Lines

Often a string in Rust has multiple lines of data—each part is separated by a "\n" character. The lines() iterator helps us parse these strings.

With lines, we can test or print out each individual line. Or we can use map, filter or collect on the resulting iterator for more power.

Lines example

This code introduces a string that has 2 newlines separating 3 lines. By calling lines() in a for-loop, we print each part (the newline delimiters are not included).

Detail Lines() returns an iterator. This means we can call map and collect on the lines, and put the lines in a Vector of strings.

fn main() {
    let data = "a\n b\n  c";
    // Loop over and print the lines.
    for line in data.lines() {
        println!("{line}");
    }
    // Get the lines, uppercase them, and put them in a vector.
    let all_lines = data.lines().map(|x| x.to_uppercase()).collect::<Vec<String>>();
    println!("{:?}", all_lines);
}a
 b
  c
["A", " B", "  C"]

Filter

Suppose you have a string that has many lines in it, and you just want to use some of them. We can use filter() on the Lines iterator.

Here We use filter and filter out all lines except those that start with the lowercase letter B.

fn main() {
    let data = "blue\nbird\nfrog";
    // Get lines, and filter the lines.
    // Only print lines starting with a certain letter.
    for line in data.lines().filter(|x| x.starts_with('b')) {
        println!("{line}");
    }
}blue
bird

A summary

If we have a string containing multiple lines, and want to loop over them, the lines() iterator is ideal. It can be used on str or String types.