問題描述
標準 rand() 函數給出的數字對我來說不夠大:我需要 unsigned long long 的.我們如何獲得真正大的隨機數?我嘗試修改一個簡單的散列函數，但它太大大，運行時間太長，而且永遠不會產生小于 1e5 的數字！！
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!!
推薦答案
這是一個返回隨機 64 位數字的便攜式 C99 解決方案:
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;
}
說明:rand() 返回 0 到 RAND_MAX 范圍內的整數，并且 RAND_MAX 只保證至少為 32,767(15 個隨機位).long long 保證有 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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