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srand((unsigned)time(NULL))是初始化随机函数种子：

1、是拿当前系统时间作为种子，由于时间是变化的，种子变化，可以产生不相同的随机数。

计算机中的随机数实际上都不是真正的随机数，如果两次给的种子一样，是会生成同样的随机序列的。 所以，一般都会以当前的时间作为种子来生成随机数，这样更加的随机。 

2、使用时，参数可以是unsigned型的任意数据，比如srand（10）； 3、如果不使用srand，用rand（）产生的随机数，在多次运行，结果是一样的。

void test_rand(void)

{           unsigned long n;          srand((unsigned)time(NULL));          for(int i = 0; i < 100; i++)          {                n = rand();                printf("d\n", n);           }}

int rand (void);  
  
 
    Generate random number 
 Returns a pseudo-random integral number in the range between 0 and .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 .  is a constant defined in . A typical way to generate trivial pseudo-random numbers in a determined range using rand is to use the modulo of the returned value by the range span and add the initial value of the range:  
   
  
   
    
     123 
     v1 = rand() % 100;         // v1 in the range 0 to 99v2 = rand() % 100 + 1;     // v2 in the range 1 to 100v3 = rand() % 30 + 1985;   // v3 in the range 1985-2014 
       
    
   
   
 Notice though that this modulo operation does not generate uniformly distributed random numbers in the span (since in most cases this operation makes lower numbers slightly more likely)

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