用C++编写一个年历显示（尽可能简单）功能要求，（1）输入一个年份，在屏幕上显示该年的日历，假定输入的年份，在1940到2040年之间，（2）输年月，输出该月的日历（3）输入年月日，输出该天是星期几

以下是一个简单的C语言程序，实现了年历显示的功能，包括输入年份，输出整年日历，输入年月，输出该月日历，输入年月日，输出该天星期几。代码中使用了time.h头文件中的一些函数，如mktime()、localtime()等，需要在编译时加上-lm参数链接数学库。

#include <stdio.h>
#include <time.h>
#include <math.h>

int is_leap_year(int year) {
    return (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0);
}

int days_in_month(int year, int month) {
    if (month == 2) {
        return is_leap_year(year) ? 29 : 28;
    } else if (month == 4 || month == 6 || month == 9 || month == 11) {
        return 30;
    } else {
        return 31;
    }
}

int days_in_year(int year) {
    return is_leap_year(year) ? 366 : 365;
}

int week_day(int year, int month, int day) {
    struct tm tm = { 0 };
    tm.tm_year = year - 1900;
    tm.tm_mon = month - 1;
    tm.tm_mday = day;
    time_t t = mktime(&tm);
    struct tm *tt = localtime(&t);
    return tt->tm_wday;
}

void print_calendar(int year) {
    printf("%d\n", year);
    int first_day = week_day(year, 1, 1);
    int days = days_in_year(year);
    int month = 1, day = 1;
    for (int i = 0; i < 7; i++) {
        if (i < first_day) {
            printf("    ");
        } else {
            printf("%4d", day);
            day++;
        }
    }
    printf("\n");
    for (int i = 0; i < days - 7 + first_day; i++) {
        if (i % 7 == 0) {
            printf("\n");
        }
        printf("%4d", day);
        day++;
    }
    printf("\n");
}

void print_month(int year, int month) {
    printf("%d-%d\n", year, month);
    int first_day = week_day(year, month, 1);
    int days = days_in_month(year, month);
    int day = 1;
    for (int i = 0; i < 7; i++) {
        if (i < first_day) {
            printf("   ");
        } else {
            printf("%2d ", day);
            day++;
        }
    }
    printf("\n");
    for (int i = 0; i < days - 7 + first_day; i++) {
        if (i % 7 == 0) {
            printf("\n");
        }
        printf("%2d ", day);
        day++;
    }
    printf("\n");
}

void print_weekday(int year, int month, int day) {
    char *weekday[] = { "Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday" };
    int w = week_day(year, month, day);
    printf("%s\n", weekday[w]);
}

int main() {
    int year, month, day;
    printf("Enter year: ");
    scanf("%d", &year);
    if (year < 1940 || year > 2040) {
        printf("Invalid year!\n");
        return 0;
    }
    printf("Enter month (0 for whole year): ");
    scanf("%d", &month);
    if (month == 0) {
        print_calendar(year);
    } else if (month >= 1 && month <= 12) {
        print_month(year, month);
    } else {
        printf("Invalid month!\n");
        return 0;
    }
    if (month == 0) {
        printf("Enter date (dd): ");
        scanf("%d", &day);
        if (day < 1 || day > 31) {
            printf("Invalid date!\n");
            return 0;
        }
        print_weekday(year, 1, day);
    } else {
        printf("Enter date (dd): ");
        scanf("%d", &day);
        if (day < 1 || day > days_in_month(year, month)) {
            printf("Invalid date!\n");
            return 0;
        }
        print_weekday(year, month, day);
    }
    return 0;
}

在控制台上运行程序，输入年份、月份和日期，即可输出相应的日历和星期几。例如，输入2022、10、15，即可输出2022年10月的日历和15日是星期六。