How do you iterate over a string by character

Question

I have a string and I need to scan for every occurrence of "foo" and read all the text following it until a second ". Since Rust does not have a contains function for strings, I need to iterate by characters scanning for it. How would I do this?

Edit: Rust's &str has a contains() and find() method.

Prioress

Could you show an example of some inputs with your desired outputs? It will help us see more clearly what you are trying to accomplish exactly. – Speos

there's an example of such loop here rustbyexample.org/loops.html, although I think there're easier means for that. – Bostick

Note that there are many string search algorithms, and their time complexity is not that of a straightforward approach (O(n*m)). en.wikipedia.org/wiki/String_searching_algorithm – Colima

This sounds like regex. – Jamijamie

Answer 1

I need to iterate by characters scanning for it.

The .chars() method returns an iterator over characters in a string. e.g.

for c in my_str.chars() { 
    // do something with `c`
}

for (i, c) in my_str.chars().enumerate() {
    // do something with character `c` and index `i`
}

If you are interested in the byte offsets of each char, you can use char_indices.

Look into .peekable(), and use peek() for looking ahead. It's wrapped like this because it supports UTF-8 codepoints instead of being a simple vector of characters.

You could also create a vector of chars and work on it from there, but that's more time and space intensive:

let my_chars: Vec<_> = mystr.chars().collect();

Stambul

Beware that the characters obtained this way might not correspond to an intuitive definition of a character as perceived by humans. See github.com/unicode-rs/unicode-segmentation for more details. – Mahmud

Does this destroy/consume the string? – Blackbeard

@FreelanceConsultant: No, as the signature of the method shows, it takes an immutable reference to self, not self itself or a mutable reference to it. It iterates the underlying data directly, without changing it in any way. – Perturbation

Answer 2

The concept of a "character" is very ambiguous and can mean many different things depending on the type of data you are working with. The most obvious answer is the chars method. However, this does not work as advertised. What looks like a single "character" to you may actually be made up of multiple Unicode code points, which can lead to unexpected results:

"a̐".chars() // => ['a', '\u{310}']

For a lot of string processing, you want to work with graphemes. A grapheme consists of one or more unicode code points represented as a string slice. These map better to the human perception of "characters". To create an iterator of graphemes, you can use the unicode-segmentation crate:

use unicode_segmentation::UnicodeSegmentation;

for grapheme in my_str.graphemes(true) {
    // ...
}

If you are working with raw ASCII then none of the above applies to you, and you can simply use the bytes iterator:

for byte in my_str.bytes() {
    // ...
}

Although, if you are working with ASCII then arguably you shouldn't be using String/&str at all and instead use Vec<u8>/&[u8] directly.

Pinion

Interesting last sentence statement. Any pointers to why String is not advised for simple ASCII? I'm learning Rust. – Gait

@DawidLaszuk A String cannot be indexed directly, and has to perform extra utf8 checks to be manipulated. It's easier, and less overhead to work with bytes directly. – Pinion

Answer 3

fn main() {
let s = "Rust is a programming language";
for i in s.chars() {
    print!("{}", i);
}}

Output: Rust is a programming language

I use the chars() method to iterate over each element of the string.

Subcritical

Sorry, but what does this add to the accepted answer? – Ieper