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Reading and writing files

Read project files with fs::read_to_string, return io::Result from main, and write temporary files under /tmp.

Runs in Zerethon IDE · Rust 1.87 Official Rust documentation ↗

A run sees the project's files in its working folder, read-only. A program may write files under /tmp; they are deleted when the run ends.

main.rs 
use std::fs;

fn main() -> std::io::Result<()> {
    let text = fs::read_to_string("data.csv")?;
    let mut count = 0;
    for line in text.lines().skip(1) {
        let fields: Vec<&str> = line.split(',').collect();
        println!("{} {}", fields[0], fields[1]);
        count += 1;
    }

    fs::write("/tmp/report.txt", format!("count={count}\n"))?;
    print!("{}", fs::read_to_string("/tmp/report.txt")?);
    Ok(())
}
data.csv 
name,score
ada,90
alan,85

Output

ada 90
alan 85
count=2
  • main may return std::io::Result<()>; then ? works inside it, and an error ends the program with a message.
  • Paths are relative to the project root: fs::read_to_string("data/input.txt") reads the file shown in the explorer as data/input.txt.
  • Writing next to your code fails with Read-only file system — use /tmp/out.txt.
  • There is no csv or serde crate; split simple CSV lines yourself as above.

Related topics

Official references