在 C/C++ 中获取大随机数

问题描述

标准 rand() 函数给出的数字对我来说不够大:我需要 unsigned long long 的.我们如何获得真正大的随机数?我尝试修改一个简单的散列函数，但它太大大，运行时间太长，而且永远不会产生小于 1e5 的数字！！

Standard rand() function gives numbers not big enough for me: I need unsigned long long ones. How do we get really big random numbers? I tried modifying a simple hash function but it's too big, takes too long to run and never produces numbers which are less than 1e5!!

推荐答案

这是一个返回随机 64 位数字的便携式 C99 解决方案:

Here's a portable C99 solution that returns a random 64-bit number:

unsigned long long llrand() {
    unsigned long long r = 0;

    for (int i = 0; i < 5; ++i) {
        r = (r << 15) | (rand() & 0x7FFF);
    }

    return r & 0xFFFFFFFFFFFFFFFFULL;
}

说明:rand() 返回 0 到 RAND_MAX 范围内的整数，并且 RAND_MAX 只保证至少为 32,767(15 个随机位).long long 保证有 64 位，但可能更大.

Explanation: rand() returns integers in the range 0 to RAND_MAX and RAND_MAX is only guaranteed to be at least 32,767 (15 random bits). long long is guaranteed to have 64 bits but may be larger.

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