笔记 指针数组和数组指针 数组查表法

int main()
{
	int a = 1, b = 2, c = 3, d = 4;
	int* ar[5] = { &a,&b,&c,&d };//数组指针 是一个数组，数组中的每个元素可以保存地址
	int(*pa)[5];//这个指针变量可以存放数组的地址

	*(ar[0]) = 100;
	*ar[1] = 200;

	return 0;
}

查表法：将一些事先计算好的结果，储存在数组中，用到是直接按下标取数据，以节省运行时的计算时间，以空间换时间。

int Get_YM_Day(int year,int month)
{
    int days[] = { 29,31,28,31,30,31,30,31,31,30,31,30,31 };
    //              0  1  2  3  4  5 6  7  8  9  10 11 12
    if (month == 2 && IsLeap(year))
    {
        month = 0;
    }
    return days[month];
}
int main()
{
    int year, month;
    scanf("%d %d \n", &year, &month);
    printf("day :%d\n", Get_YM_Day(year, month));
    return 0;
}

给出一个日期，计算它是这一年的第几天

#define YEARERROR -1
#define MONTHERROR -2
#define DAYERROR -3
int Get_YMD_Total(int year, int month, int day)
{
	int da = 0;
	static const int sum[] = {0,0,31,59,90,120,151,181,212,243,273,304,334,365};
	//                                       0 1 2   3   4   5     6     7    8     9      10   11  12   13
	if (year < 1)return YEARERROR;
	if (month < 1 || month>12)return MONTHERROR;
	if (day < 1 || day>Get_YM_Day(year, month))return DAYERROR;
	if (month > 2 && IsLeap(year))
	{
		da = 1;
	}
	return sum[month] + day+da;
}
int main()
{
	int year, month, day;
	int sum = 0;
	printf("input year month day:\n");
	scanf("%d %d %d", &year, &month, &day);
	sum = Get_YMD_Total(year, month, day);
	switch (sum)
	{
	case YEARERROR:
		printf("year input error %d\n", year);
		break;
	case MONTHERROR:
		printf("month input error %d\n", month);
		break;
	case DAYERROR:
		printf("day input error %d\n", day);
		break;
	default:
		printf("%d year %d month %d day => %d total \n", year, month, day, sum);
	}
	printf("%d year %d month %d day=>%d sum\n", year, month, day, sum);
	return 0;
}