How to get i-th character from String in Rust

December 27, 2020 2 minute read

Intro

fn main() {
  let string = String::from("127.0.0.1:8080"); 
  let string_slice = &string[10..]; 
  let string_borrow: &str = &string; 
  let string_literal = "1234"; 

  dbg!(&string); 
  dbg!(string_slice); 
  dbg!(string_borrow); 
  dbg!(string_literal);
}

This code shows this result.

[src/main.rs:11] &string = "127.0.0.1:8080"
[src/main.rs:12] string_slice = "8080"
[src/main.rs:13] string_borrow = "127.0.0.1:8080"
[src/main.rs:14] string_literal = "1234"

Accessing the characters by using the direct index is dangerous since we have no idea how much space is required to store string data. We can say string without what the type of a string will be. The string data is quite complex to store machine-friendly way. All characters are translated into the digital way; numbers. Fundamentally, it should be represented by combining multiple zeros and ones.

Encoding

How to do that? With a code map, we can call it encoding scheme; ASCII, UTF-8, and Unicode. In the table of those encoding system, we can find some examples of characters. Common English letters have the same value. Since the encoding map is growing with developing computer memory, the successor should keep the rule. Here’s an interesting post about encoding[4]; ASCII, Unicode, UTF-8, and UTF-16.

In Rust

fn main() {
    let eng = "f";
    let emoji = "🔥";
    println!("{:?} {}", eng, emoji);
}

Let me use the debugger to see what’s going on. result

The debugger tells us about how to store string type data in memory. We need to catch each digit number index (a direct index) represent 1 byte. The “emoji” variable shows how to store the fire emoji. The first index is 0. In the 0, ‘\xf0’ is stored. It is a hexadecimal expression to show the raw binary value(11110000). The length is 8. We need 8 bits to store the value(11110000). For the fire emoji, if we try to access it directly, the Rust compiler shows an error. How to do it in Rust? result

Rust access n-th character

As suggested in the error message, we are going to call chars().nth() method and also bytes().nth().

fn main() {
    let eng = "f";
    let emoji = "🔥";
    println!("{:?} {}", eng, emoji.chars().nth(0).unwrap());

    println!("{:?} {}", eng, emoji.bytes().nth(0).unwrap());
}

"f" 🔥
"f" 240

The ‘chars’ and ‘nth’ is not far away from the main idea. But what ‘unwrap’ mean? Rust has no null value. To handle this, Rust introduces ‘Some’ type that can represent an absence of value. The ‘unwrap’ consuming the value in it or panic. So, its use is generally discouraged. [7]

Here is the code to print the ‘Some’ type.

fn main() {
    let eng = "f";
    let emoji = "🔥";
    println!("{:?} {:?}", eng, emoji.chars().nth(0));

    println!("{:?} {:?}", eng, emoji.bytes().nth(0));
}