I have the following code:
extern crate rand;
use rand::{thread_rng, Rng};
fn main() {
let mut vec: Vec<u32> = (0..10).collect();
let mut slice: &[u32] = vec.as_mut_slice();
thread_rng().shuffle(slice);
}
and get the following error:
error[E0308]: mismatched types
--> src/main.rs:9:26
|
9 | thread_rng().shuffle(slice);
| ^^^^^ types differ in mutability
|
= note: expected type `&mut [_]`
found type `&[u32]`
I think I understand that the content of vectors and slices is immutable and that causes the error here, but I'm unsure.
The signature of as_mut_slice
is pub fn as_mut_slice<'a>(&'a mut self) -> &'a mut [T]
, so the slice should be mutable, but it somehow isn't.
I know that there must be an easy fix, but I tried my best and couldn't get it to work.
The Rng::shuffle
method is now deprecated; rand::seq::SliceRandom
trait should be used. It provides the shuffle()
method on all slices, which accepts an Rng
instance:
// Rust edition 2018 no longer needs extern crate use rand::thread_rng; use rand::seq::SliceRandom; fn main() { let mut vec: Vec<u32> = (0..10).collect(); vec.shuffle(&mut thread_rng()); println!("{:?}", vec); }
See it on Playground.
You're very close. This should work:
extern crate rand; use rand::{thread_rng, Rng}; fn main() { let mut vec: Vec<u32> = (0..10).collect(); let slice: &mut [u32] = &mut vec; thread_rng().shuffle(slice); }
&mut [T]
is implicitly coercible to &[T]
, and you annotated the slice
variable with &[u32]
, so the slice became immutable: &mut [u32]
was coerced to &[u32]
. mut
on the variable is not relevant here because slices are just borrows into data owned by someone else, so they do not have inherited mutability - their mutability is encoded in their types.
In fact, you don't need an annotation on slice
at all. This works as well:
extern crate rand; use rand::{thread_rng, Rng}; fn main() { let mut vec: Vec<u32> = (0..10).collect(); let slice = vec.as_mut_slice(); thread_rng().shuffle(slice); }
You don't even need the intermediate variable:
extern crate rand; use rand::{thread_rng, Rng}; fn main() { let mut vec: Vec<u32> = (0..10).collect(); thread_rng().shuffle(&mut vec); }
You should read The Rust Programming Language as it explains the concepts of ownership and borrowing and how they interact with mutability.
You can use shuffle
like this:
extern crate rand;
use rand::Rng;
fn main() {
let mut vec: Vec<usize> = (0..10).collect();
println!("{:?}", vec);
rand::thread_rng().shuffle(&mut vec);
println!("{:?}", vec);
}
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