初中级C语言水平自测：srand((int)time(0)); 有没有错？

sonicling

回复 86# pmerofc


举个例子吧。

一个很简单的猜数字游戏（事先声明：没有输入检查，只说明问题，不做教学用途，别批我）

1. #include <stdio.h>
   
2. #include <time.h>
   
3. 
   
4. int main(int argc, const char *argv[])
   
5. {
   
6.         srand(time(0));
   
7.         if (argc == 2)
   
8.         {
   
9.                 int number = rand() % 100;
   
10.                 int guess;
    
11.                 sscanf(argv[1], "%d", &guess);
    
12.                 printf("number=%d, you guess %d, you %s.\n",
    
13.                         number, guess, number==guess?"won":"lost");
    
14.         }
    
15.         return 0;
    
16. }
    

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自己猜太麻烦了，让电脑自己玩吧。

1. #!/bin/sh
   
2. 
   
3. for ((n=0; i<100; i=i+1)) do
   
4.         ./rand $i
   
5. done

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最后一定可以猜中一个，原因自己看：

[root@localhost test]# vi rand.c
[root@localhost test]# vi guess.sh
[root@localhost test]# chmod +x guess.sh
[root@localhost test]# ./guess.sh
number=38, you guess 1, you lost.
number=38, you guess 2, you lost.
number=38, you guess 3, you lost.
number=38, you guess 4, you lost.
number=38, you guess 5, you lost.
number=38, you guess 6, you lost.
number=38, you guess 7, you lost.
number=38, you guess 8, you lost.
number=38, you guess 9, you lost.
number=38, you guess 10, you lost.
number=38, you guess 11, you lost.
number=38, you guess 12, you lost.
number=38, you guess 13, you lost.
number=38, you guess 14, you lost.
number=38, you guess 15, you lost.
number=38, you guess 16, you lost.
number=38, you guess 17, you lost.
number=38, you guess 18, you lost.
number=38, you guess 19, you lost.
number=38, you guess 20, you lost.
number=38, you guess 21, you lost.
number=38, you guess 22, you lost.
number=38, you guess 23, you lost.
number=38, you guess 24, you lost.
number=38, you guess 25, you lost.
number=38, you guess 26, you lost.
number=38, you guess 27, you lost.
number=38, you guess 28, you lost.
number=38, you guess 29, you lost.
number=38, you guess 30, you lost.
number=38, you guess 31, you lost.
number=38, you guess 32, you lost.
number=38, you guess 33, you lost.
number=38, you guess 34, you lost.
number=38, you guess 35, you lost.
number=38, you guess 36, you lost.
number=38, you guess 37, you lost.
number=38, you guess 38, you won.
number=38, you guess 39, you lost.
number=38, you guess 40, you lost.
number=38, you guess 41, you lost.
number=38, you guess 42, you lost.
number=38, you guess 43, you lost.
number=38, you guess 44, you lost.
number=38, you guess 45, you lost.
number=38, you guess 46, you lost.
number=38, you guess 47, you lost.
number=38, you guess 48, you lost.
number=38, you guess 49, you lost.
number=38, you guess 50, you lost.
number=38, you guess 51, you lost.
number=38, you guess 52, you lost.
number=38, you guess 53, you lost.
number=38, you guess 54, you lost.
number=38, you guess 55, you lost.
number=38, you guess 56, you lost.
number=38, you guess 57, you lost.
number=38, you guess 58, you lost.
number=38, you guess 59, you lost.
number=38, you guess 60, you lost.
number=38, you guess 61, you lost.
number=38, you guess 62, you lost.
number=38, you guess 63, you lost.
number=38, you guess 64, you lost.
number=38, you guess 65, you lost.
number=38, you guess 66, you lost.
number=38, you guess 67, you lost.
number=38, you guess 68, you lost.
number=38, you guess 69, you lost.
number=38, you guess 70, you lost.
number=38, you guess 71, you lost.
number=38, you guess 72, you lost.
number=38, you guess 73, you lost.
number=38, you guess 74, you lost.
number=38, you guess 75, you lost.
number=38, you guess 76, you lost.
number=38, you guess 77, you lost.
number=78, you guess 78, you won.
number=78, you guess 79, you lost.
number=78, you guess 80, you lost.
number=78, you guess 81, you lost.
number=78, you guess 82, you lost.
number=78, you guess 83, you lost.
number=78, you guess 84, you lost.
number=78, you guess 85, you lost.
number=78, you guess 86, you lost.
number=78, you guess 87, you lost.
number=78, you guess 88, you lost.
number=78, you guess 89, you lost.
number=78, you guess 90, you lost.
number=78, you guess 91, you lost.
number=78, you guess 92, you lost.
number=78, you guess 93, you lost.
number=78, you guess 94, you lost.
number=78, you guess 95, you lost.
number=78, you guess 96, you lost.
number=78, you guess 97, you lost.
number=78, you guess 98, you lost.
number=78, you guess 99, you lost.
[root@localhost test]#

哎呀，居然跨越秒，还猜中两次，这运气真是好。

sonicling

刚好在秒滴答的时候猜中一次，这运气真的是逆天了！

sonicling

我说错了一个地方，一旦跨越秒，就不一定能猜中。

玩了多次，绝大多数情况下都没有跨越秒，好不容易出现了一次都没猜中的情况：

1. number=70, you guess 59, you lost.
   
2. number=70, you guess 60, you lost.
   
3. number=9, you guess 61, you lost.
   
4. number=9, you guess 62, you lost.

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前后就不贴了。

sonicling

pmerofc 发表于 2012-10-14 00:00
如果seed数目少，序列的数目也少（这个是我们的分歧之所在吧）

序列数目少一半也不是什么问题。40亿的一半也有20亿，总是用不完的。

Update：而且每个序列都是无穷的。

唯一的问题就是陈记错了srand的参数类型，还特地转换成int，结果画蛇添足了。

gvim

回复 82# pmerofc


少不了，现在的rand算法是 n = n * 16807 mod 2147483647 实现都只需要31bit就可以了，因为2^31 - 1 = 2147483647 是梅森素数，最适合32和64位系统（下一个是2^67-1）。另外rand的幻数16807选择需要能遍历1 --- 2^31-1之间的所有数，并且一个连续的遍历周期内只能出现一次（虽然16807遍历出来的数之间的相关性相比有些幻数不是最好）。
所以参数 n ， 也就是种子给多了也没用，只要是32位和64位的机器，给不给符号不影响随机数的随机性也不影响产生的序列长度(超不出1 --- 2^31-1之间)，或者哪怕产生了所谓的UB，也不影响随机性，这里随机性的来源是取时间的随机。种子的随机性导致rand结果的随机性才是使用rand的意义，而不是参数 n符号的 正确与否(因为错误不影响随机性)。

当然，这个话题当作转型，溢出，UB的讨论是成功的，呵呵。

starwing83

回复 94# sonicling


    我想知道seed少和数列少有什么关系………………

你给的是个unsigned，你哪怕给全0或者全1都能给你生成随机的数列，问题仅仅在于随机性不太一样。

如果seed非要达到多少bit的话，就不需要rand了，光seed自己的随机性就能保证一些应用了= =

starwing83

回复 97# pmerofc


    还是不对，因为srand的参数是unsigned，即使强制转换到int，也会最终转到unsigned，最终srand得到的数据量是一样多的。