Copy from slice

Suppose we want to copy data into an array from a slice. The slice could be from another array or a vector. Copy_from_slice can help here.

With this function, we use range syntax to copy data. The implementation is accelerated and will often perform faster than a for-loop.

First example

Here we demonstrate the usage of copy_from_slice in a simple Rust program. We copy a part of the data vector into an array.

Part 1 This is the data we are copying into our array. The vector here has 4 elements and is created with the vec macro.

Part 2 This is the "empty" array of 4 elements we want to populate with copied data.

Part 3 Here we invoke the copy_from_slice() function and copy only the first 2 elements from the data array into our empty array.

Important The length of the 2 ranges in the copy_from_slice invocation must be equal (the starting indexes can be different though).

fn main() {
    // Part 1: The data we wish to copy.
    let data = vec![10, 20, 30, 40];

    // Part 2: The memory we want to copy the data to.
    let mut empty = [0, 0, 0, 0];

    // Part 3: Use copy from slice with slices.
    empty[0..2].copy_from_slice(&data[0..2]);
    println!("{:?}", empty);
}
[10, 20, 0, 0]

`HashMap` keys

Sometimes in Rust we want to quickly look up keys in a HashMap. We can use copy_from_slice() to perform part of this task.

Step 1 We create a HashMap with 3-element array keys, and str values. We only add 1 entry.

Step 2 We copy into our "key" array by using the copy_from_slice function. We fill up the key with 3 elements.

Step 3 We use get() and if-let syntax to access the value from the HashMap at the key we filled with copy_from_slice.

use std::collections::HashMap;

fn main() {
    // Step 1: Create HashMap with array keys.
    let mut h = HashMap::new();
    h.insert([5, 15, 25], "OK");

    // Step 2: Copy data into key with copy from slice.
    let data = [5, 15, 25, 35, 45];
    let mut key = [0, 0, 0];
    key.copy_from_slice(&data[0..3]);

    // Step 3: Access HashMap value with key.
    if let Some(value) = h.get(&key) {
        println!("{value}");
    }
}
OK

Final notes

Much like extend_from_slice(), copy_from_slice has optimizations that help it perform faster than a loop. Thus it should be preferred when copying from an array or vector.

Copy from slice

First example

HashMap keys

Final notes

`HashMap` keys