为什么Random()不是随机的? [英] Why isn't Random() random?

问题描述

可能重复:
为什么此随机数生成器不是随机的?

我有这个测试程序:

static void Main(string[] args)
{
    var randomNumbers = new Dictionary<int, int>();
    foreach (var s in Enumerable.Range(1, 500))
    {
        var rand = Rand5();
        if (!randomNumbers.ContainsKey(rand))
            randomNumbers.Add(rand, 1);
        else
            randomNumbers[rand] += 1;
    }

    randomNumbers
        .ToList()
        .ForEach(x => Console.WriteLine("{0}: {1}", x.Key, x.Value));
    Console.ReadLine();
}

static int Rand5()
{
    System.Threading.Thread.Sleep(1);
    return new Random().Next(1, 6);
}

如果我将System.Threading.Thread.Sleep(1);注释掉，我将得到

5: 500

但是，如果我取消注释该行，则会得到随机数.

2: 87
4: 94
1: 116
5: 108
3: 95

为什么代码行很重要?谢谢！

解决方案

正如其他人所说，new Random()从当前系统时间播种随机数生成器.

我有一篇文章对此进行了更详细的描述，包括解决问题的方法，您可能会发现这很有用.基本上，您想多次使用相同的Random实例-但要注意，它不是线程安全的.

Possible Duplicate:
Why does this Random Number Generator not random?

I have this test program:

static void Main(string[] args)
{
    var randomNumbers = new Dictionary<int, int>();
    foreach (var s in Enumerable.Range(1, 500))
    {
        var rand = Rand5();
        if (!randomNumbers.ContainsKey(rand))
            randomNumbers.Add(rand, 1);
        else
            randomNumbers[rand] += 1;
    }

    randomNumbers
        .ToList()
        .ForEach(x => Console.WriteLine("{0}: {1}", x.Key, x.Value));
    Console.ReadLine();
}

static int Rand5()
{
    System.Threading.Thread.Sleep(1);
    return new Random().Next(1, 6);
}

If I comment out System.Threading.Thread.Sleep(1);, I get

5: 500

But if I uncomment that line, I do get random numbers.

2: 87
4: 94
1: 116
5: 108
3: 95

Why does the line of code matter? Thanks!

解决方案

As others have said, new Random() seeds the random number generator from the current system time.

I have an article describing this in more detail, including solutions to the problem, which you may find useful. Basically you want to use the same instance of Random multiple times - but observing that it's not thread-safe.

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