Println strip out ^M

help
1

Hello
I'm learning Rust and trying to re-write "head" for this learning experience.
I have noticed that GNU coreutils "head" is passing around the special character ^M.
And in my case I'm using println!() macro to output the lines from input files, also println!() seems to be stripping out ^M from the output or the way I'm reading the file...
I'm curious how I could pass out this character through println().
This is the piece of code where I read the file and output through println!() the lines.

fn open(filename: &str) -> MyResult<Box<dyn BufRead>> {
    match filename {
        "-" => Ok(Box::new(BufReader::new(io::stdin()))),
        _ => Ok(Box::new(BufReader::new(File::open(filename)?))),
    }
}

match open(&filename) {
                     Err(err) => eprintln!("{}:{}", filename, err),
                     Ok(file) => {
                         let max_output_line = &config.lines;
                         let index = &config.files.iter().position(|r| r == filename).unwrap();
                         let number_of_files = &config.files.len();
                         let first_file: usize = 0;
                         if *number_of_files > 1 {
                            if index == &first_file {
                               println!("==> {} <==", &filename);
                            } else {
                               println!("\n==> {} <==", &filename);
                            }
                         }
                         for (line_num, line) in file.lines().enumerate(){
                             let line = line?;
                             if &line_num == max_output_line {
                                 break;
                             }
                             println!("{}",line);
                         }
                     }
                     }

For reference here is my input:

image
image407×517 22.5 KB

In the output I don't have the ^M anymore

PS: ^M is the Windows carriage return character (0xD) as interpreted by VIM when showing all characters.

Thanks in advance for any advice or help.

2

It's not println!, it's Lines that strips the carriage return (\r, ^M).

If you want to preserve the \r of a \r\n, you'll need something besides Lines. Perhaps read_line or read_until.

5 Likes
4

Thank you ! I missed that one effectively.
Will look at read_line, and fix that.

5

Thanks for the tips, also I'm probably missing a concept here because it seems that unlike lines(), read_line() does not return an iterator.
I have the feeling that I should re-create lines() without consuming the return character.
Still investigating, by looking at the standard library doc. And source code of lines().

6

You could, but that would be unnecessary work. It's fairly easy to call read_line in a loop:

// let mut reader = ...
let mut buf = String::with_capacity(100);
loop {
    buf.clear();
    let len = reader.read_line(&mut buf)?;
    if len == 0 {
        break;
    }
    let line = buf.trim_end_matches('\n');
    // note that \r is still present in the line.
    println!("{}", line);
}
2 Likes
7

Here's another example of using read_line based on your OP.

            let mut line_num = 0;
            let mut line = String::new();
            loop {
                line.clear();
                if 0 == file.read_line(&mut line)? || &line_num == max_output_line {
                    break;
                }
                print!("{}", line);
                line_num += 1;
            }
2 Likes
8

Thank you guys !
This is awesome, I didn't know the keyword loop very happy to discover it !
And very nice to see this modern syntax ( I know this is not that modern anymore ) on the if statement.

I still can picture/figure out, how my program here is correctly iterating through each line of my file instead of looping always to the first line ?
Could it be because I'm using File ?

9

All the read methods in the BufRead and Read traits advance the position in the underlying file or buffer.

1 Like
10

Thank you will look at those two more in depth.

1 Like
11

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