How to make a vector of received size?

Question

I have a vector data with size unknown at compile time. I want to create a new vector of the exact that size. These variants don't work:

let size = data.len();

let mut try1: Vec<u32> = vec![0 .. size]; //ah, you need compile-time constant
let mut try2: Vec<u32> = Vec::new(size); //ah, there is no constructors with arguments

I'm a bit frustrated - there is no any information in Rust API, book, reference or rustbyexample.com about how to do such simple base task with vector. This solution works but I don't think it is good to do so, it is strange to generate elements one by one and I don't have need in any exact values of elements:

let mut temp: Vec<u32> = range(0u32, data.len() as u32).collect();

Answer 1

The recommended way of doing this is in fact to form an iterator and collect it to a vector. What you want is not precisely clear, however; if you want [0, 1, 2, …, size - 1], you would create a range and collect it to a vector:

let x = (0..size).collect::<Vec<_>>();

(range(0, size) is better written (0..size) now; the range function will be disappearing from the prelude soon.)

If you wish a vector of zeroes, you would instead write it thus:

let x = std::iter::repeat(0).take(size).collect::<Vec<_>>();

If you merely want to preallocate the appropriate amount of space but not push values onto the vector, Vec::with_capacity(capacity) is what you want.

You should also consider whether you need it to be a vector or whether you can work directly with the iterator.

Answer 2

You can use Vec::with_capacity() constructor followed by an unsafe set_len() call:

let n = 128;
let v: Vec<u32> = Vec::with_capacity(n);
unsafe { v.set_len(n); }
v[12] = 64;  // won't panic

This way the vector will "extend" over the uninitialized memory. If you're going to use it as a buffer it is a valid approach, as long as the type of elements is Copy (primitives are ok, but it will break horribly if the type has a destructor).