Related
Suppose I have a modify_vec() function that takes a vector reference and returns a new vector. It doesn't really matter what it does so for the sake of example it will just append a given number.
fn modify_vec(v: &Vec<i64>, i: i64) -> Vec<i64> {
let mut new_v = v.clone();
new_v.push(i);
new_v
}
Now, suppose I want to run this modify_vec a few times, I could do it like that:
fn main() {
let v = vec![1, 2, 3];
println!("{:?}", run(&v)); // Outputs [1, 2, 3, 4, 5, 6]
}
fn run(v: &Vec<i64>) -> Vec<i64> {
let new_1 = modify_vec(&v, 4);
let new_2 = modify_vec(&new_1, 5);
modify_vec(&new_2, 6)
}
Or I can do it like that
fn run(v: &Vec<i64>) -> Vec<i64> {
let mut new_v = modify_vec(&v, 4);
new_v = modify_vec(&new_v, 5);
modify_vec(&new_v, 6)
}
However it would obviously be better to use a loop. The problem is that the run function starts with a reference of the vector, but it must declare a new owned variable to store the returned value of modify_vec. The only way I found to initialize that owned variable outside of the loop was doing a .clone(), like this.
fn run(v: &Vec<i64>) -> Vec<i64> {
let mut new_v = v.clone();
for i in 4..7 {
new_v = modify_vec(&new_v, i);
}
new_v
}
But this seems "wasteful", as it is my understanding that it makes a deep copy. In the previous unlooped examples, it was not necessary to clone the vector prior to passing it to the first call to modify_vec.Is there a better way?
modify_vec should take a Vec by value instead of a reference. Let the caller clone the Vec if it doesn't want to (or cannot) transfer ownership – do not impose the cost of a clone on all callers.
fn main() {
let v = vec![1, 2, 3];
println!("{:?}", run(&v)); // Outputs [1, 2, 3, 4, 5, 6]
}
fn run(v: &Vec<i64>) -> Vec<i64> {
let mut new_v = v.clone();
for i in 4..7 {
new_v = modify_vec(new_v, i);
}
new_v
}
fn modify_vec(mut v: Vec<i64>, i: i64) -> Vec<i64> {
v.push(i);
v
}
Perhaps the example is too contrived, but there is no need to clone any Vec at all:
fn main() {
let v = vec![1, 2, 3];
println!("{:?}", run(v)); // Outputs [1, 2, 3, 4, 5, 6]
// ^ this could be replaced with `v.clone()`
// if `v` needs to be used later
}
fn run(mut v: Vec<i64>) -> Vec<i64> {
for i in 4..7 {
v = modify_vec(v, i);
}
v
}
fn modify_vec(mut v: Vec<i64>, i: i64) -> Vec<i64> {
v.push(i);
v
}
I'm trying to eliminate duplicates in a sorted vector with the dedup function, but it returns empty.
Here's my code:
fn main() {
let mut input = vec![1, 1, 2, 3, 3];
remove_dup(&mut input);
}
fn remove_dup(input: &mut Vec<u32>) {
let result = input.dedup();
println!("Result: {:?}", result);
}
Here's the output:
Result: ()
The dedup() method operates in-place, updating the original vector.
fn main() {
let mut input = vec![1, 1, 2, 3, 3];
remove_dup(&mut input);
}
fn remove_dup(input: &mut Vec<u32>) {
input.dedup();
println!("Result: {:?}", input);
}
Playground.
In general, when you see a function that returns () (no return type), it's a good sign that it's a modifying operation.
I have an array of an unknown size, and I would like to get a slice of that array and convert it to a statically sized array:
fn pop(barry: &[u8]) -> [u8; 3] {
barry[0..3] // expected array `[u8; 3]`, found slice `[u8]`
}
How would I do this?
You can easily do this with the TryInto trait (which was stabilized in Rust 1.34):
// Before Rust 2021, you need to import the trait:
// use std::convert::TryInto;
fn pop(barry: &[u8]) -> [u8; 3] {
barry.try_into().expect("slice with incorrect length")
}
But even better: there is no need to clone/copy your elements! It is actually possible to get a &[u8; 3] from a &[u8]:
fn pop(barry: &[u8]) -> &[u8; 3] {
barry.try_into().expect("slice with incorrect length")
}
As mentioned in the other answers, you probably don't want to panic if the length of barry is not 3, but instead handle this error gracefully.
This works thanks to these impls of the related trait TryFrom (before Rust 1.47, these only existed for arrays up to length 32):
impl<'_, T, const N: usize> TryFrom<&'_ [T]> for [T; N]
where
T: Copy,
impl<'a, T, const N: usize> TryFrom<&'a [T]> for &'a [T; N]
impl<'a, T, const N: usize> TryFrom<&'a mut [T]> for &'a mut [T; N]
Thanks to #malbarbo we can use this helper function:
use std::convert::AsMut;
fn clone_into_array<A, T>(slice: &[T]) -> A
where
A: Default + AsMut<[T]>,
T: Clone,
{
let mut a = A::default();
<A as AsMut<[T]>>::as_mut(&mut a).clone_from_slice(slice);
a
}
to get a much neater syntax:
fn main() {
let original = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
let e = Example {
a: clone_into_array(&original[0..4]),
b: clone_into_array(&original[4..10]),
};
println!("{:?}", e);
}
as long as T: Default + Clone.
If you know your type implements Copy, you can use this form:
use std::convert::AsMut;
fn copy_into_array<A, T>(slice: &[T]) -> A
where
A: Default + AsMut<[T]>,
T: Copy,
{
let mut a = A::default();
<A as AsMut<[T]>>::as_mut(&mut a).copy_from_slice(slice);
a
}
Both variants will panic! if the target array and the passed-in slice do not have the same length.
I recommend using the crate arrayref, which has a handy macro for doing just this.
Note that, using this crate, you create a reference to an array, &[u8; 3], because it doesn't clone any data!
If you do want to clone the data, then you can still use the macro, but call clone at the end:
#[macro_use]
extern crate arrayref;
fn pop(barry: &[u8]) -> &[u8; 3] {
array_ref!(barry, 0, 3)
}
or
#[macro_use]
extern crate arrayref;
fn pop(barry: &[u8]) -> [u8; 3] {
array_ref!(barry, 0, 3).clone()
}
You can manually create the array and return it.
Here is a function that can easily scale if you want to get more (or less) than 3 elements.
Note that if the slice is too small, the end terms of the array will be 0's.
fn pop(barry: &[u8]) -> [u8; 3] {
let mut array = [0u8; 3];
for (&x, p) in barry.iter().zip(array.iter_mut()) {
*p = x;
}
array
}
Here's a function that matches the type signature you asked for.
fn pop(barry: &[u8]) -> [u8; 3] {
[barry[0], barry[1], barry[2]]
}
But since barry could have fewer than three elements, you may want to return an Option<[u8; 3]> rather than a [u8; 3].
fn pop(barry: &[u8]) -> Option<[u8; 3]> {
if barry.len() < 3 {
None
} else {
Some([barry[0], barry[1], barry[2]])
}
}
I was unhappy with other answers because I needed several functions that return varying length fixed u8 arrays. I wrote a macro that produces functions specific for the task. Hope it helps someone.
#[macro_export]
macro_rules! vec_arr_func {
($name:ident, $type:ty, $size:expr) => {
pub fn $name(data: std::vec::Vec<$type>) -> [$type; $size] {
let mut arr = [0; $size];
arr.copy_from_slice(&data[0..$size]);
arr
}
};
}
//usage - pass in a name for the fn, type of array, length
vec_arr_func!(v32, u8, 32);
v32(data); //where data is std::vec::Vec<u8>
The nice common thing between Vec, 'Slice' and Array is Iter, so you can zip and map both together, as simple as:
let x = vec![1, 2, 3];
let mut y: [u8; 3] = [Default::default(); 3];
println!("y at startup: {:?}", y);
x.iter().zip(y.iter_mut()).map(|(&x, y)| *y = x).count();
println!("y copied from vec: {:?}", y);
This is as the array is 1 dimensional array.
To test all together, vec, slice and array, here you go:
let a = [1, 2, 3, 4, 5];
let slice = &a[1..4];
let mut x: Vec<u8> = vec![Default::default(); 3];
println!("X at startup: {:?}", x);
slice.iter().zip(x.iter_mut()).map(|(&s, x)| *x = s).count();
println!("X copied from vec: {:?}", x);
Another option which should be faster than byte-by-byte copy is:
y[..x.len()].copy_from_slice(&x);
Which is applicable for all, below is example:
let a = [1, 2, 3, 4, 5];
let mut b: Vec<u8> = vec![Default::default(); 5];
b[..a.len()].copy_from_slice(&a);
println!("Copy array a into vector b: {:?}", b);
let x: Vec<u8> = vec![1, 2, 3, 4, 5];
let mut y: [u8; 5] = [Default::default(); 5];
y[..x.len()].copy_from_slice(&x);
println!("Copy vector x into array y: {:?}", y);
I have a function called new_vec. It takes two vectors and creates a new one, by performing an elementwise operation on the pair of elements from the zipped vectors.
fn main() {
let v1s = vec![1, 0, 1];
let v2s = vec![0, 1, 1];
let v3s = new_vec(v1s, v2s);
println!("{:?}", v3s) // [1, 1, 2]
}
fn new_vec(v1s: Vec<i32>, v2s: Vec<i32>) -> Vec<i32> {
let mut v3s = Vec::<i32>::new();
for (v1, v2) in v1s.iter().zip(v2s.iter()) {
v3s.push(v1 + v2) // would also like to use -
}
v3s
}
I want to have a new_vec function for the common binary operation that is possible to use on two integers, such as +, -, /, *.
How do I do this? I can imagine two ways: macros and closures. A minimal example of how to do this in the best way, for example with + and - would be appreciated.
I would pass a closure:
fn new_vec<F>(v1s: &[i32], v2s: &[i32], foo: F) -> Vec<i32>
where F: Fn(i32, i32) -> i32
{
let mut v3s = Vec::<i32>::new();
for (&v1, &v2) in v1s.iter().zip(v2s.iter()) {
v3s.push(foo(v1, v2))
}
v3s
}
fn main() {
let v1s = vec![1, 0, 1];
let v2s = vec![0, 1, 1];
let v3s = new_vec(&v1s, &v2s, |x, y| x - y);
let v4s = new_vec(&v1s, &v2s, |x, y| x + y);
println!("{:?}", v3s); // [1, -1, 0]
println!("{:?}", v4s); // [1, 1, 2]
}
Note the change in the first two parameters; if your function doesn't need to consume its arguments, references are preferable to Vectors - in this case &[i32].
This implementation is not too efficient because the resulting Vector is extended incrementally; it's better if you modified it as follows to reduce the number of allocations:
fn new_vec<F>(v1s: &[i32], v2s: &[i32], foo: F) -> Vec<i32>
where F: Fn(i32, i32) -> i32
{
v1s.iter().zip(v2s.iter()).map(|(&x, &y)| foo(x, y)).collect()
}
Is there any straightforward way to insert or replace multiple elements from &[T] and/or Vec<T> in the middle or at the beginning of a Vec in linear time?
I could only find std::vec::Vec::insert, but that's only for inserting a single element in O(n) time, so I obviously cannot call that in a loop.
I could do a split_off at that index, extend the new elements into the left half of the split, and then extend the second half into the first, but is there a better way?
As of Rust 1.21.0, Vec::splice is available and allows inserting at any point, including fully prepending:
let mut vec = vec![1, 5];
let slice = &[2, 3, 4];
vec.splice(1..1, slice.iter().cloned());
println!("{:?}", vec); // [1, 2, 3, 4, 5]
The docs state:
Note 4: This is optimal if:
The tail (elements in the vector after range) is empty
or replace_with yields fewer elements than range’s length
or the lower bound of its size_hint() is exact.
In this case, the lower bound of the slice's iterator should be exact, so it should perform one memory move.
splice is a bit more powerful in that it allows you to remove a range of values (the first argument), insert new values (the second argument), and optionally get the old values (the result of the call).
Replacing a set of items
let mut vec = vec![0, 1, 5];
let slice = &[2, 3, 4];
vec.splice(..2, slice.iter().cloned());
println!("{:?}", vec); // [2, 3, 4, 5]
Getting the previous values
let mut vec = vec![0, 1, 2, 3, 4];
let slice = &[9, 8, 7];
let old: Vec<_> = vec.splice(3.., slice.iter().cloned()).collect();
println!("{:?}", vec); // [0, 1, 2, 9, 8, 7]
println!("{:?}", old); // [3, 4]
Okay, there is no appropriate method in Vec interface (as I can see). But we can always implement the same thing ourselves.
memmove
When T is Copy, probably the most obvious way is to move the memory, like this:
fn push_all_at<T>(v: &mut Vec<T>, offset: usize, s: &[T]) where T: Copy {
match (v.len(), s.len()) {
(_, 0) => (),
(current_len, _) => {
v.reserve_exact(s.len());
unsafe {
v.set_len(current_len + s.len());
let to_move = current_len - offset;
let src = v.as_mut_ptr().offset(offset as isize);
if to_move > 0 {
let dst = src.offset(s.len() as isize);
std::ptr::copy_memory(dst, src, to_move);
}
std::ptr::copy_nonoverlapping_memory(src, s.as_ptr(), s.len());
}
},
}
}
shuffle
If T is not copy, but it implements Clone, we can append given slice to the end of the Vec, and move it to the required position using swaps in linear time:
fn push_all_at<T>(v: &mut Vec<T>, mut offset: usize, s: &[T]) where T: Clone + Default {
match (v.len(), s.len()) {
(_, 0) => (),
(0, _) => { v.push_all(s); },
(_, _) => {
assert!(offset <= v.len());
let pad = s.len() - ((v.len() - offset) % s.len());
v.extend(repeat(Default::default()).take(pad));
v.push_all(s);
let total = v.len();
while total - offset >= s.len() {
for i in 0 .. s.len() { v.swap(offset + i, total - s.len() + i); }
offset += s.len();
}
v.truncate(total - pad);
},
}
}
iterators concat
Maybe the best choice will be to not modify Vec at all. For example, if you are going to access the result via iterator, we can just build iterators chain from our chunks:
let v: &[usize] = &[0, 1, 2];
let s: &[usize] = &[3, 4, 5, 6];
let offset = 2;
let chain = v.iter().take(offset).chain(s.iter()).chain(v.iter().skip(offset));
let result: Vec<_> = chain.collect();
println!("Result: {:?}", result);
I was trying to prepend to a vector in rust and found this closed question that was linked here, (despite this question being both prepend and insert AND efficiency. I think my answer would be better as an answer for that other, more precises question because I can't attest to the efficiency), but the following code helped me prepend, (and the opposite.) [I'm sure that the other two answers are more efficient, but the way that I learn, I like having answers that can be cut-n-pasted with examples that demonstrate an application of the answer.]
pub trait Unshift<T> { fn unshift(&mut self, s: &[T]) -> (); }
pub trait UnshiftVec<T> { fn unshift_vec(&mut self, s: Vec<T>) -> (); }
pub trait UnshiftMemoryHog<T> { fn unshift_memory_hog(&mut self, s: Vec<T>) -> (); }
pub trait Shift<T> { fn shift(&mut self) -> (); }
pub trait ShiftN<T> { fn shift_n(&mut self, s: usize) -> (); }
impl<T: std::clone::Clone> ShiftN<T> for Vec<T> {
fn shift_n(&mut self, s: usize) -> ()
// where
// T: std::clone::Clone,
{
self.drain(0..s);
}
}
impl<T: std::clone::Clone> Shift<T> for Vec<T> {
fn shift(&mut self) -> ()
// where
// T: std::clone::Clone,
{
self.drain(0..1);
}
}
impl<T: std::clone::Clone> Unshift<T> for Vec<T> {
fn unshift(&mut self, s: &[T]) -> ()
// where
// T: std::clone::Clone,
{
self.splice(0..0, s.to_vec());
}
}
impl<T: std::clone::Clone> UnshiftVec<T> for Vec<T> {
fn unshift_vec(&mut self, s: Vec<T>) -> ()
where
T: std::clone::Clone,
{
self.splice(0..0, s);
}
}
impl<T: std::clone::Clone> UnshiftMemoryHog<T> for Vec<T> {
fn unshift_memory_hog(&mut self, s: Vec<T>) -> ()
where
T: std::clone::Clone,
{
let mut tmp: Vec<_> = s.to_owned();
//let mut tmp: Vec<_> = s.clone(); // this also works for some data types
/*
let local_s: Vec<_> = self.clone(); // explicit clone()
tmp.extend(local_s); // to vec is possible
*/
tmp.extend(self.clone());
*self = tmp;
//*self = (*tmp).to_vec(); // Just because it compiles, doesn't make it right.
}
}
// this works for: v = unshift(v, &vec![8]);
// (If you don't want to impl Unshift for Vec<T>)
#[allow(dead_code)]
fn unshift_fn<T>(v: Vec<T>, s: &[T]) -> Vec<T>
where
T: Clone,
{
// create a mutable vec and fill it
// with a clone of the array that we want
// at the start of the vec.
let mut tmp: Vec<_> = s.to_owned();
// then we add the existing vector to the end
// of the temporary vector.
tmp.extend(v);
// return the tmp vec that is identitcal
// to unshift-ing the original vec.
tmp
}
/*
N.B. It is sometimes (often?) more memory efficient to reverse
the vector and use push/pop, rather than splice/drain;
Especially if you create your vectors in "stack order" to begin with.
*/
fn main() {
let mut v: Vec<usize> = vec![1, 2, 3];
println!("Before push:\t {:?}", v);
v.push(0);
println!("After push:\t {:?}", v);
v.pop();
println!("popped:\t\t {:?}", v);
v.drain(0..1);
println!("drain(0..1)\t {:?}", v);
/*
// We could use a function
let c = v.clone();
v = unshift_fn(c, &vec![0]);
*/
v.splice(0..0, vec![0]);
println!("splice(0..0, vec![0]) {:?}", v);
v.shift_n(1);
println!("shift\t\t {:?}", v);
v.unshift_memory_hog(vec![8, 16, 31, 1]);
println!("MEMORY guzzler unshift {:?}", v);
//v.drain(0..3);
v.drain(0..=2);
println!("back to the start: {:?}", v);
v.unshift_vec(vec![0]);
println!("zerothed with unshift: {:?}", v);
let mut w = vec![4, 5, 6];
/*
let prepend_this = &[1, 2, 3];
w.unshift_vec(prepend_this.to_vec());
*/
w.unshift(&[1, 2, 3]);
assert_eq!(&w, &[1, 2, 3, 4, 5, 6]);
println!("{:?} == {:?}", &w, &[1, 2, 3, 4, 5, 6]);
}