Writing a file
- Rust course
- Intermediate
- Lesson 90/115
- 4 min
Write a file with std::fs::write.
Writing files
fs::write creates the file or overwrites it. To write piece by piece, File::create and the Write trait are used; the writeln! macro can write to files too:
use std::fs::File;
use std::io::Write;
let mut f = File::create("sayilar.txt")?;
for i in 1..=3 {
writeln!(f, "{i}")?;
}To use write!/writeln!, std::io::Write must be in scope. The file closes automatically when f goes out of scope.
Buffered writing
Many small writes mean many calls to the operating system. BufWriter collects writes in memory and sends them in bulk:
use std::io::{BufWriter, Write};
let f = File::create("buyuk.txt")?;
let mut w = BufWriter::new(f);
for i in 0..10_000 {
writeln!(w, "{i}")?;
}
w.flush()?; // send the remaining buffer and catch the errorBufWriter also flushes when dropped, but an error at that moment is ignored; that's why an explicit flush is recommended.
Practice Write numbers
Open sayilar.txt in the temp folder with File::create and write the numbers 1 to 3 with writeln!. Read the file back and print its lines joined with commas.
Starter code
use std::env;
use std::fs::{self, File};
use std::io::{self, Write};
fn main() {
// buraya yaz
}Sample solution
use std::env;
use std::fs::{self, File};
use std::io::{self, Write};
fn main() -> io::Result<()> {
let yol = env::temp_dir().join("sayilar.txt");
{
let mut f = File::create(&yol)?;
for i in 1..=3 {
writeln!(f, "{}", i)?;
}
}
let icerik = fs::read_to_string(&yol)?;
let parcalar: Vec<&str> = icerik.lines().collect();
println!("{}", parcalar.join(","));
Ok(())
}Expected output
1,2,3
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does <code>File::create</code> do with an existing file?
- Errors
- Makes a backup
- Erases its content (truncates it to zero length) Correct answer
- Appends to the end
Explanation: To protect an existing file, use File::create_new (new file only) or OpenOptions with append.
2 Write a line
let mut f = File::create("sayilar.txt")?;
____(f, "{}", 42)?;let mut f = File::create("sayilar.txt")?;
writeln!(f, "{}", 42)?;Explanation: writeln! writes a line to any target implementing Write.
3 Operation and effect
| Term | Meaning |
|---|---|
fs::write |
Creates or overwrites |
File::create |
Opens an empty file |
BufWriter::new(f) |
Bulk writing |
w.flush() |
Flushes the buffer |