I couldn't find a simple thing on google. How to convert a simple Rust array to a pointer?
How to get pointer to [u8; 3]? I tried doing as *mut u8 but it wouldn't work.
Use as_ptr() or as_mut_ptr().
fn main() {
let a: [u8; 3] = [1, 2, 3];
println!("{:p}", a.as_ptr());
}
0x7ffc97350edd
Arrays coerce to slices, so any slice method may be called on an array.
Note that arrays in Rust are just blobs of memory. They does not point on some stored objects, like an arrays in C do, they are a sequence of objects.
If you have some data and want to get a pointer to it, you'll usually create a reference instead, since only references (and other pointers) can be cast to pointers with as:
fn main() {
let a: [u8; 3] = [1, 2, 3]; // a blob of data on the stack...
let a_ref = &a; // a shared reference to this data...
let a_ptr = a_ref as *const u8; // and a pointer, created from the reference
println!("{:p}", a_ptr);
}
Playground
Related
Here is my code:
struct MyStruct;
impl MyStruct {
static static_value: Vec<i32> = vec![1, 2, 3];
}
fn main() {
for elem in MyStruct::static_value {
println!("{}", elem);
}
}
And the compiler gives an error:
error: associated `static` items are not allowed
--> test.rs:3:5
|
3 | static static_value: Vec<i32> = vec![1, 2, 3];
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
error: aborting due to previous error
Though in this case, I can simply replace the Vec<i32> with an integer array, still I would like to know why the rust compiler will prohibit such code and if there is any ways I can define a static vector.
An array is practically a constant vector.
The main difference ( in terms of usability ) between a vector and an array is the vectors ability to have a dynamic size which of course would be lost if it were constant.
static ARR: [i32; 3] = [1, 2, 3];
should be exactly what you are looking for.
As user4815162342 said "A vector is heap-allocated, so it cannot be static, i.e. baked into the executable."
This is the low level reason why a static vector is not possible.
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 an fn that looks something like this:
use std::io::Read;
fn read_something(file: &mut impl Read) {
let _ = file.read(&mut [0; 8]);
}
When I pass a slice of a Vec into it like that:
fn main() {
let vector = vec![1, 2, 3, 4];
read_something(&mut &vector[..]);
}
it works fine. But when I save the slice into a variable first it does not compile:
fn main() {
let vector = vec![1, 2, 3, 4];
let slice = &vector[..];
read_something(&mut slice);
}
The compiler tells me, that I
cannot borrow as mutable
Playground
How do these two cases differ?
Why does the first example work even though the vector is not declared as mutable either?
How can I pass a &[u8] slice into that fn, that eats a &mut impl Read?
How can I pass a &[u8] slice into that fn, that eats a &mut impl Read?
Add mut to the variable that holds the slice:
let vector = vec![1, 2, 3, 4];
let mut slice = &vector[..];
read_something(&mut slice);
Note that this makes the slice mutable, meaning it can be modified to refer to different data (which read_something() makes use of). It doesn't grant the slice ability to mutate the immutable vector.
How do these two cases differ? Why does the first example work even though the vector is not declared as mutable either?
In the first case the vector is not mutable, but the unnamed temporary that holds the slice is. Your first case desugars to something like:
let vector = vec![1, 2, 3, 4]; // immutable Vec
let mut tmp = &vector[..]; // mutable slice referring to immutable Vec
read_something(&mut tmp);
// here `tmp` may be pointing to different content
// (e.g. a subset of the vector, or something static)
The vector doesn't need to be mutable because the Read::read() impl for &[u8] doesn't attempt to modify the contents of the slice, it only modifies the slice (conceptually a <pointer, length> tuple) itself. It can do that because Read is implemented for &[u8], so when <&[u8] as Read>::read() receives &mut self, its full type is &mut &[u8] - a mutable reference to some slice of immutable data.
What Read::read() for &[u8] does is replace the received slice with a smaller slice which encompasses the as-yet-unread portion of the slice. (This allows "the same" slice to be passed to read() to eventually exhaust all data.) Since the 8-byte buffer you read into is larger than the 4-byte slice that serves as reading source, the modified tmp slice will be empty after read_something() returns (playground).
Can someone explain why this compiles:
fn main() {
let a = vec![1, 2, 3];
println!("{:?}", a[4]);
}
When running it, I got:
thread '' panicked at 'index out of bounds: the len is 3 but the index is 4', ../src/libcollections/vec.rs:1132
If you would like to access elements of the Vec with index checking, you can use the Vec as a slice and then use its get method. For example, consider the following code.
fn main() {
let a = vec![1, 2, 3];
println!("{:?}", a.get(2));
println!("{:?}", a.get(4));
}
This outputs:
Some(3)
None
In order to understand the issue, you have to think about it in terms of what the compiler sees.
Typically, a compiler never reasons about the value of an expression, only about its type. Thus:
a is of type Vec<i32>
4 is of an unknown integral type
Vec<i32> implements subscripting, so a[4] type checks
Having a compiler reasoning about values is not unknown, and there are various ways to get it.
you can allow evaluation of some expression at compile-time (C++ constexpr for example)
you can encode value into types (C++ non-type template parameters, using Peano's numbers)
you can use dependent typing which bridges the gap between types and values
Rust does not support any of these at this point in time, and while there has been interest for the former two it will certainly not be done before 1.0.
Thus, the values are checked at runtime, and the implementation of Vec correctly bails out (here failing).
Note that the following is a compile time error:
fn main() {
let a = [1, 2, 3];
println!("{:?}", a[4]);
}
error: this operation will panic at runtime
--> src/main.rs:3:22
|
3 | println!("{:?}", a[4]);
| ^^^^ index out of bounds: the length is 3 but the index is 4
|
= note: `#[deny(unconditional_panic)]` on by default
This works because without the vec!, the type is [i32; 3], which does actually carry length information.
With the vec!, it's now of type Vec<i32>, which no longer carries length information. Its length is only known at runtime.
Maybe what you mean is :
fn main() {
let a = vec![1, 2, 3];
println!("{:?}", a[4]);
}
This returns an Option so it will return Some or None. Compare this to:
fn main() {
let a = vec![1, 2, 3];
println!("{:?}", &a[4]);
}
This accesses by reference so it directly accesses the address and causes the panic in your program.
I'm just playing around with rust for the first time, implementing quicksort, and I'm stuck on references to dynamically sized arrays (I had no problem with fixed size arrays).
I would like to have an indefinitely sized array of integers to sort, which I gather I can create with something like:
let array = ~[1,2,3,4,3,2,1];
However, I am not sure how I can pass this by reference into a partition function.
partition ( a : &mut ~[uint], p: uint, i: uint) {
// partition a, in place
}
As soon as I try to reorder any elements in a, the compiler complains:
error: cannot assign to immutable vec content a[..]
You should use a mutable borrow of the vector rather than a mutable borrow of a pointer to a vector, so you should get rid of that ~ pointer in the type of your partition function. For example:
fn partition(_: &mut [uint], _: uint, _: uint) { }
fn main() {
let mut array = ~[1, 2, 3, 4, 3, 2, 1];
partition(array, 0, 0);
}
Notice that array is automatically passed as a mutable borrowed pointer.
If you use a Vec instead, then you need to explicitly create a slice:
fn partition(_: &mut [uint], _: uint, _: uint) { }
fn main() {
let mut array = vec!(1, 2, 3, 4, 3, 2, 1);
partition(array.as_mut_slice(), 0, 0);
}
Both code snippets should compile on the latest Rust (from tip).
With 0.10 the language is undergoing some changes to array types at the moment, so things are a bit messy. Vec<T> is Rust's intended dynamically sized array type.
let vec = vec!(1u,2,3,4,3,2,1);
partition ( a : &mut Vec<uint>, p: uint, i: uint) {
// partition a, in place
}
Note that indexing of a Vec via brackets is currently only possible by first calling .as_slice() or .as_mut_slice() on it since the respective traits are not yet implemented.