How to read subset of lines (like tail) from a file

[bck01215]

Currently, I'm using this function in my code:

fn lines_from_file(filename: impl AsRef<Path>) -> Vec<String> {
    let file = File::open(filename).expect("no such file");
    let buf = BufReader::new(file);
    buf.lines().map(|l| l.expect("Could not parse line")).collect()
}

How can I safely read the last x lines only?

[jvcmarcenes]

I don't really know how to help you rather than providing the code I'd write for it, so:

fn read_last_n_lines(path: impl AsRef<Path>, n: usize) -> Vec<String> {
    // it's simpler to use the provided helper function than dealing with the File ourselves
    let s = std::fs::read_to_string(path).unwrap();
   
    // we collect all the lines into a vec
    let s = s.lines().collect::<Vec<_>>();
   
    // then take only the last n lines
    s[s.len().saturating_sub(n)..s.len()].iter().map(|&s| String::from(s)).collect()
}

If there's anything unclear about the code above, feel free to ask me any questions about it

[bck01215]

Still trying to understand some of the lower levels of rust. Wouldn't s contain the entire contents of the file? Most times that's not problematic, but I'm work with potentially files large than my RAM.

[semicoleon]

Yes. If you want to actually read only the last n lines of data from the file I think you'll have to write the code for that yourself. As far as I know the standard library doesn't have any facilities for reading lines from the end of a file back towards the beginning.

[bck01215]

I ended up using rev_buffer_reader and only take certain number items with the iterator

fn lines_from_file(file: &File, limit: usize) -> Vec<String> {
    let buf = RevBufReader::new(file);
    buf.lines().take(limit).map(|l| l.expect("Could not parse line")).collect()
}