Fahrenheit and Celsius converter in Rust

Question

I made a program that converts between Celsius and Fahrenheit. Is there any way that I could make this code more efficient and cleaner?

use std::io;

// C to F: F = C*(9/5) + 32
// F to C: C = (F-32)*(5/9)

/**********Converts between Fahrenheit and Celsius*********/

fn main() -> () {
    println!("Do you want to convert to Celsius or Fahrenheit? Input C or F");
    let mut convert_type = String::new();

    io::stdin().read_line(&mut convert_type)
        .expect("Failed to conversion type.");

    let t = String::from(convert_type);

    println!("You want to convert to: {}", t);
    println!("What temperature would you like to convert?");
    let mut temp = String::new();

    io::stdin().read_line(&mut temp)
        .expect("Failed to read temperature.");

    let temp: i32 = match temp.trim().parse() {
        Ok(temp) => temp,
        Err(_e) => {
            -1
        }
    };

    match t.as_str() {
        "C\n" => println!("{}", ftoc(temp)),
        "F\n" => println!("{}", ctof(temp)),
        _ => println!("t = {:?}", t),
}
}

// Celsius to Fahrenheit
fn ctof(c: i32) -> i32 {
    (c * (9 / 5)) + 32

}

//Fahrenheit to Celsius
fn ftoc(f: i32) -> i32 {
    (f-32) * (5 / 9)
}

Output:

    cargo run
   Compiling ftoc v0.1.0 (/Users/roberthayek/rustprojects/ftoc)
    Finished dev [unoptimized + debuginfo] target(s) in 2.64s
     Running `target/debug/ftoc`
Do you want to convert to Celsius or Fahrenheit? Input C or F
F
You want to convert to: F

What temperature would you like to convert?
0
32

Answer 1

The first thing I always do is running clippy.

You will catch some things that are not neccessary, e.g.

• fn main() -> () can be reduced to fn main()
• let t = String::from(convert_type); is simply let t = convert_type

The bad things are

• c * (9 / 5) which is always c because of integer arithmetic. You probably want f64::from(c) * (9.0 / 5.0)
• same for (f - 32) * (5 / 9) should be (f64::from(f) - 32.0) * (5.0 / 9.0)) as i32

You may want to add some unit-tests to your program to verify that ctof and ftoc actually work.

Answer 2

Hellow got most of the good parts, but, I'd also recommend figuring out how you might want to signal the user so that an invalid input isn't accepted, since -1 is a temperature someone might want to convert.

In this case I would do something like replacing

let temp: i32 = match temp.trim().parse() {
    Ok(temp) => temp,
    Err(_e) => {
        -1
    }
};

with

let temp= match temp.trim().parse() {
    Err(_e) =>{
        panic!("That wasn't valid input! Temperatures can only be integers!");
    }
    Ok(i)=>i
};

If I knew this program were to be used solely interactively, then I'd consider adding a loop to give the user another attempt should their input fail.