Is there a method like JavaScript's substr in Rust?

Question

I looked at the Rust docs for String but I can\'t find a way to extract a substring.

Is there a method like JavaScript\'s substr in Rust? If not, how would you implement

Answer 1

This code performs both substring-ing and string-slicing, without panicking nor allocating:

use std::ops::{Bound, RangeBounds};

trait StringUtils {
    fn substring(&self, start: usize, len: usize) -> &str;
    fn slice(&self, range: impl RangeBounds<usize>) -> &str;
}

impl StringUtils for str {
    fn substring(&self, start: usize, len: usize) -> &str {
        let mut char_pos = 0;
        let mut byte_start = 0;
        let mut it = self.chars();
        loop {
            if char_pos == start { break; }
            if let Some(c) = it.next() {
                char_pos += 1;
                byte_start += c.len_utf8();
            }
            else { break; }
        }
        char_pos = 0;
        let mut byte_end = byte_start;
        loop {
            if char_pos == len { break; }
            if let Some(c) = it.next() {
                char_pos += 1;
                byte_end += c.len_utf8();
            }
            else { break; }
        }
        &self[byte_start..byte_end]
    }
    fn slice(&self, range: impl RangeBounds<usize>) -> &str {
        let start = match range.start_bound() {
            Bound::Included(bound) | Bound::Excluded(bound) => *bound,
            Bound::Unbounded => 0,
        };
        let len = match range.end_bound() {
            Bound::Included(bound) => *bound + 1,
            Bound::Excluded(bound) => *bound,
            Bound::Unbounded => self.len(),
        } - start;
        self.substring(start, len)
    }
}

fn main() {
    let s = "abcdèfghij";
    // All three statements should print:
    // "abcdè, abcdèfghij, dèfgh, dèfghij."
    println!("{}, {}, {}, {}.",
        s.substring(0, 5),
        s.substring(0, 50),
        s.substring(3, 5),
        s.substring(3, 50));
    println!("{}, {}, {}, {}.",
        s.slice(..5),
        s.slice(..50),
        s.slice(3..8),
        s.slice(3..));
    println!("{}, {}, {}, {}.",
        s.slice(..=4),
        s.slice(..=49),
        s.slice(3..=7),
        s.slice(3..));
}

Answer 2

You can use the as_str method on the Chars iterator to get back a &str slice after you have stepped on the iterator. So to skip the first start chars, you can call

let s = "Some text to slice into";
let mut iter = s.chars();
iter.by_ref().nth(start); // eat up start values
let slice = iter.as_str(); // get back a slice of the rest of the iterator

Now if you also want to limit the length, you first need to figure out the byte-position of the length character:

let end_pos = slice.char_indices().nth(length).map(|(n, _)| n).unwrap_or(0);
let substr = &slice[..end_pos];

This might feel a little roundabout, but Rust is not hiding anything from you that might take up CPU cycles. That said, I wonder why there's no crate yet that offers a substr method.

Answer 3

The solution given by oli_obk does not handle last index of string slice. It can be fixed with .chain(once(s.len())).

Here function substr implements a substring slice with error handling. If invalid index is passed to function, then a valid part of string slice is returned with Err-variant. All corner cases should be handled correctly.

fn substr(s: &str, begin: usize, length: Option<usize>) -> Result<&str, &str> {
    use std::iter::once;
    let mut itr = s.char_indices().map(|(n, _)| n).chain(once(s.len()));
    let beg = itr.nth(begin);
    if beg.is_none() {
        return Err("");
    } else if length == Some(0) {
        return Ok("");
    }
    let end = length.map_or(Some(s.len()), |l| itr.nth(l-1));
    if let Some(end) = end {
        return Ok(&s[beg.unwrap()..end]);
    } else {
        return Err(&s[beg.unwrap()..s.len()]);
    }
}
let s = "abc                                                                    

                                                        
            

Answer 4

I would suggest you use the crate substring. (And look at its source code if you want to learn how to do this properly.)

Answer 5

For characters, you can use s.chars().skip(pos).take(len):

fn main() {
    let s = "Hello, world!";
    let ss: String = s.chars().skip(7).take(5).collect();
    println!("{}", ss);
}

Beware of the definition of Unicode characters though.

For bytes, you can use the slice syntax:

fn main() {
    let s = b"Hello, world!";
    let ss = &s[7..12];
    println!("{:?}", ss);
}

Answer 6

You can also use .to_string()[ <range> ].

This example takes an immutable slice of the original string, then mutates that string to demonstrate the original slice is preserved.

let mut s: String = "Hello, world!".to_string();

let substring: &str = &s.to_string()[..6];

s.replace_range(..6, "Goodbye,");

println!("{}   {} universe!", s, substring);

//    Goodbye, world!   Hello, universe!