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 it?

str.substr(start[, length])

The closest is probably slice_unchecked but it uses byte offsets instead of character indexes and is marked unsafe.

Answer 1

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 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

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 4

For my_string.substring(start, len)-like syntax, you can write a custom trait:

trait StringUtils {
    fn substring(&self, start: usize, len: usize) -> Self;
}

impl StringUtils for String {
    fn substring(&self, start: usize, len: usize) -> Self {
        self.chars().skip(start).take(len).collect()
    }
}

// Usage:
fn main() {
    let phrase: String = "this is a string".to_string();
    println!("{}", phrase.substring(5, 8)); // prints "is a str"
}

Answer 5

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";
assert_eq!(Ok("bc"), substr(s, 1, Some(2)));
assert_eq!(Ok("c"), substr(s, 2, Some(2)));
assert_eq!(Ok("c"), substr(s, 2, None));
assert_eq!(Err("c"), substr(s, 2, Some(99)));
assert_eq!(Ok(""), substr(s, 2, Some(0)));
assert_eq!(Err(""), substr(s, 5, Some(4)));

Note that this does not handle unicode grapheme clusters. For example, "y̆es" contains 4 unicode chars but 3 grapheme clusters. Crate unicode-segmentation solves this problem. Unicode grapheme clusters are handled correctly if part

let mut itr = s.char_indices()...

is replaced with

use unicode_segmentation::UnicodeSegmentation;
let mut itr = s.grapheme_indices(true)...

Then also following works

assert_eq!(Ok("y̆"), substr("y̆es", 0, Some(1)));

Answer 6

Knowing about the various syntaxes of the slice type might be beneficial for some of the readers.

• Reference to a part of a string
  &s[6..11]
• If you start at index 0, you can omit the value
  &s[0..1] ^= &s[..1]
• Equivalent if your substring contains the last byte of the string
  &s[3..s.len()] ^= &s[3..]
• This also applies when the slice encompasses the entire string
  &s[..]
• You can also use the range inclusive operator to include the last value
  &s[..=1]

Link to docs: https://doc.rust-lang.org/book/ch04-03-slices.html

Answer 7

I'm not very experienced in Rust but I gave it a try. If someone could correct my answer please don't hesitate.

fn substring(string:String, start:u32, end:u32) -> String {
    let mut substr = String::new();
    let mut i = start;
    while i < end + 1 {
        substr.push_str(&*(string.chars().nth(i as usize).unwrap().to_string()));
        i += 1;
    }
    return substr;
}

Here is a playground

Answer 8

I couldn't find the exact substr implementation that I'm familiar with from other programming languages like: JavaScript, Dart, and etc.

Here is possible implementation of method substr to &str and String

Let's define a trait for making able to implement functions to default types, (like extensions in Dart).

trait Substr {
    fn substr(&self, start: usize, end: usize) -> String; 
}

Then implement this trait for &str

impl<'a> Substr for &'a str {
  fn substr(&self, start: usize, end: usize) -> String {
      if start > end || start == end {
          return String::new();
      }
      
      self.chars().skip(start).take(end - start).collect()
  }
}

Try:

fn main() {
    let string = "Hello, world!";
    let substring = string.substr(0, 4);
    println!("{}", substring); // Hell
}

Answer 9

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!