File create

Suppose in Rust we have a file containing bytes (u8 elements) and want to modify the data. We then want to write the file in another location.

With u8 vectors, we can read in an existing file, and loop over the elements, building up a new vector. Then we can call File::create() to write the new data to disk.

Example

This example code uses the Vec::new() function to create new vectors of u8 elements. These 2 vectors store the bytes read, and the bytes we will write.

Info To read the existing data, we must pass a mutable reference to our vector to read_to_end.

Note The for-loop is where we modify the existing data, and push new byte elements (u8) as we go along.

Detail We open a file for writing by calling File::create() and then convert the u8 vector to a slice by calling as_slice.

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

fn main() -> io::Result<()> {
    // Read the file.
    let f = File::open("/Users/sam/file.txt")?;
    let mut reader = BufReader::new(f);
    let mut buffer = Vec::new();
    reader.read_to_end(&mut buffer)?;

    // Loop over and modify the data.
    let mut buffer_modified = Vec::new();
    for value in buffer {
        println!("BYTE: {}", value);
        if value == 'a' as u8 {
            let value_modified = 'z' as u8;
            buffer_modified.push(value_modified);
            println!("MODIFIED: {}", value_modified);
        } else {
            buffer_modified.push(value);
        }
    }

    // Add additional letter.
    buffer_modified.push('.' as u8);

    // Write the modified data.
    let mut f = File::create("/Users/sam/file-modified.txt")?;
    f.write_all(buffer_modified.as_slice())?;
    Ok(())
}BYTE: 97
MODIFIED: 122
BYTE: 98
BYTE: 99
abc
zbc.

Modify existing file

If we pass the same file name to the File::create method that we used for File::open, we can modify the same file. No additional file is needed.

A summary

By using u8 Vectors, we can read in a file and modify it in memory. Then we can write the modified data to a different file, or the same file, thus mutating the byte file.