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: https://stackoverflow.com/q/26185618/1233251 — E_net4, Jul 20 '19 at 15:26
Of course you can limit the mutability to a nested scope: `let w = { let mut array = ...; ...; array };`. — Sven Marnach, Jul 20 '19 at 16:05
@E_net4 does `for v in &mut w {` work if `w` is not declated as mutable to begin with? — Mark, Jul 20 '19 at 19:54
it does not without declaring `w` as mutable, thus why I'm asking. — E_net4, Jul 20 '19 at 20:03
Possible duplicate of [How do I create and initialize an immutable array?](https://stackoverflow.com/questions/26435046/how-do-i-create-and-initialize-an-immutable-array) — E_net4, Jul 21 '19 at 01:30
@E_net4 Not a duplicate. The questions are distinctly different. — Listerone, Jul 21 '19 at 15:55
What about this one? https://stackoverflow.com/q/26757355/1233251 — E_net4, Jul 21 '19 at 16:42

score 0 · Answer 1 · answered Jul 24 '19 at 08:11

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 '19 at 08:11

Kornel

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score -1 · Answer 2 · answered Jul 20 '19 at 23:05

-1

The answer is NO. ⠀

answered Jul 20 '19 at 23:05

Listerone

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How to populate an array without using mut?

2 Answers2