为什么Rand()总是生成具有固定唯一组计数的随机数? [英] Why Rand() always generates random numbers with fixed Unique Group count?

问题描述

尽管如此，我还是再次执行以下代码&再次，我得到相同的输出.但是，我认为不应该:

Even though , I execute the below code again & again I get the same Output. But , I think it should not:

int ObjectCount =500;
NSMutableArray *mut_arr = [[NSMutableArray alloc]initWithCapacity:0];
for (int i = 0; i<ObjectCount ; i++)
{    
 [mut_arr addObject:[NSNumber numberWithInt: rand()%ObjectCount]];
}
NSSet* uniqueSet = [NSSet setWithArray:mut_arr];    
NSLog(@"Array of %d objects generates %d Unique Objects",[mut_arr count],[uniqueSet count]);

输出如下:

由500个对象组成的数组会生成317个唯一对象

Array of 500 objects generates 317 Unique Objects

在这里，由于数组包含随机数，因此唯一集计数应再次相同&再次使用相同的ObjectCount.

Here, Since the array contains random numbers the unique set count should be same again & again for same ObjectCount.

推荐答案

您实际上并没有生成唯一的NSNumber对象.其中一些是平等的.

You're not actually generating unique NSNumber objects; some of them are equal.

NSArray可以包含多个相等的对象. NSSet不能.这就是从数组创建的对象较少的原因.

A NSArray can contain multiple objects that are equal. A NSSet can not. This is why the set created from your array has less objects.

总是获得317个对象的原因是您使用的是rand()而没有种子:

The reason why you're always getting 317 objects is that you're using rand() without seeding: Why do I always get the same sequence of random numbers with rand()?

考虑改用arc4random()，它会自动播种.

Consider using arc4random() instead, which is seeding automatically.

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