smaller or larger. At compile time, those guarantees allow the size aspect of the array to be removed, and the array only takes up N * sizeof(element) space.
That means that [T; N] and [T; M] are different types (when N != M) and you cannot convert a reference of one to the other.
The idiomatic solution is to use a slice instead:
fn foo(array: &[u32; 10]) -> &[u32] {
&array[0..5]
}
A slice contains a pointer to the data and the length of the data, thus moving that logic from compile time to run time.
Nightly Rust
You can perform a runtime check that the slice is the correct length and convert it to an array in one step:
#![feature(try_from)]
use std::convert::TryInto;
fn foo(array: &[u32; 10]) -> &[u32; 5] {
array[0..5].try_into().unwrap()
}
fn main() {}
Unsafe Rust
Because someone might want to do this the unsafe way in an earlier version of Rust, I'll present code based on the standard library implementation:
fn foo(array: &[u32; 10]) -> &[u32; 5] {
let slice = &array[0..5];
if slice.len() == 5 {
let ptr = slice.as_ptr() as *const [u32; 5];
unsafe { &*ptr }
} else {
panic!("Needs to be length 5")
}
}
fn main() {
let input = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
let output = foo(&input);
println!("{:?}", output);
}
I have a boxed array of structs and I want to consume this array and insert it into a vector.
My current approach would be to convert the array into a vector, but the corresponding library function does not seem to work the way I expected.
let foo = Box::new([1, 2, 3, 4]);
let bar = foo.into_vec();
The compiler error states
no method named into_vec found for type Box<[_; 4]> in the current scope
I've found specifications here that look like
fn into_vec(self: Box<[T]>) -> Vec<T>
Converts self into a vector without clones or allocation.
... but I am not quite sure how to apply it. Any suggestions?
I think there's more cleaner way to do it. When you initialize foo, add type to it. Playground
fn main() {
let foo: Box<[u32]> = Box::new([1, 2, 3, 4]);
let bar = foo.into_vec();
println!("{:?}", bar);
}
The documentation you link to is for slices, i.e., [T], while what you have is an array of length 4: [T; 4].
You can, however, simply convert those, since an array of length 4 kinda is a slice. This works:
let foo = Box::new([1, 2, 3, 4]);
let bar = (foo as Box<[_]>).into_vec();
In rustc 1.0.0, I'd like to write a function that mutates a two dimensional array supplied by the caller. I was hoping this would work:
fn foo(x: &mut [[u8]]) {
x[0][0] = 42;
}
fn main() {
let mut x: [[u8; 3]; 3] = [[0; 3]; 3];
foo(&mut x);
}
It fails to compile:
$ rustc fail2d.rs
fail2d.rs:7:9: 7:15 error: mismatched types:
expected `&mut [[u8]]`,
found `&mut [[u8; 3]; 3]`
(expected slice,
found array of 3 elements) [E0308]
fail2d.rs:7 foo(&mut x);
^~~~~~
error: aborting due to previous error
I believe this is telling me I need to somehow feed the function a slice of slices, but I don't know how to construct this.
It "works" if I hard-code the nested array's length in the function signature. This isn't acceptable because I want the function to operate on multidimensional arrays of arbitrary dimension:
fn foo(x: &mut [[u8; 3]]) { // FIXME: don't want to hard code length of nested array
x[0][0] = 42;
}
fn main() {
let mut x: [[u8; 3]; 3] = [[0; 3]; 3];
foo(&mut x);
}
tldr; any zero-cost ways of passing a reference to a multidimensional array such that the function use statements like $x[1][2] = 3;$?
This comes down to a matter of memory layout. Assuming a type T with a size known at compile time (this constraint can be written T: Sized), the size of [T; n] is known at compile time (it takes n times as much memory as T does); but [T] is an unsized type; its length is not known at compile time. Therefore it can only be used through some form of indirection, such as a reference (&[T]) or a box (Box<[T]>, though this is of limited practical value, with Vec<T> which allows you to add and remove items without needing to reallocate every single time by using overallocation).
A slice of an unsized type doesn’t make sense; it’s permitted for reasons that are not clear to me, but you can never actually have an instance of it. (Vec<T>, by comparison, requires T: Sized.)
&[T; n] can coerce to &[T], and &mut [T; n] to &mut [T], but this only applies at the outermost level; the contents of slice is fixed (you’d need to create a new array or vector to achieve such a transformation, because the memory layout of each item is different). The effect of this is that arrays work for single‐dimensional work, but for multi‐dimensional work they fall apart. Arrays are currently very much second‐class citizens in Rust, and will be until the language supports making slices generic over length, which it is likely to eventually.
I recommend that you use either a single‐dimensional array (suitable for square matrices, indexed by x * width + y or similar), or vectors (Vec<Vec<T>>). There may also be libraries already out there abstracting over a suitable solution.