Comparing numbers

Parse the guess into a number and compare with Ordering.

Converting to a number

parse is used to turn text into a number; the target type must be given. The result is a Result because texts like "abc" can't be converted:

let tahmin: u32 = tahmin.trim().parse().expect("Bir sayı yaz!");

Here the old tahmin (a String) is shadowed by a new u32 with the same name; there's no need to invent another name.

cmp and Ordering

a.cmp(&b) returns one of three values of the type std::cmp::Ordering: Less, Greater, Equal. match forces you to handle all three:

use std::cmp::Ordering;

match tahmin.cmp(&gizli) {
    Ordering::Less => println!("Çok küçük"),
    Ordering::Greater => println!("Çok büyük"),
    Ordering::Equal => println!("Bildin!"),
}

If you forget an arm, the compiler gives a "non-exhaustive patterns" error.

Exercise First comparison

Convert the text "30" with parse::<u32>() (unwrap) and compare it with let gizli: u32 = 42; using cmp. Print 30: Çok küçük for Less, Çok büyük for Greater and Bildin! for Equal.

Show example solution

use std::cmp::Ordering;

fn main() {
    let gizli: u32 = 42;
    let tahmin: u32 = "30".parse().unwrap();
    match tahmin.cmp(&gizli) {
        Ordering::Less => println!("{}: Çok küçük", tahmin),
        Ordering::Greater => println!("{}: Çok büyük", tahmin),
        Ordering::Equal => println!("{}: Bildin!", tahmin),
    }
}

Test yourself

What does <code>"-5".parse::<u32>()</code> return?
- Err(...)
- Ok(5)
- Ok(-5)
- Panic
Show answer

Err(...) — u32 is unsigned; the minus sign is an invalid digit and Err is returned.

Compare

match tahmin.____(&gizli) {
    Ordering::Less => println!("Çok küçük"),
    Ordering::Greater => println!("Çok büyük"),
    Ordering::Equal => println!("Bildin!"),
}

Show answer

match tahmin.cmp(&gizli) {
    Ordering::Less => println!("Çok küçük"),
    Ordering::Greater => println!("Çok büyük"),
    Ordering::Equal => println!("Bildin!"),
}

cmp result

3.cmp(&5)

Less
5.cmp(&3)

Greater
5.cmp(&5)

Equal
"x".parse::<u32>()

Err

Next lesson: Random number

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