Replace some characters in a string with the next unicode character

Question

I have an input text as following:

 inputtext = "This is a test";

I need to replace some of the character (based on a certain criteria) to next unicode character

 let i = 0;
 for c in inputtext.chars() {
   if (somecondition){
     // Replace char here
     inputtext.replace_range(i..i+1, newchar);
     // println!("{}", c);
 }

What is the best way to do this?

Answer 1

You can't easily update a string in-place because a Rust string is not just an array of characters, it's an array of bytes (in UTF-8 encoding), and different characters may use different numbers of bytes. For example, the character ? (U+07FF "Nko Taman Sign") uses two bytes, whereas the next Unicode character ? (U+0800 "Samaritan Letter Alaf") uses three.

It's therefore simplest to turn the string into an iterator of characters (using .chars()), manipulate that iterator as appropriate, and then construct a new string using .collect().

For example:

let old = "abcdef";

let new = old.chars()
    // note: there's an edge case if ch == char::MAX which we must decide
    //       how to handle. in this case I chose to not change the
    //       character, but this may be different from what you need.
    .map(|ch| {
        if somecondition {
            char::from_u32(ch as u32 + 1).unwrap_or(ch)
        } else {
            ch
        }
    })
    .collect::<String>();