Find The Last Occurrence Of A Character In A String In Rust

Determining the position of the last occurrence of a specific character in a string is a common task in programming. In this short article, we will look at how to accomplish this in the Rust language.

Code snippet - Finding the last occurrence of a character in a string

    fn find_last(s: &str, c: char) -> Option<usize> {
        s.char_indices().rfind(|&(_, x)| x == c).map(|(i, _)| i)
    }

    fn main() {
        let text = "hello";
        find_last(&text, 'l').map(|pos| println!("{}", pos)); 
        // This will print 3
    }

Code Explanation - Finding the last occurrence of a character in a string

In the above code snippet, we start with declaring a function find_last(s: &str, c: char) -> Option<usize>, which returns the last index of a given character in a string.

First, we are using the char_indices() function which gives us an iterator over (index, char) pairs. So, we enumerate each character of the string along with its index.

The rfind function is then used to find the last element (i.e., the last pair of (index, char)) that satisfies a certain condition. In this case, we are checking for the last pair where the character matches our specified character. This will return Some((i, c)) where i is the index and c is the character, or None if no matching character is found.

Next, we use map(|(i, _)| i) to turn Some((i, c)) into Some(i), essentially stripping off the character and retaining the index only. If no match was found (if rfind returned None) then map does nothing and None is returned by the function.

Finally, in the main function, we call the find_last function with the text 'hello' and the character l. The map on the result is used to print the position if a match was found.

This rust function provides an efficient and simple way to find the last occurrence of a character in a string in Rust.