How to write a vector to a json file?

Question

How to write a vector to a JSON file in rust?

Code:

use std::fs::File;
use std::io::prelude::*;

let vec1 = vec![1.0,2.0,2.1,0.6];

let mut file = File::create("results.json").unwrap();
let data = serde_json::to_string(&vec1).unwrap();

file.write(&data);

Error:

mismatched types
expected reference `&[u8]`
   found reference `&std::string::String`rustc(E0308)

Answer 1

Instead of writing the data to an in-memory string first, you can also write it directly to the file:

use std::fs::File;
use std::io::{BufWriter, Write};

fn main() -> std::io::Result<()> {
    let vec = vec![1, 2, 3];
    let file = File::create("a")?;
    let mut writer = BufWriter::new(file);
    serde_json::to_writer(&mut writer, &vec)?;
    writer.flush()?;
    Ok(())
}

This approach has a lower memory footprint and is generally preferred. Note that you should use buffered writes to the file, since serialization could otherwise result in many small writes to the file, which would severely reduce performance.

If you need to write the data to memory first for some reason, I suggest using serde_json::to_vec() instead of serde_json::to_string(), since that function will give you a Vec<u8> immediately.