File

Without data persistence, programs have limited usefulness. With files, we can store data, and persist data from memory for next time.

We can implement file handling directly with low-level types, but things like BufReader can simplify this. BufReader helps with reading lines, and reading entire files.

Lines example

To begin, consider this example—perform the steps necessary to read in a file on the disk. We then print out its contents to the screen.

Step 1 We often a file with File open(). Please change the path to one that exists on your computer—any text file will do.

Step 2 We use the BufReader we created from the file. We call lines() and print each line in a for-loop.

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

fn main() {
    // Step 1: open and unwrap a file.
    let file = File::open("/home/sam/programs/test.txt").unwrap();
    let reader = BufReader::new(file);

    // Step 2: loop over lines and print them.
    for line in reader.lines() {
        println!("LINE: {}", line.unwrap());
    }
}LINE: Hello
LINE: friends

Read all text

This example uses the read_until function on BufReader. It gets the entire contents of the file, and places it into a vector of bytes.

Note From the byte vector, we can get a string representation with String from_utf8.

Note 2 The file here contains the text "orange cat," which has the byte representation shown.

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

fn main() {
    let file = File::open("/home/sam/programs/test.txt").unwrap();
    let mut reader = BufReader::new(file);
    let mut buf = vec![];

    // Use read_until to read until EOF.
    reader.read_until(u8::MIN, &mut buf);
    println!("BYTES: {:?}", buf);

    // Convert vector of bytes to string.
    let data = String::from_utf8(buf).unwrap();
    println!("STRING: {}", data);
}BYTES: [111, 114, 97, 110, 103, 101, 32, 99, 97, 116, 13, 10]
STRING: orange cat

File exists

To see if a file exists, we can use the Path type and call exists() on it. Here we check a file that does not exist, and one that exists on the test system.

use std::path::Path;

fn main() {
    // Check that this file does not exist.
    if !Path::new("/Users/sam/missing").exists() {
        println!("NOT FOUND");
    }
    // This file exists.
    if Path::new("/Users/sam/example.txt").exists() {
        println!("FOUND");
    }
}NOT FOUND
FOUND

File handling in Rust is done in a similar way to many other languages. We can handle byte data, and then convert it to strings. And functions like lines() on BufReader are helpful.