Compare structs for ordering (sorting) - Ord

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• In order to be able to decide which object is "bigger" or "smaller" than the other one we need the Ord trait that requires PartialOrd trait and the Eq and PartialEq traits.
• This will allow use to sort the values.
• Comaring the fields happen in the order the appear in the definition of ths struct. In our case Rust forst compares the 'number' fields. The 'name' fields are only compared if the 'number' fields are equal.

examples/struct/compare-structs-for-ordering/src/main.rs

#[derive(PartialEq, Eq, PartialOrd, Ord)]
struct Thing {
    number: i32,
    name: String,
}

fn main() {
    let a = Thing {
        number: 42,
        name: String::from("Foo"),
    };

    let b = Thing {
        number: 43,
        name: String::from("Foo"),
    };

    let c = Thing {
        number: 42,
        name: String::from("Bar"),
    };

    let d = Thing {
        number: 43,
        name: String::from("Bar"),
    };

    println!("{}", a < b); // becasue 42 < 43
    println!("{}", c < b); // becasue 42 < 43 ( Bar < Foo is not even checked here)
    println!("{}", c < a); // because Bar < Foo
    println!("{}", c < d); // because 42 < 43
    println!("{}", a < d); // because 42 < 43
                           // the comparisions is in the order of declaraion of the fields
                           // So the `name` field is not checked here
}

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