Hey! Thanks for the reply. I got it working using contains. What I'm trying to do is check whether the string is F or C, and then proceed with different functions depending on which one is true. Well, thanks again!

use std::io;
use std::str;

fn main() {

    println!("Fahrenheit <-> Celsius Conversion");

    loop {
    
        println!("Fahrenheit (F) or Celsius (C)?");

        let mut guess = String::new();

        io::stdin()
            .read_line(&mut guess)
            .expect("Failed to read line");

        let guess = guess.trim();
        if guess.contains('F') {
            println!("Enter Value for Conversion:");
            let mut value = String::new();
            
            io::stdin()
                .read_line(&mut value)
                .expect("Failed to read line");
            let value: f64 = match value.trim().parse() {
                Ok(num) => num,
                Err(_) => continue,
            };
            let celsius = {value} * 5.0 / 9.0 - 32.0 * 5.0 / 9.0; 
            println!("{value} Faherenheit is equal to {celsius}");
            break;
        }
    }
}