学习笔记_用户输入行数，显示对应的菱形

            bool flag = false;
            int a = 0;
            for (int d = 0; d < 20; d++)
            {
                while (flag == false)
                {
                    Console.WriteLine("请输入行数");
                    try
                    {
                        a = Convert.ToInt32(Console.ReadLine());
                        flag = true;
                    }
                    catch
                    {
                        Console.WriteLine("您输入的有问题，请重新输入");
                        flag = false;//如果用户输入的不为数字，则再次循环
                    }
                }
                while (a % 2 == 0)
                {
                    Console.WriteLine("请输入单数");
                    try
                    {
                        a = Convert.ToInt32(Console.ReadLine());
                    }
                    catch
                    {
                        Console.WriteLine("程序结束");
                        flag = false;
                        break;
                    }
                }
                if (flag == false)
                {
                    break;
                }
                for (int i = 0; i <= a / 2; i++)//先做出上半部分
                {
                    for (int c = a / 2; c > i; c--)
                    {
                        Console.Write(" ");//先填入空格
                    }
                    for (int b = 0; b < i * 2 + 1; b++)
                    {
                        Console.Write("*");
                    }
                    Console.WriteLine();
                }
                for (int i = 0; i < a / 2; i++)//再做出下半部分
                {
                    for (int c = 0; c <= i; c++)
                    {
                        Console.Write(" ");
                    }
                    for (int b = a - (i * 2 + 2); b > 0; b--)
                    {
                        Console.Write("*");
                    }
                    Console.WriteLine();
                }
                Console.WriteLine("是否结束");//判断是否要结束循环
                string input = Console.ReadLine();
                try
                {
                    if (Convert.ToInt32(input) == 1)//如果输入"1",则退出循环.否则继续
                    {
                        break;
                    }
                    flag = false;
                }
                catch
                {
                    flag = false;
                }
            }