サンプルコードで身につける: Option<T> — null の代わり

fn main() {
    let found: Option<i32> = Some(7);
    match found {
        Some(n) => println!("値あり: {}", n),
        None => println!("値なし"),
    }

    let missing: Option<i32> = None;
    match missing {
        Some(n) => println!("値あり: {}", n),
        None => println!("値なし"),
    }
}

fn safe_divide(a: i32, b: i32) -> Option<i32> {
    if b == 0 {
        None // 結果が存在しないことを値で表す
    } else {
        Some(a / b)
    }
}

fn main() {
    match safe_divide(10, 2) {
        Some(n) => println!("10 / 2 = {}", n),
        None => println!("0 では割れません"),
    }
    match safe_divide(10, 0) {
        Some(n) => println!("10 / 0 = {}", n),
        None => println!("0 では割れません"),
    }
}

fn main() {
    let numbers = vec![10, 20, 30];

    match numbers.get(1) {
        Some(n) => println!("添字1の値: {}", n),
        None => println!("添字1は範囲外"),
    }

    match numbers.get(5) {
        Some(n) => println!("添字5の値: {}", n),
        None => println!("添字5は範囲外でも panic しない"),
    }
}

use std::collections::HashMap;

fn main() {
    let mut stock = HashMap::new();
    stock.insert(String::from("apple"), 3);
    stock.insert(String::from("banana"), 5);

    match stock.get("apple") {
        Some(count) => println!("apple の在庫: {}", count),
        None => println!("apple は未登録"),
    }
    match stock.get("melon") {
        Some(count) => println!("melon の在庫: {}", count),
        None => println!("melon は未登録"),
    }
}

struct User {
    name: String,
    nickname: Option<String>, // 未設定かもしれない項目
}

fn display_name(user: &User) -> String {
    if let Some(nick) = &user.nickname {
        format!("{} ({})", nick, user.name)
    } else {
        user.name.clone() // なければ本名で代用
    }
}

fn main() {
    let a = User { name: String::from("田中"), nickname: Some(String::from("タロー")) };
    let b = User { name: String::from("鈴木"), nickname: None };
    println!("{}", display_name(&a));
    println!("{}", display_name(&b));
}