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rand() 生成相同的数字 –即使在我的主程序中使用 srand(time(NULL)) ！

发布于 2024-09-05 02:50:30 字数 939 浏览 7 评论 0原文

所以，我试图创建一个随机向量（想想几何，而不是可扩展数组），每次调用随机向量函数时，我都会得到相同的 x 值，尽管 y 和 z 不同。

int main () {
    srand ( (unsigned)time(NULL));
    Vector<double> a;
    a.randvec();
    cout << a << endl;
    return 0;
}

使用函数

//random Vector
template <class T>
void Vector<T>::randvec()
{
    const int min=-10, max=10;
    int randx, randy, randz;

    const int bucket_size = RAND_MAX/(max-min);

    do randx = (rand()/bucket_size)+min;
    while (randx <= min && randx >= max);
    x = randx;

    do randy = (rand()/bucket_size)+min;
    while (randy <= min && randy >= max);
    y = randy;

    do randz = (rand()/bucket_size)+min;
    while (randz <= min && randz >= max);
    z = randz;
}

出于某种原因，randx 将始终返回 8，而其他数字似乎完全遵循（伪）随机性。但是，如果我在 randx 之前调用定义（例如 randy），randy 将始终返回 8。

为什么我的第一个随机数始终是 8？难道是我播种方式不对？

原文

So, I'm trying to create a random vector (think geometry, not an expandable array), and every time I call my random vector function I get the same x value, though y and z are different.

int main () {
    srand ( (unsigned)time(NULL));
    Vector<double> a;
    a.randvec();
    cout << a << endl;
    return 0;
}

using the function

//random Vector
template <class T>
void Vector<T>::randvec()
{
    const int min=-10, max=10;
    int randx, randy, randz;

    const int bucket_size = RAND_MAX/(max-min);

    do randx = (rand()/bucket_size)+min;
    while (randx <= min && randx >= max);
    x = randx;

    do randy = (rand()/bucket_size)+min;
    while (randy <= min && randy >= max);
    y = randy;

    do randz = (rand()/bucket_size)+min;
    while (randz <= min && randz >= max);
    z = randz;
}

For some reason, randx will consistently return 8, whereas the other numbers seem to be following the (pseudo) randomness perfectly. However, if I put the call to define, say, randy before randx, randy will always return 8.

Why is my first random number always 8? Am I seeding incorrectly?

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评论（8）

拿命拼未来 2024-09-12 02:50:31

我也有同样的问题。我通过移动 srand() 调用来修复它，因此它只在我的程序中调用一次（之前我一直在函数调用的顶部播种它）。
不太明白技术细节 - 但问题已经解决了。

I had the same problem exactly. I fixed it by moving the srand() call so it was only called once in my program (previously I had been seeding it at the top of a function call).
Don't really understand the technicalities - but it was problem solved.

回复 原文
笛声青案梦长安 2024-09-12 02:50:31

另外值得一提的是，您甚至可以摆脱那个奇怪的 bucket_size 变量，并使用以下方法生成从 a 到 b 的数字包含：

srand ((unsigned)time(NULL));

const int a = -1;
const int b = 1;

int x = rand() % ((b - a) + 1) + a;
int y = rand() % ((b - a) + 1) + a;
int z = rand() % ((b - a) + 1) + a;

Also to mention, you can even get rid of that strange bucket_size variable and use the following method to generate numbers from a to b inclusively:

srand ((unsigned)time(NULL));

const int a = -1;
const int b = 1;

int x = rand() % ((b - a) + 1) + a;
int y = rand() % ((b - a) + 1) + a;
int z = rand() % ((b - a) + 1) + a;
回复 原文
不一样的天空 2024-09-12 02:50:31

一个简单的快速修复方法是在播种后调用 rand 几次。

int main ()
{
    srand ( (unsigned)time(NULL));
    rand(); rand(); rand();

    Vector<double> a;
    a.randvec();
    cout << a << endl;
    return 0;
}

为了更好地解释，在测试程序的四次连续运行中第一次调用 rand() 给出了以下输出：

27592
27595
27598
27602

注意它们有多相似？例如，如果将 rand() 除以 100，您将连续 3 次得到相同的数字。现在看一下 rand() 在四次连续运行中的第二个结果：

11520
22268
248
10997

这看起来好多了，不是吗？我真的看不出有什么理由投反对票。

A simple quickfix is to call rand a few times after seeding.

int main ()
{
    srand ( (unsigned)time(NULL));
    rand(); rand(); rand();

    Vector<double> a;
    a.randvec();
    cout << a << endl;
    return 0;
}

Just to explain better, the first call to rand() in four sequential runs of a test program gave the following output:

27592
27595
27598
27602

Notice how similar they are? For example, if you divide rand() by 100, you will get the same number 3 times in a row. Now take a look at the second result of rand() in four sequential runs:

11520
22268
248
10997

This looks much better, doesn't it? I really don't see any reason for the downvotes.

回复 原文
埖埖迣鎅 2024-09-12 02:50:31

您的实现通过整数除法忽略随机数的最小 4-5 位。由于您的 RNG 是用系统时间作为种子的，因此您从中获得的第一个值只会（平均）每 20 秒改变一次。

这应该有效：

randx = (min) + (int) ((max - min) * rand() / (RAND_MAX + 1.0));

[0, 1) 中的随机双精度值在哪里

rand() / (RAND_MAX + 1.0)

，其余的只是将其移动。

Your implementation, through integer division, ignores the smallest 4-5 bit of the random number. Since your RNG is seeded with the system time, the first value you get out of it will change only (on average) every 20 seconds.

This should work:

randx = (min) + (int) ((max - min) * rand() / (RAND_MAX + 1.0));

where

rand() / (RAND_MAX + 1.0)

is a random double value in [0, 1) and the rest is just shifting it around.

回复 原文
耀眼的星火 2024-09-12 02:50:31

与这个问题中的代码没有直接关系，但我在使用时遇到了同样的问题
srand ((unsigned)time(NULL)) 并且仍然具有从以下对 rand() 的调用返回的值的相同序列。

事实证明， srand 需要分别调用您正在使用它的每个线程。我有一个正在生成随机内容的加载线程（由于种子问题，这不是随机的）。我只是在主线程中使用 srand 而不是加载线程。因此添加另一个 srand ((unsigned)time(NULL)) 来开始加载线程解决了这个问题。

Not directly related to the code in this question, but I had same issue with using
srand ((unsigned)time(NULL)) and still having same sequence of values being returned from following calls to rand().

It turned out that srand needs to called on each thread you are using it on separately. I had a loading thread that was generating random content (that wasn't random cuz of the seed issue). I had just using srand in the main thread and not the loading thread. So added another srand ((unsigned)time(NULL)) to start of loading thread fixed this issue.

回复 原文
很酷不放纵 2024-09-12 02:50:31

播种后，为什么第一次调用 rand 总是生成相同的数字？

需要调查三个潜在原因。一个或多个可能是促成因素。

  1. rand 的糟糕实现
  2. 种子的糟糕选择
  3. 将 rand 生成的值映射到所需输出范围的问题

自从 OP 编写以来的 14 年里发生了很多变化2010年，当时C++11还处于草案阶段，还没有被正式采用。单独的随机数引擎和随机数分布的概念尚未普及。

OP 问道，“[为什么] rand() 生成相同的数字 - 即使在我的 main 中使用 srand(time(NULL)) ？”他的意思是，“为什么在函数 randvec 中编码的随机数分布总是在播种 rand 后的第一次调用时生成相同的数字>？”正如下面的数据所示，只要给它不同的种子，rand，它本身确实生成至少有一点不同的数字。

1.rand有问题吗？

在评论中，OP @Nick Sweet 透露他在 2010 年使用的是 Xcode。交叉检查日期，这意味着他的系统运行的是 OS X。 根据 Wikipedia，Apple 当时对 rand 的实现是 CarbonLib。它使用了 Park 和 Miller 于 1988 年以 MINSTD 形式发布的乘法同余生成器 (MCG)。可以在标头 中找到相同的生成器，如 std::minstd_rand0。

在 20 世纪 80 年代，当微处理器仍在使用 16 位字并且时钟速率以兆赫为单位时，MINSTD 占有一席之地。如今，它已经失宠了。除其他外，许多人认为 2^31 的周期短得令人无法接受。

是 MINSTD 的问题吗？为了找到答案，我测试了五个随机数引擎：

  • std::minstd_rand0 – MINSTD (Park & Miller, 1988)
  • std::minstd_rand – MINSTD (Park & Miller) , 1993)
  • tbx::knuth_lcg – pcg32 使用的 64 位 LCG（线性同余生成器）
  • tbx::pcg32_engine – 32 位 PCG （置换同余生成器）由 ME O'Neill 设计。它实现了 O'Neill 的 pcg32_oneseq。
  • std::mt19937 – 32 位 Mersenne Twister 引擎

tbx::pcg_engine 的源代码太长，无法包含在此处。它改编自ME O'Neill 发布的程序。

然而，tbx::knuth_lcg 的源代码非常短。由于其周期为 2^64 以及精心选择的参数，tbx::knuth.lcg 优于 MINSTD。

#pragma once
// tbx.random.knuth_lcg.h
#include <cstdint>
#include <random>
namespace tbx
{
    // Donald E. Knuth
    // The Art of Computer Programming, Volume 2
    // Second Edition, 1981
    // 
    // Section 3.3.4 Table 1
    // Page 102: Table 1 lists LCG constants for a variety of generators.
    // Page 105: "Line 30 (whose excellent multiplier 6364136223846793005 
    // is too big to fit in the column) is due to C.E. Haynes." 
    // 
    // Knuth's book gives no mention of the increment 1442695040888963407. 
    // 
    // Melissa E. O'Neill describes this as "Knuth's preferred 64-bit LCG."
    // https://www.pcg-random.org/posts/cpp-seeding-surprises.html

    using knuth_lcg = std::linear_congruential_engine
        < std::uint64_t                 // result_type
        , 6'364'136'223'846'793'005ull  // a = multiplier
        , 1'442'695'040'888'963'407ull  // c = increment
        , 0ull                          // m = modulus (true modulus is 2^64)
        >;
}
// end file: tbx.random.knuth_lcg.h

2. std::time(NULL) 有多糟糕？

在许多系统上，std::time(NULL) 返回的时间值具有一秒的分辨率。当快速运行的程序多次调用 srand(time(NULL)) 时，很有可能所有都将使用相同的种子！

如果用作种子的时间值不相同，它们可能非常相似，可能仅相隔几秒。在这种情况下，如果 PRNG 不以某种方式条件种子来进一步随机化它，那么 PRNG 的初始输出可能是相似的。将发电机循环“少量”次数可以减轻这种播种的影响。

是种子的问题吗？为了找到答案，每个随机数引擎都使用两种播种方法进行了测试。测试以一秒的间隔运行。

  • std::srand(std::time(NULL))
  • std::srand(std::random_device{}());

std::random_device< /code> 是 标头的一部分。它生成 32 位无符号值，旨在用作种子。根据编译器的不同，它生成的值可以是在硬件中生成的真正随机值，也可以是通常由加密安全伪随机数生成器 (CSPRNG) 生成的高质量伪随机值。

注意：std::random_device 也有自己的一系列问题。特别是，9.2 版本之前的 GCC MinGW 发行版存在错误。这是报告。这些版本使用 std::mt19937 和固定种子，并且 std::random_device 每次都会生成相同的序列。

3. 随机数分布有问题吗？

2010 年的许多评论者没有意识到函数 randvec 实现了拒绝采样算法，以生成均匀分布的随机值。他们中令人惊讶的数量建议放弃它使用的循环，转而支持由以下方式产生的有偏见的值：

inline int biased_rand(int const a, int const b) {
    // Genenerate a random value on the closed interval [a, b].
    return a + std::rand() % (b - a + 1);  // BIASED! - not uniformly distributed
}

很难责怪他们。 OP 中的拒绝测试编码错误，使用的是 &&，而不是 ||。难怪人们看不到发生了什么。

根据 OP，函数 randvec 改编自 Andrew Koenig & 出版于 Accelerated C++ (2000) 中的算法。芭芭拉·穆 (Barbara E. Moo)该算法在该书的第 7 章中以函数 nrand 的形式实现。与上面的偏置技术相比，它使用商而不是余数将 rand 返回的值映射到输出范围。因此，它对来自 rand 的高阶位敏感，而不是低阶位。当 rand 中的两个值共享相同的高位时，nrand 返回的值通常会相同。相反，当高位发生变化时，nrand 返回的值也会发生变化。它对低位的变化不太敏感。

以下是 Accelerated C++ 的源代码：

//======================================================================
// nrand
// return a random integer in the range [0, n)
// Source: Andrew Koenig & Barbara E. Moo – 'Accelerated C++' (2000)
//         Section 7.4.4 Selecting a random element
// Algorithm: Use "rejection sampling" to guarantee that generated 
//         values are uniformly distributed.
//======================================================================
int nrand(int n)
{
    if (n <= 0 || n > RAND_MAX)
        throw domain_error("Argument to nrand is out of range");
    const int bucket_size = RAND_MAX / n;
    int r;
    do r = rand() / bucket_size;
    while (r >= n);
    return r;
}

nrand 的工作原理是将输入范围 [0, RAND_MAX] 划分为 n 个大小相等的“桶”，编号为 0 到 n-1。每个存储桶都包含与输入范围不同的 bucket_size 值。只允许装满桶。如果 [0, RAND_MAX] 中存在未分配给任何存储桶的剩余值，则每当调用 rand 生成其中一个值时，它们就会被拒绝。

整数商rand()/bucket_size 产生范围[0, n] 上的结果。在程序中，商称为r，但您可以将其视为bucket_number。当r等于n时，它被拒绝，程序循环返回，再次调用rand。否则，r 是位于半开区间 [0, n) 上的“桶号”。

注意：虽然nrand中的算法是正确的，但它返回的值位于半开区间[0, n)上。当使用 n 等于 RAND_MAX 调用 nrand 时，它永远不会返回 RAND_MAX。请参阅下文，了解在 nrand 中扩展算法的方法，以便可以返回 RAND_MAX。

以下函数使用 nrand 在闭范围 [a, b] 上生成均匀分布的值。但请注意，您不能使用 a 等于 0 且 b 等于 RAND_MAX 来调用它。如果这样做，nrand 的参数将为 RAND_MAX + 1，并且会发生以下两种情况之一：

  • 当 RAND_MAX 小于 >INT_MAX、nrand 将抛出异常。
  • 否则，RAND_MAX + 1 的计算将溢出，并且将出现未定义的行为。
inline int uniform_rand(int const a, int const b) {
    // Generate a random value on the closed interval [a, b].
    return a + nrand(b - a + 1);  // values are uniformly distributed on [a, b]
}

给定这些函数，来自OP的函数randvec可以编码如下：

// random Vector - from the OP
template <class T>
void Vector<T>::randvec()
{
    const int a = -10, b = 10;
    x = uniform_rand(a, b);
    y = uniform_rand(a, b);
    z = uniform_rand(a, b);
}

nrand是randvec在之后的第一次调用失败的原因吗？播种？为了找到答案，我使用上面给出的两个随机数函数测试了 RNG 和种子的每种组合：

  • biased_rand(-10, 10)
  • uniform_rand(-10, 10)

这里是uniform_rand的改进版本，可以返回RAND_MAX。鉴于最好避免使用 std::rand ，这比其他任何事情都更令人好奇。但是，如果您确实选择使用 rand，则应考虑使用此函数。对于生产工作，您应该使用 std::uniform_int_distribution。

#pragma once
// tbx.random.uniform_rand.h
namespace tbx
{
    int uniform_rand(int const a, int const b);
}
// end file: tbx.random.uniform_rand.h

// tbx.random.uniform_rand.cpp
#include <cstdlib>    // rand, srand. RAND_MAX
#include <random>     // random_device
#include <stdexcept>  // runtime_error
#include "tbx.random.uniform_rand.h"

namespace tbx
{
    //==================================================================
    // uniform_rand
    // Return a uniformly distributed, pseudo-random integer on the 
    // closed interval [a, b]. This function uses "rejection sampling."
    // 
    // Precondition: (a <= b) && (b - a <= RAND_MAX)
    // 
    // It is implementation-defined whether `std::rand` is thread-safe. 
    // If `std::rand` is thread-safe, then `tbx::uniform_rand` will 
    // also be thread-safe, but not otherwise.
    //==================================================================
    int uniform_rand(int const a, int const b)
    {
        // Call `srand` on the first pass, and never again, thereafter.
        static thread_local bool const is_seeded{
            []() { std::srand(std::random_device{}()); return true; }()
        };
        if (a > b) {
            throw std::runtime_error("tbx::uniform_rand: a > b");
        }
        static_assert(sizeof(long long) > sizeof(int));
        auto const aa{ static_cast<long long>(a) };
        auto const bb{ static_cast<long long>(b) };
        auto const rand_max{ static_cast<long long>(RAND_MAX) };
        if (bb - aa > rand_max) {
            throw std::runtime_error("tbx::uniform_rand: b - a > RAND_MAX");
        }
        unsigned const desired_range{ 1u + static_cast<unsigned>(bb - aa) };
        unsigned const range_of_rand{ 1u + static_cast<unsigned>(RAND_MAX) };
        unsigned const bucket_size{ range_of_rand / desired_range };
        unsigned n;  // "bucket number"
        do {
            n = static_cast<unsigned>(std::rand()) / bucket_size;
        } while (n >= desired_range);
        return a + static_cast<int>(n);
    }
}
// end file: tbx.random.uniform_rand.cpp

那么，给我看看数据吧！

std::minstd_rand0

第一个表显示了 MINSTD 的输出。前五列包含种子 std::time(NULL) 的数据。最后四列将种子设置为 std::random_device{}()。

在第 1 列和第 2 列中，每行时间值增加 1 秒。那是因为我将线程编程为在行之间休眠 1 秒。

在第 3 列中，rand 返回的值非常相似。它们均以 366 开头，第四位数字为 3、4、5 或 6。对于 MINSTD，单独使用当前时间作为种子并不是一个好主意。

第 4 列显示了播种后第一次调用时 uniform_rand 返回的值。它永远不会改变。 RNG和seed的这个组合是失败的。

第 5 列显示 biased_rand 返回的值。注意第 5 列中的重复模式。这也是一个失败。

现在我们已经得到了 OP 中两个问题的答案。

为什么我的第一个随机数总是 8？我播种的方式不对吗？

答案是： (1) 当对 std::time(NULL) 的紧密调用用作 MINSTD 的种子时，其第一次调用返回的结果不是随机的。 （2）是的。请参阅 ME 的 简单可移植 C++ 种子熵奥尼尔提出了一些关于更好播种的想法。否则，只需使用 std::random_device{}()。

我未经检验的假设是加性常数（MINSTD 中为 0）是导致第 3-5 列失败的一个因素。相比之下，tbx::knuth_lcg 做得更好。

MINSTD 是一个乘法同余生成器 (MCG)。除了模数之外，MINSTD 中的状态转换函数只有一个参数：乘数。对于tbx::knuth_lcg，有两个：乘数和加法常数。这种组合意味着相似的种子在 tbx::knuth_lcg 的单个循环中比在 MINSTD 中分歧更多。

至少，这是假设。

接下来的四列展示了 MINSTD 是可用的，但前提是您有更好的种子。

第 6 列显示了 std::random_device 返回的种子。有趣的是，它们看起来很好而且很随意。第一次调用 rand 返回的值也是如此，如第 7 列所示。

第 8 列和第 9 列显示函数 uniform_rand 和 biased_rand< 生成的输出/代码>。有趣的是，这两列似乎都是随机的，但我们知道严格的测试会表明第 9 列中的值存在微小偏差。

minstd_rand0   a: -10   b: 10
                                 1st call                                    seed with     1st call
std::time(NULL)         seed      to rand   uniform_rand   biased_rand   random_device      to rand   uniform_rand   biased_rand
---------------   ----------   ----------   ------------   -----------   -------------   ----------   ------------   -----------
     1725470667   1725470667    366331181             -7            10        79934811   1287089102              2            -5
     1725470668   1725470668    366347988             -7            -4       523355155   2084555620             10            -3
     1725470670   1725470670    366381602             -7            10       814741099    997917621             -1             5
     1725470671   1725470671    366398409             -7            -4      3582022895    502236267             -6             5
     1725470672   1725470672    366415216             -7             3      3765154492   1050920895              0            -7
     1725470673   1725470673    366432023             -7            10      2560642594   1147791478              1            -9
     1725470674   1725470674    366448830             -7            -4      1060201997   1143144420              1             2
     1725470675   1725470675    366465637             -7             3      3896449311    109754712             -9             8
     1725470676   1725470676    366482444             -7            10       499904143    932904337             -1            -9
     1725470677   1725470677    366499251             -7            -4      2931344698   1687993459              6             9
     1725470678   1725470678    366516058             -7             3      2601193000   1926149021              8            10
     1725470679   1725470679    366532865             -7            10       415279526    281140732             -8             9
     1725470680   1725470680    366549672             -7            -4      1042354394   1826191379              7            -5
     1725470681   1725470681    366566479             -7             3      1405921226    555477441             -5             5
     1725470682   1725470682    366583286             -7            10      3238867965   1238403599              2            10
     1725470683   1725470683    366600093             -7            -4      4147558193    691368131             -4            10
     1725470684   1725470684    366616900             -7             3      1312675989   1045841492              0             7
     1725470685   1725470685    366633707             -7            10      4202573649   1918168913              8             7
     1725470686   1725470686    366650514             -7            -4        66238746    878074876             -2            -3
     1725470687   1725470687    366667321             -7             3      3397492534     66845208            -10            -7

std::minstd_rand 的数据（未显示）与上面的数据具有相同的失败（和成功）。

tbx::knuth_lcg

tbx::knuth_lcg 表现优于 MINSTD。当面对一系列相似的种子（第 2 列）时，它会生成看起来像是随机输出的内容（据说是随机输出）（第 3 列）。

然而，tbx::knuth_lcg 也有其自身的问题。肉眼看来，第 3 列中的值看起来是随机的，但第 4 列中的值引起了怀疑。重复次数不足。想想生日问题。

我们从 21 个总体中抽取了 20 个值，并且只有两个重复（0 和 -5）。第 8 列，播种效果更好，更令人满意。它有很多重复，甚至有几个三重。

在程序的多次运行（但不是全部）中，出现了相同的模式。尽管我只是使用“眼球”测试来评估数据，但这件事的发生足以让我担心。

当种子良好时，如第 6 列所示，tbx::knuth_lcg 会产生良好的结果。

knuth_lcg   a: -10   b: 10
                                              1st call                                    seed with                  1st call
std::time(NULL)         seed                   to rand   uniform_rand   biased_rand   random_device                   to rand   uniform_rand   biased_rand
---------------   ----------   -----------------------   ------------   -----------   -------------   -----------------------   ------------   -----------
     1725470729   1725470729       7682619102818709988              7            -9      2761315232       8143472074341520495              8             9
     1725470730   1725470730       4823383289810727185              0            -5      1940100256       8838089308847210863             10             0
     1725470731   1725470731       1964147476802744382             -6            -1      4177479439       1647040385966567410             -7            -3
     1725470732   1725470732       8328283700649537387              8           -10      4232132119       7189879011191188570              6             3
     1725470733   1725470733       5469047887641554584              2            -6      1506009362       1118517872796630905             -8             1
     1725470734   1725470734       2609812074633571781             -5            -2      1514642789       6095452229502519056              3            -5
     1725470735   1725470735       8973948298480364786             10            10      1095415460        119404177483429923            -10           -10
     1725470736   1725470736       6114712485472381983              3            -7       583154339       8072170026490244342              8             7
     1725470737   1725470737       3255476672464399180             -3            -3      3460774016       6240305398129356751              4            -9
     1725470738   1725470738        396240859456416377            -10             1      4039019375       1192875450421810386             -8           -10
     1725470739   1725470739       6760377083303209382              5            -8      3183640201       3021312392947218020             -4             4
     1725470740   1725470740       3901141270295226579             -2            -4      2846194354       6061817392193233049              3            -2
     1725470741   1725470741       1041905457287243776             -8             0        12515812       3252120412882307683             -3             0
     1725470742   1725470742       7406041681134036781              6            -9      4079972030       3154528753519180469             -3            -5
     1725470743   1725470743       4546805868126053978              0            -5      2809010452       8336560618007352787              8            -3
     1725470744   1725470744       1687570055118071175             -7            -1      2377801474        747086193202967209             -9            -6
     1725470745   1725470745       8051706278964864180              8           -10      4231172747       1009655311069690046             -8            10
     1725470746   1725470746       5192470465956881377              1            -6      2072539433       7046472142399192196              6            -2
     1725470747   1725470747       2333234652948898574             -5            -2      2237413946       8140891742275797889              8             3
     1725470748   1725470748       8697370876795691579              9            10       386168962       4228694408572382761             -1             9

std::mt19937 的Mersenne Twister

数据一致良好。

它在播种不良的情况下表现良好的原因可能是因为它在将种子安装到引擎之前调节您提供的种子。

当您使用单个（无符号）整数为 std::mt19937 提供种子时，它会将该种子推送到种子序列，以填充引擎使用的 624 个状态变量。从外观上看，第 2 列中的种子看起来都很相似。然而，当它们撞击引擎时，它们已经被显着地“随机化”了。

mt19937   a: -10   b: 10
                                 1st call                                    seed with     1st call
std::time(NULL)         seed      to rand   uniform_rand   biased_rand   random_device      to rand   uniform_rand   biased_rand
---------------   ----------   ----------   ------------   -----------   -------------   ----------   ------------   -----------
     1725470688   1725470688   1846392152              8             7      3488227490   1683360094              6             0
     1725470689   1725470689    923719378             -1            -9      4172931053   1019958242             -1            -8
     1725470690   1725470690   1919984915              8            -8      3443978328   1172527630              1            -9
     1725470691   1725470691   1612674401              5            -8      3542463677    926495713             -1             0
     1725470692   1725470692    339483995             -7             1      2534471012   1768656412              7             9
     1725470693   1725470693    881359337             -2            -5        38428959     47618872            -10            -3
     1725470694   1725470694   1481684138              4             1      3098883754    727394473             -3            -9
     1725470695   1725470695   1289557349              2             7      1460710575    351019927             -7             3
     1725470696   1725470696    593798350             -5             9       167206676   1070657304              0            -7
     1725470697   1725470697    695392031             -4            -5      1215347282    924400893             -1            -7
     1725470698   1725470698   1152322541              1            10      3860368725   1741906414              7             0
     1725470699   1725470699    111164029             -9            -6      2743553328    377480583             -7             8
     1725470700   1725470700   1998001782              9           -10      2268784299    827285329             -2             0
     1725470701   1725470701    814093545             -3            -4      1593534606    548682246             -5           -10
     1725470702   1725470702   1511789889              4             5       142519159   1539300695              5            -8
     1725470703   1725470703   1885533215              8            10      3120646034    798241165             -3            -9
     1725470704   1725470704     61821576            -10             2      2487963867    659747497             -4             0
     1725470705   1725470705   1211428588              1             3       531291314   1551904191              5             2
     1725470706   1725470706   1724047358              6            -5      1915141704    367916844             -7             5
     1725470707   1725470707   1375067752              3             6      1484660056   2086820278             10            -6

pcg_engine（也调节其种子）的结果类似。

结论

主要结论是 std::time(NULL) 是一个较差的随机种子源。当一个程序多次调用它时，它返回的种子很可能非常相似。在最坏的情况下（这并不罕见），种子将是相同的。

当 std::time(NULL) 返回的值不相同时，它们在用作 std::mt19937 和 pcg_engine 的种子时起作用。然而，他们的其他引擎却失败了。当然，当这些值相同时，所有引擎都会失败。

std::time(NULL) 仍然可以用作种子，但它应该与其他“熵”源结合起来。这可以通过将其与内存中某些对象的地址进行异或或调用 std::random_device （或两者）来完成。请参阅 简单可移植 C++ 种子熵，作者：我奥尼尔。

否则，只需使用 std::random_device 来生成种子。

尽管 std::rand 有着复杂的历史，但它可能不会像 std::time(NULL) 那样成为问题。当然，这取决于您的系统。即使使用低级 MINSTD 生成器实现 rand（如 OP 中所示），上述数据也表明正确的播种可以产生有用的结果。如果有选择，您应该更喜欢 std::mt19937 或 pcg32_engine 之类的东西。然而，对于轻量级应用程序，rand 可能就可以了。

至于函数nrand，随机数分布，这是一种垃圾输入，垃圾输出的情况。当您向其提供一系列相似的随机值（其中高位是恒定的）时，nrand 将会陷入困境。有趣的是，在 std::minstd_rand0 和 std::minstd_rand 的测试中，nrand 陷入困境的时刻也是 biased_rand 也有自己的问题。

After seeding, why does the first call to rand always generate the same number?

There are three potential causes to investigate. One or more may be contributing factors.

  1. A poor implementation of rand
  2. A poor choice of seeds
  3. Problems mapping the values generated by rand onto the desired output range

A lot has changed in the 14 years since the OP was written in 2010. Back then, C++11 was still in the draft stage, and <random> had not yet been officially adopted. The concept of separate random number engines and random number distributions had not yet taken hold.

The OP asks, "[Why is] rand() generating the same number – even with srand(time(NULL)) in my main?" What he means is, "Why does the random number distribution coded in function randvec always generate the same number, on the first call, after seeding rand?" As is shown in the data below, so long as it is given different seeds, rand, itself, does generate numbers that are, at least, a little bit different.

1. Are there problems with rand?

In the comments, the OP, @Nick Sweet, revealed that he was using Xcode in 2010. Cross-checking dates, that means his system was running OS X. According to Wikipedia, Apple's implementation of rand at that time was part of CarbonLib. It used the multiplicative congruential generator (MCG) published as MINSTD, in 1988, by Park and Miller. The same generator can be found in header <random>, as std::minstd_rand0.

In the 1980s, when microprocessors were still using 16-bit words, and clock rates were measured in megahertz, MINSTD had a place. Today, it has fallen out of favor. Among other things, its period of 2^31 is considered by many to be unacceptably short.

Is MINSTD the problem? To find out, I tested five random number engines:

  • std::minstd_rand0 – MINSTD (Park & Miller, 1988)
  • std::minstd_rand – MINSTD (Park & Miller, 1993)
  • tbx::knuth_lcg – 64-bit LCG (linear congruential generator) used by pcg32
  • tbx::pcg32_engine – 32-bit PCG (permuted congruential generator) designed by M.E. O'Neill. It implements O'Neill's pcg32_oneseq.
  • std::mt19937 – 32-bit Mersenne Twister engine

Source code for tbx::pcg_engine is too long to be included here. It was adapted from the program published by M.E. O'Neill.

Source code for tbx::knuth_lcg, however, is quite short. Due to its period of 2^64, and its well-chosen parameters, tbx::knuth.lcg is superior to MINSTD.

#pragma once
// tbx.random.knuth_lcg.h
#include <cstdint>
#include <random>
namespace tbx
{
    // Donald E. Knuth
    // The Art of Computer Programming, Volume 2
    // Second Edition, 1981
    // 
    // Section 3.3.4 Table 1
    // Page 102: Table 1 lists LCG constants for a variety of generators.
    // Page 105: "Line 30 (whose excellent multiplier 6364136223846793005 
    // is too big to fit in the column) is due to C.E. Haynes." 
    // 
    // Knuth's book gives no mention of the increment 1442695040888963407. 
    // 
    // Melissa E. O'Neill describes this as "Knuth's preferred 64-bit LCG."
    // https://www.pcg-random.org/posts/cpp-seeding-surprises.html

    using knuth_lcg = std::linear_congruential_engine
        < std::uint64_t                 // result_type
        , 6'364'136'223'846'793'005ull  // a = multiplier
        , 1'442'695'040'888'963'407ull  // c = increment
        , 0ull                          // m = modulus (true modulus is 2^64)
        >;
}
// end file: tbx.random.knuth_lcg.h

2. How bad is std::time(NULL)?

On many systems, the time values returned by std::time(NULL) have a one-second resolution. When a fast-running program makes several calls to srand(time(NULL)), there is a significant chance that all of them will use the same seed!

If the time values used as seeds are not identical, they may be very similar, perhaps separated by only a few seconds. In that case, if the PRNG does not condition the seed in some way, to further randomize it, then the initial outputs from the PRNG could be similar. Cycling the generator a "small" number of times can lessen the effect of such a seeding.

Is the seed the problem? To find out, each random number engine was tested with two seeding methods. Tests were run at one-second intervals.

  • std::srand(std::time(NULL))
  • std::srand(std::random_device{}());

std::random_device is part of the <random> header. It generates 32-bit unsigned values that are intended to be used as seeds. Depending on the compiler, the values it generates can be truly random values, generated in hardware, or high-quality, pseudorandom values, often generated by a cryptographically secure pseudorandom number generator (CSPRNG).

Note: std::random_device comes with its own set of issues. In particular, MinGW distributions of GCC, prior to version 9.2, were buggy. Here is the report. Those versions used std::mt19937 with a fixed seed, and std::random_device generated the same sequence, every time.

3. Are there problems with the random number distribution?

Many of the commenters from 2010 did not recognize that function randvec implements a rejection sampling algorithm, to generate uniformly distributed random values. A surprising number of them recommend ditching the loops that it uses, in favor of the biased values produced by:

inline int biased_rand(int const a, int const b) {
    // Genenerate a random value on the closed interval [a, b].
    return a + std::rand() % (b - a + 1);  // BIASED! - not uniformly distributed
}

It's hard to blame them. The rejection tests in the OP were miscoded, using &&, instead of ||. No wonder folks couldn't see what was happening.

According to the OP, function randvec was adapted from an algorithm published in Accelerated C++ (2000), by Andrew Koenig & Barbara E. Moo. The algorithm was implemented as function nrand, in Chapter 7 of that book. Compared to the biased technique above, it uses a quotient, rather than a remainder, to map the values returned by rand onto the output range. Because of this, it is sensitive to the high-order bits coming from rand, rather than the low-order bits. When two values from rand share the same high-order bits, the values returned by nrand will often be the same. Conversely, when the high-order bits vary, so will the values returned by nrand. It is less sensitive to changes in the low-order bits.

Here is the source code from Accelerated C++:

//======================================================================
// nrand
// return a random integer in the range [0, n)
// Source: Andrew Koenig & Barbara E. Moo – 'Accelerated C++' (2000)
//         Section 7.4.4 Selecting a random element
// Algorithm: Use "rejection sampling" to guarantee that generated 
//         values are uniformly distributed.
//======================================================================
int nrand(int n)
{
    if (n <= 0 || n > RAND_MAX)
        throw domain_error("Argument to nrand is out of range");
    const int bucket_size = RAND_MAX / n;
    int r;
    do r = rand() / bucket_size;
    while (r >= n);
    return r;
}

nrand works by dividing the input range [0, RAND_MAX] into n equally sized "buckets," numbered 0 to n-1. Each bucket contains bucket_size distinct values from the input range. Only full buckets are allowed. If there are leftover values from [0, RAND_MAX], which are not assigned to any bucket, they are rejected, whenever one of them is produced by a call to rand.

The integer quotient rand() / bucket_size produces a result on the range [0, n]. In the program, the quotient is called r, but you can think of it as bucket_number. When r equals n, it is rejected, and the program loops back, to call rand again. Otherwise, r is a "bucket number" that lies on the half-open interval [0, n).

Note: Although the algorithm in nrand is correct, the values it returns lie on the half-open interval [0, n). When nrand is invoked with n equals RAND_MAX, it can never return RAND_MAX. See below for a way to extend the algorithm in nrand, so that RAND_MAX can be returned.

The following function uses nrand to generate uniformly distributed values on the closed range [a, b]. Note, however, that you cannot call it with a equals 0, and b equals RAND_MAX. If you do, the argument to nrand will be RAND_MAX + 1, and one of two things will happen:

  • When RAND_MAX is less than INT_MAX, nrand will throw an exception.
  • Otherwise, the computation of RAND_MAX + 1 will overflow, and undefined behavior will follow.
inline int uniform_rand(int const a, int const b) {
    // Generate a random value on the closed interval [a, b].
    return a + nrand(b - a + 1);  // values are uniformly distributed on [a, b]
}

Given these functions, function randvec, from the OP, can be coded as follows:

// random Vector - from the OP
template <class T>
void Vector<T>::randvec()
{
    const int a = -10, b = 10;
    x = uniform_rand(a, b);
    y = uniform_rand(a, b);
    z = uniform_rand(a, b);
}

Is nrand the reason why randvec fails on the first call after seeding? To find out, I tested each combination of RNG and seed with the two random number functions given above:

  • biased_rand(-10, 10)
  • uniform_rand(-10, 10)

Here is an improved version of uniform_rand, which can return RAND_MAX. Given that std::rand is best avoided, it is more of a curiosity than anything else. If, however, you do choose to use rand, you should consider using this function. For production work, you should use std::uniform_int_distribution, instead.

#pragma once
// tbx.random.uniform_rand.h
namespace tbx
{
    int uniform_rand(int const a, int const b);
}
// end file: tbx.random.uniform_rand.h

// tbx.random.uniform_rand.cpp
#include <cstdlib>    // rand, srand. RAND_MAX
#include <random>     // random_device
#include <stdexcept>  // runtime_error
#include "tbx.random.uniform_rand.h"

namespace tbx
{
    //==================================================================
    // uniform_rand
    // Return a uniformly distributed, pseudo-random integer on the 
    // closed interval [a, b]. This function uses "rejection sampling."
    // 
    // Precondition: (a <= b) && (b - a <= RAND_MAX)
    // 
    // It is implementation-defined whether `std::rand` is thread-safe. 
    // If `std::rand` is thread-safe, then `tbx::uniform_rand` will 
    // also be thread-safe, but not otherwise.
    //==================================================================
    int uniform_rand(int const a, int const b)
    {
        // Call `srand` on the first pass, and never again, thereafter.
        static thread_local bool const is_seeded{
            []() { std::srand(std::random_device{}()); return true; }()
        };
        if (a > b) {
            throw std::runtime_error("tbx::uniform_rand: a > b");
        }
        static_assert(sizeof(long long) > sizeof(int));
        auto const aa{ static_cast<long long>(a) };
        auto const bb{ static_cast<long long>(b) };
        auto const rand_max{ static_cast<long long>(RAND_MAX) };
        if (bb - aa > rand_max) {
            throw std::runtime_error("tbx::uniform_rand: b - a > RAND_MAX");
        }
        unsigned const desired_range{ 1u + static_cast<unsigned>(bb - aa) };
        unsigned const range_of_rand{ 1u + static_cast<unsigned>(RAND_MAX) };
        unsigned const bucket_size{ range_of_rand / desired_range };
        unsigned n;  // "bucket number"
        do {
            n = static_cast<unsigned>(std::rand()) / bucket_size;
        } while (n >= desired_range);
        return a + static_cast<int>(n);
    }
}
// end file: tbx.random.uniform_rand.cpp

So, show me the data!

std::minstd_rand0

This first table shows the output from MINSTD. The first five columns contain data for the seed std::time(NULL). The final four columns, have the seed set to std::random_device{}().

In columns 1 and 2, the time values increase by 1 second for each row. That's because I programmed the thread to sleep for 1 second between rows.

In column 3, the values returned by rand are very similar. They all begin with 366, and the fourth digit is either 3, 4, 5 or 6. With MINSTD, using the current time, alone, as a seed, is not a good idea.

Columns 4 shows the values returned by uniform_rand on the first call after seeding. It never changes. This combination of RNG and seed is a failure.

Column 5 shows the values returned by biased_rand. Note the repeating pattern in column 5. This is also a failure.

We now have the answers to the two questions in the OP.

Why is my first random number always 8? Am I seeding incorrectly?

The answers are: (1) When closely spaced calls to std::time(NULL) are used as seeds for MINSTD, the results returned by its first invocation are less than random. (2) Yes. See Simple Portable C++ Seed Entropy by M.E. O'Neill for some ideas on a better seeding. Otherwise, just use std::random_device{}().

My untested hypothesis is the the additive constant, which is 0 in MINSTD, is a contributing factor in the failures in columns 3-5. Comparitively, tbx::knuth_lcg did a better job.

MINSTD is a multiplicative congruential generator (MCG). In addition to its modulus, the state transition function in MINSTD has only one parameter: its multiplier. With tbx::knuth_lcg, there are two: a multiplier and an additive constant. The combination means that similar seeds diverge more in a single cycle of tbx::knuth_lcg than they do in MINSTD.

At least, that's the hypothesis.

The next four columns demonstrate that MINSTD is usable, but only if you have a better seed.

Column 6 shows the seeds returned by std::random_device. Anecdotally, they appear to be nice and random. So do the values returned from the first call to rand, which are shown in column 7.

Columns 8 and 9 show the output generated by functions uniform_rand and biased_rand. Anecdotally, both columns appear to be random, but we know that rigorous testing would reveal that the values in column 9 have a tiny bias.

minstd_rand0   a: -10   b: 10
                                 1st call                                    seed with     1st call
std::time(NULL)         seed      to rand   uniform_rand   biased_rand   random_device      to rand   uniform_rand   biased_rand
---------------   ----------   ----------   ------------   -----------   -------------   ----------   ------------   -----------
     1725470667   1725470667    366331181             -7            10        79934811   1287089102              2            -5
     1725470668   1725470668    366347988             -7            -4       523355155   2084555620             10            -3
     1725470670   1725470670    366381602             -7            10       814741099    997917621             -1             5
     1725470671   1725470671    366398409             -7            -4      3582022895    502236267             -6             5
     1725470672   1725470672    366415216             -7             3      3765154492   1050920895              0            -7
     1725470673   1725470673    366432023             -7            10      2560642594   1147791478              1            -9
     1725470674   1725470674    366448830             -7            -4      1060201997   1143144420              1             2
     1725470675   1725470675    366465637             -7             3      3896449311    109754712             -9             8
     1725470676   1725470676    366482444             -7            10       499904143    932904337             -1            -9
     1725470677   1725470677    366499251             -7            -4      2931344698   1687993459              6             9
     1725470678   1725470678    366516058             -7             3      2601193000   1926149021              8            10
     1725470679   1725470679    366532865             -7            10       415279526    281140732             -8             9
     1725470680   1725470680    366549672             -7            -4      1042354394   1826191379              7            -5
     1725470681   1725470681    366566479             -7             3      1405921226    555477441             -5             5
     1725470682   1725470682    366583286             -7            10      3238867965   1238403599              2            10
     1725470683   1725470683    366600093             -7            -4      4147558193    691368131             -4            10
     1725470684   1725470684    366616900             -7             3      1312675989   1045841492              0             7
     1725470685   1725470685    366633707             -7            10      4202573649   1918168913              8             7
     1725470686   1725470686    366650514             -7            -4        66238746    878074876             -2            -3
     1725470687   1725470687    366667321             -7             3      3397492534     66845208            -10            -7

Data for std::minstd_rand (not shown), have the same failings (and successes) as the data above.

tbx::knuth_lcg

tbx::knuth_lcg fared better than MINSTD. When faced with a series of similar seeds (column 2), it generated what appears, anecdotally, to be random output (column 3).

tbx::knuth_lcg, however, has its own problems. To the naked eye, the values in column 3 look random, but the values in column 4 raise suspicions. There are not enough repeats. Think of the Birthday Problem.

We drew 20 values, from a population of 21, and there were only two repeats (0 and -5). Column 8, where the seeding is better, is more satisfying. It has many repeats, and even a couple of triples.

In many runs of the program (but not all), the same pattern appeared. It happened enough to concern me, even though I am just using the "eyeball" test to evaluate the data.

When the seeds are good, as in column 6, tbx::knuth_lcg yields good results.

knuth_lcg   a: -10   b: 10
                                              1st call                                    seed with                  1st call
std::time(NULL)         seed                   to rand   uniform_rand   biased_rand   random_device                   to rand   uniform_rand   biased_rand
---------------   ----------   -----------------------   ------------   -----------   -------------   -----------------------   ------------   -----------
     1725470729   1725470729       7682619102818709988              7            -9      2761315232       8143472074341520495              8             9
     1725470730   1725470730       4823383289810727185              0            -5      1940100256       8838089308847210863             10             0
     1725470731   1725470731       1964147476802744382             -6            -1      4177479439       1647040385966567410             -7            -3
     1725470732   1725470732       8328283700649537387              8           -10      4232132119       7189879011191188570              6             3
     1725470733   1725470733       5469047887641554584              2            -6      1506009362       1118517872796630905             -8             1
     1725470734   1725470734       2609812074633571781             -5            -2      1514642789       6095452229502519056              3            -5
     1725470735   1725470735       8973948298480364786             10            10      1095415460        119404177483429923            -10           -10
     1725470736   1725470736       6114712485472381983              3            -7       583154339       8072170026490244342              8             7
     1725470737   1725470737       3255476672464399180             -3            -3      3460774016       6240305398129356751              4            -9
     1725470738   1725470738        396240859456416377            -10             1      4039019375       1192875450421810386             -8           -10
     1725470739   1725470739       6760377083303209382              5            -8      3183640201       3021312392947218020             -4             4
     1725470740   1725470740       3901141270295226579             -2            -4      2846194354       6061817392193233049              3            -2
     1725470741   1725470741       1041905457287243776             -8             0        12515812       3252120412882307683             -3             0
     1725470742   1725470742       7406041681134036781              6            -9      4079972030       3154528753519180469             -3            -5
     1725470743   1725470743       4546805868126053978              0            -5      2809010452       8336560618007352787              8            -3
     1725470744   1725470744       1687570055118071175             -7            -1      2377801474        747086193202967209             -9            -6
     1725470745   1725470745       8051706278964864180              8           -10      4231172747       1009655311069690046             -8            10
     1725470746   1725470746       5192470465956881377              1            -6      2072539433       7046472142399192196              6            -2
     1725470747   1725470747       2333234652948898574             -5            -2      2237413946       8140891742275797889              8             3
     1725470748   1725470748       8697370876795691579              9            10       386168962       4228694408572382761             -1             9

Mersenne Twister

Data for std::mt19937 were uniformly good.

The reason it does well with poor seeding is likely due to the fact that it conditions the seeds you give it, before installing them in the engine.

When you seed std::mt19937 with a single (unsigned) integer, it pushes that seed through a seed sequence, to fill the 624 state variables used by the engine. On the outside, the seeds in column 2 all look similar. By the time they hit the engine, however, they have been significantly "randomized."

mt19937   a: -10   b: 10
                                 1st call                                    seed with     1st call
std::time(NULL)         seed      to rand   uniform_rand   biased_rand   random_device      to rand   uniform_rand   biased_rand
---------------   ----------   ----------   ------------   -----------   -------------   ----------   ------------   -----------
     1725470688   1725470688   1846392152              8             7      3488227490   1683360094              6             0
     1725470689   1725470689    923719378             -1            -9      4172931053   1019958242             -1            -8
     1725470690   1725470690   1919984915              8            -8      3443978328   1172527630              1            -9
     1725470691   1725470691   1612674401              5            -8      3542463677    926495713             -1             0
     1725470692   1725470692    339483995             -7             1      2534471012   1768656412              7             9
     1725470693   1725470693    881359337             -2            -5        38428959     47618872            -10            -3
     1725470694   1725470694   1481684138              4             1      3098883754    727394473             -3            -9
     1725470695   1725470695   1289557349              2             7      1460710575    351019927             -7             3
     1725470696   1725470696    593798350             -5             9       167206676   1070657304              0            -7
     1725470697   1725470697    695392031             -4            -5      1215347282    924400893             -1            -7
     1725470698   1725470698   1152322541              1            10      3860368725   1741906414              7             0
     1725470699   1725470699    111164029             -9            -6      2743553328    377480583             -7             8
     1725470700   1725470700   1998001782              9           -10      2268784299    827285329             -2             0
     1725470701   1725470701    814093545             -3            -4      1593534606    548682246             -5           -10
     1725470702   1725470702   1511789889              4             5       142519159   1539300695              5            -8
     1725470703   1725470703   1885533215              8            10      3120646034    798241165             -3            -9
     1725470704   1725470704     61821576            -10             2      2487963867    659747497             -4             0
     1725470705   1725470705   1211428588              1             3       531291314   1551904191              5             2
     1725470706   1725470706   1724047358              6            -5      1915141704    367916844             -7             5
     1725470707   1725470707   1375067752              3             6      1484660056   2086820278             10            -6

Results for pcg_engine, which also conditions its seeds, were similar.

Conclusions

The primary conclusion is that std::time(NULL) is a poor source of random seeds. When a program makes several calls to it, the odds are good that the seeds it returns will be very similar. In the worst case, which is not uncommon, the seeds will be identical.

When the values returned by std::time(NULL) were not identical, they worked when used as seeds for std::mt19937 and pcg_engine. They failed, however, with the other engines. When the values were identical, of course, they failed for all engines.

std::time(NULL) can still be used as a seed, but it should be combined with other sources of "entropy." This can be accomplished by exclusive-or'ing it with the addresses of certain objects in memory or with a call to std::random_device (or both). See Simple Portable C++ Seed Entropy, by M.E. O'Neill.

Otherwise, just use std::random_device to generate your seeds.

Although it has a checkered history, std::rand might not be as much of a problem as std::time(NULL). That depends, of course, on your system. Even when rand is implemented using the lowly MINSTD generator, as in the OP, the foregoing data show that proper seeding can yield serviceable results. Given a choice, you should prefer something like std::mt19937 or pcg32_engine. For lightweight applications, however, rand may be fine.

As for function nrand, the random number distribution, that's a case of garbage-in, garbage-out. When you feed it a series of similar random values, in which the high-order bits are constant, it is to be expected that nrand will struggle. Interestingly, in tests of both std::minstd_rand0 and std::minstd_rand, the times when nrand struggled were also the times when biased_rand had problems of its own.

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转身泪倾城 2024-09-12 02:50:30

问题是随机数生成器使用非常接近的值进行播种 - 程序的每次运行只会少量更改 time() 的返回值 - 可能是 1 秒，甚至可能没有！然后，相当差的标准随机数生成器使用这些相似的种子值来生成明显相同的初始随机数。基本上，您需要一个比 time() 更好的初始种子生成器和一个比 rand() 更好的随机数生成器。

我认为实际使用的循环算法是从 Accelerated C++ 中提取出来的，其目的是在所需范围内产生比使用 ​​mod 运算符更好的数字分布。但它无法弥补总是（有效地）给予相同的种子。

The issue is that the random number generator is being seeded with a values that are very close together - each run of the program only changes the return value of time() by a small amount - maybe 1 second, maybe even none! The rather poor standard random number generator then uses these similar seed values to generate apparently identical initial random numbers. Basically, you need a better initial seed generator than time() and a better random number generator than rand().

The actual looping algorithm used is I think lifted from Accelerated C++ and is intended to produce a better spread of numbers over the required range than say using the mod operator would. But it can't compensate for always being (effectively) given the same seed.

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蝶…霜飞 2024-09-12 02:50:30

我没有发现您的 srand() 有任何问题，并且当我尝试运行极其相似的代码时，我没有在第一个 rand() 中重复获得相同的数字。然而，我确实注意到了另一个可能的问题。

do randx = (rand()/bucket_size)+min;
while (randx <= min && randx >= max);

该行可能没有达到您的预期。只要 min < max（而且总是应该如此），randx 不可能既小于或等于 min 又大于或等于 max。另外，您根本不需要循环。相反，您可以使用以下方法获取最小值和最大值之间的值：

randx = rand() % (max - min) + min;

I don't see any problem with your srand(), and when I tried running extremely similar code, I did not repeatedly get the same number with the first rand(). However, I did notice another possible issue.

do randx = (rand()/bucket_size)+min;
while (randx <= min && randx >= max);

This line probably does not do what you intended. As long as min < max (and it always should be), it's impossible for randx to be both less than or equal to min and greater than or equal to max. Plus, you don't need to loop at all. Instead, you can get a value in between min and max using:

randx = rand() % (max - min) + min;
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