iOS 生成自定义位数随机数

#pragma mark生成随机数

- (NSString *)generateTradeNO

{

   static int kNumber =15;

   NSString *sourceStr = @"0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";

   NSMutableString *resultStr = [[NSMutableStringalloc] init];

   srand((unsigned)time(0));

   for (int i =0; i < kNumber; i++)

    {

       unsigned index = rand() % [sourceStrlength];

       NSString *oneStr = [sourceStr substringWithRange:NSMakeRange(index,1)];

        [resultStrappendString:oneStr];

    }

   return resultStr;

}