Issue when creating a Vector and assigning it later on [closed] - rust

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After reading a LOT of documentation I figured out that I am facing a problem of scope, but I have no idea how to solve it. See the example code below:
fn main() {
let mut bytes_buf:Vec<u8> = Vec::new(); // 1) where I declare the Vector, the compiler force me to initialize it.
loop {
match socket.recv_from(&mut buf) {
Ok((size, src)) => {
if count == 0 {
chunks_cnt = ...
bytes_buf = vec![0; MAX_CHUNK_SIZE * chunks_cnt as usize]; // 2) where I want to set vector size, only ONCE, and after knowing chunks_cnt
}
bytes_buf[start..end].copy_from_slice(buf); // 3) where I want to gradually fill the vector
}
}
}
}
For convenience, you can check the full code here

Answer 1

Possible solution
Here the socket fills the slice buf. If it fails an error message is shown. If is succeeds it will enter the loop.
On each iteration of the loop, the buf is converted to a Vec<u8> and appended to bytes_buf. Then if this is the first iteration, then the size value is inserted into the first position. Then the first flag is set to false. After that all iterations will continue appending data to the vector.
The following minimal example should compile fine:
use std::net::{UdpSocket};
const UDP_HEADER: usize = 8;
const IP_HEADER: usize = 20;
const MAX_DATA_LENGTH: usize = (64 * 1024 - 1) - UDP_HEADER - IP_HEADER;
fn main() {
let socket = UdpSocket::bind("0.0.0.0:8888").expect("Could not bind socket");
let mut buf= [0u8; MAX_DATA_LENGTH]; // Slice that will be filled by recv_from.
let mut bytes_buf:Vec<u8> = Vec::new(); // Vector where the data will be moved.
let mut first = true; // Flag that indicates if this is our first iteration.
loop {
match socket.recv_from(&mut buf) {
Ok((_size, _src)) => {
// Convert the slice to a vector (to_vec function) and append it to the bytes_buf.
bytes_buf.append(&mut buf.to_vec());
if first {
// Insert function inserts the element at the specified position and shifts
// all elements after it to the right.
bytes_buf.insert(0, 10u8); // IDK What value you need here.
}
first = false; // Set first to false
},
Err(err) => eprintln!("Error: {}", err) // If we fail display the error.
}
}
}
Side note
Your example was missing lots of variables and context. Despite this, I managed to create a minimal working example of what I believe you are trying to achieve thanks to the link you shared despite being quite different. Please next time provide a minimal reproducible example. More information here: How to create a Minimal, Reproducible Example
Have a nice day!