Why is Rust program not returning values from if statements? [duplicate]

Question

I am making a simple console program that converts between Celsius and Fahrenheit. The program takes user input multiple times, once for the type of conversion and once for the value to convert. When I run the program it compiles and runs without error, however, it does not actually return any value.

Here is the program:

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,
    };

    if &t == "C" {
        println!("{}", ctof(temp));
    } else if &t == "F" {
        println!("{}", ftoc(temp));
    }
}

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

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

Here is a snippet of the console, as you can see it does not output the conversion:

cargo run --verbose
   Compiling ftoc v0.1.0 (/Users/roberthayek/rustprojects/ftoc)
     Running `rustc --crate-name ftoc src/main.rs --color always --crate-type bin --emit=dep-info,link -C debuginfo=2 -C metadata=8f02d379c2e5c97d -C extra-filename=-8f02d379c2e5c97d --out-dir /Users/roberthayek/rustprojects/ftoc/target/debug/deps -C incremental=/Users/roberthayek/rustprojects/ftoc/target/debug/incremental -L dependency=/Users/roberthayek/rustprojects/ftoc/target/debug/deps`
    Finished dev [unoptimized + debuginfo] target(s) in 1.16s
     Running `target/debug/ftoc`
Do you want to convert to Celsius or Fahrenheit? Input C or F
C
You want to convert to: C

What temperature would you like to convert?
0

Have you tried printing your value to see what it actually is? Like, printl!("{?}", t); ? — behnam
– behnam, Commented Nov 9, 2018 at 3:10
Please provide a MCVE in the future. You don't need to post your full program, but instead only the snippet that is interesting together will all the required functions and types. See the rust-tag for more information about rust specific mcves, e.g. using the playground — hellow
– hellow, Commented Nov 9, 2018 at 7:22

Benjamin Lindley · Accepted Answer · 2018-11-09 07:40:27Z

12

You should make a habit of handling all cases, even ones you don't expect. If you had done so, you would have found the problem. So instead of this:

if &t == "C" {
    println!("{}", ctof(temp));
} else if &t == "F" {
    println!("{}", ftoc(temp));
}

You could write this (you could also use a final else branch with no if, but match is much more appealing):

match t.as_str() {
    "C" => println!("{}", ctof(temp)),
    "F" => println!("{}", ftoc(temp)),
    _ => println!("please enter C or F"),
}

And when you ran your program, you would see that t seemed to be equal to neither "C" nor "F". That would hopefully lead you to check the value of t, by doing a debug print.

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

At which point you would see that the value of t is not "C", but "C\n" or "C\r\n". And you would then realize that read_line was not stripping the newline from your string for you.

edited Nov 9, 2018 at 7:40

answered Nov 9, 2018 at 3:26

Benjamin Lindley

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4 Comments

Matthieu M. Over a year ago

I seem to remember that unreachable!() now accepts parameters, so you may be able to just use unreachable!("t = {:?}", t"),.

hellow Over a year ago

Yes it does. Look at the declaration at unreachable!()

Benjamin Lindley Over a year ago

Interesting. Though I kind of wrote this code unthinkingly and just realized that unreachable() is entirely inappropriate in this situation where t comes from user input.

Stargateur Over a year ago

Indeed reach an unreachable code is a bug (safe in this case but still a bug).

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