看下面代码，用于生成随机数
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int main(){ cout<<"rand() traps by Sparkles."<<endl;<br> int
N = 10; for
(int i = 0; i < N; ++i) cout<< rand()%n; getchar(); return
0;} |
结果如下:
似乎并没有什么异常，但是，如果多次运行程序，会发现，每次生成的结果都是一样的。
这个问题网上解释很充分，另见：http://www.cplusplus.com/reference/cstdlib/rand/
randint rand (void);Generate random numberReturns a pseudo-random integral number in the range between
0and RAND_MAX.
This number is generated by an algorithm that returns a sequence of apparently non-related numbers each time it is called. This algorithm uses a seed to generate the series, which should be initialized to some distinctive value using function srand.
上文提到，rand()算法是由一个seed来生成随机数的，可以通过srand()来定义这个seed。而一开始的例子中，并未调到srand()，所以rand()默认将1作为seed来生成随机数。（狗屎的节奏哇）
下面是通过设置seed来生成随机数，可以看到，当seed不同的时候，随机数结果会变化，最后一种将seed设置成time(0)是网上比较通用的做法。
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int main(){ int
N = 10; cout<<"rand() traps by Sparkles."<<endl; cout<<"Set seed as 1\t\t\t"; srand(1); for
(int i = 0; i < N; ++i) cout<< rand()%N; cout<<endl<<"Set seed as 2\t\t\t"; srand(2); for
(int i = 0; i < N; ++i) cout<< rand()%N; cout<<endl<<"Set seed as current time\t"; srand(time(0)); for
(int i = 0; i < N; ++i) cout<< rand()%N; getchar(); return
0;} |
C++随机数生成函数rand()陷阱,布布扣,bubuko.com
原文：http://www.cnblogs.com/sparkles/p/3648519.html