配列からランダムに1つ選ぶには

import random
 
fruits = ["apple", "banana", "cherry", "date"]
print(random.choice(fruits))        # 例: cherry
print(random.sample(fruits, k=2))   # 例: ['date', 'apple']（重複なしで 2 個）
print(random.choices(fruits, k=3))  # 例: ['banana', 'banana', 'apple']（重複を許して 3 個）
 
try:
    random.choice([])
except IndexError as e:
    print(type(e).__name__, e)      # IndexError Cannot choose from an empty sequence

function pick(items) {
  if (items.length === 0) {
    throw new RangeError("空の配列からは選べない");
  }
  return items[Math.floor(Math.random() * items.length)];
}
 
const fruits = ["apple", "banana", "cherry", "date"];
console.log(pick(fruits)); // 例: cherry
 
// ガードが無いと、空の配列でもエラーにならず undefined が返る
const empty = [];
console.log(empty[Math.floor(Math.random() * empty.length)]); // undefined

// 空の配列では選べないので、戻り値の型に undefined を含める
function pick<T>(items: readonly T[]): T | undefined {
  return items[Math.floor(Math.random() * items.length)];
}
 
const fruits: string[] = ["apple", "banana", "cherry", "date"];
const fruit = pick(fruits); // 型は string | undefined
console.log(fruit?.toUpperCase()); // 例: CHERRY
 
console.log(pick([])); // undefined

package main
 
import (
	"fmt"
	"math/rand/v2"
)
 
func main() {
	fruits := []string{"apple", "banana", "cherry", "date"}
	fmt.Println(fruits[rand.IntN(len(fruits))]) // 例: cherry
 
	var empty []string
	defer func() {
		fmt.Println("recovered:", recover()) // recovered: invalid argument to IntN
	}()
	fmt.Println(empty[rand.IntN(len(empty))]) // IntN(0) で panic する
}

// Cargo.toml: rand = "0.10"
use rand::seq::IndexedRandom;
 
fn main() {
    let fruits = vec!["apple", "banana", "cherry", "date"];
    let mut rng = rand::rng();
 
    println!("{:?}", fruits.choose(&mut rng)); // 例: Some("cherry")
 
    let empty: Vec<&str> = Vec::new();
    println!("{:?}", empty.choose(&mut rng)); // None（空でも panic しない）
}

import java.util.List;
import java.util.concurrent.ThreadLocalRandom;
 
public class Main {
    public static void main(String[] args) {
        List<String> fruits = List.of("apple", "banana", "cherry", "date");
        String fruit = fruits.get(ThreadLocalRandom.current().nextInt(fruits.size()));
        System.out.println(fruit); // 例: cherry
 
        try {
            List<String> empty = List.of();
            empty.get(ThreadLocalRandom.current().nextInt(empty.size()));
        } catch (IllegalArgumentException e) {
            System.out.println(e.getMessage()); // bound must be positive
        }
    }
}

using System;
 
var fruits = new[] { "apple", "banana", "cherry", "date" };
Console.WriteLine(fruits[Random.Shared.Next(fruits.Length)]); // 例: cherry
 
// .NET 8 以降。重複を許して 2 個選ぶ
Console.WriteLine(string.Join(", ", Random.Shared.GetItems(fruits, 2))); // 例: date, date
 
var empty = Array.Empty<string>();
try
{
    Console.WriteLine(empty[Random.Shared.Next(empty.Length)]);
}
catch (IndexOutOfRangeException e)
{
    // Next(0) はエラーにならず 0 を返すので、添字の範囲外で初めて失敗する
    Console.WriteLine(e.GetType().Name); // IndexOutOfRangeException
}