How to populate an array without using mut?

Question

If I wanted to populate a list of numbers, I can use a vector, and hence the heap, by doing this:

let w = (0..1024).collect::<Vec<_>>();

But, if I wanted to avoid the heap I have to use an array. To use an array, I have to use a loop which means I must have a mutable variable:

let mut w = [0u32; 1024];
for i in 0..1024 {
    w[i] = i as u32;
}

Is it possible to populate an array without using mutable variables?

This question has been flagged as a dup. I'm not sure how this can possibly be confused.

"How to populate an array without using mut?" means how do I populate an array without using a mutable variable to do so. Any mut, not just the array variable itself.

"How do I create and initialize an immutable array?" means how do I create an immutable array.

Is "without using mutable variables" an artificial constraint? Creating the array and then doing for v in &mut w { or for v in w.iter_mut() would appear to be the usual way of doing this. See also: stackoverflow.com/q/26185618/1233251 — E_net4
– E_net4, Commented Jul 20, 2019 at 15:26
Of course you can limit the mutability to a nested scope: let w = { let mut array = ...; ...; array };. — Sven Marnach
– Sven Marnach, Commented Jul 20, 2019 at 16:05
@E_net4 does for v in &mut w { work if w is not declated as mutable to begin with? — Mark
– Mark, Commented Jul 20, 2019 at 19:54
it does not without declaring w as mutable, thus why I'm asking. — E_net4
– E_net4, Commented Jul 20, 2019 at 20:03

Kornel · Accepted Answer · 2019-07-24 08:11:11Z

0

You can't.

Iterator can't guarantee any specific length at compile time, so .collect() can't produce fixed-size arrays.

You can do:

let w = w;

to recreate the binding as immutable afterwards, or move the initialization to a helper function.

answered Jul 24, 2019 at 8:11

Kornel

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John Smith · Accepted Answer · 2023-12-10 06:02:38Z

0

You can create an immutable variable whose value is a &mut [T; N]:

pub fn main() {
    let a = &mut [1, 2, 3];
    a[0] = 42;
    println!("{:?}", a);
    // a = &mut [1; 3]; // error[E0384]: cannot assign twice to immutable variable `a`
    print_type_of(&a); // &mut [i32; 3]
}

fn print_type_of<T>(_: &T) {
    println!("{}", std::any::type_name::<T>())
}

edited Dec 10, 2023 at 6:02

answered Dec 10, 2023 at 5:58

John Smith

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Comments

Listerone · Accepted Answer · 2019-07-20 23:05:32Z

-1

The answer is NO. ⠀

answered Jul 20, 2019 at 23:05

Listerone

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