//0 到 N - 1 之间的随机整数
NSUInteger r = arc4random_uniform(N);
//1 到 N 之间的随机整数
NSUInteger r = arc4random_uniform(N) + 1;
//0 到 1 之间的随机浮点数（double）
 srand48(time(0));
double r = drand48(); 
//NSArray 选择一个随机元素
if ([array count] > 0) {
  id obj = array[arc4random_uniform([array count])];
}
//随机排序一个 NSArray
NSMutableArray *mutableArray = [NSMutableArray arrayWithArray:array];
NSUInteger count = [mutableArray count];
if (count > 1) {
  for (NSUInteger i = count - 1; i > 0; --i) {
      [mutableArray exchangeObjectAtIndex:i withObjectAtIndex:arc4random_uniform((int32_t)(i + 1))];
  }
}
NSArray *randomArray = [NSArray arrayWithArray:mutableArray];
//生成一个随机的小写 NSString
 NSString *letter = [NSString stringWithFormat:@"%c", arc4random_uniform(26) + 'a'];
//从一个 NSString 选择一个随机字符
NSString *vowels = @"aeiouy";
NSString *letter = [vowels substringWithRange:NSMakeRange(arc4random_uniform([vowels length]), 1)];