C語言打印某一年的日歷

// 打印日歷
#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>

char month[12][10] = { "January", "February", "March", "April", "May", "June",
 "July", "August", "September", "October", "November", "December" };


//計算某日期是當年的第幾天
int day_in_year(int y, int m, int d) {
 int r_day = 0;
 int m_days[] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };//每個月的天數
 m_days[1] += y % 400 == 0 || y % 4 == 0 && y % 100 != 0;
 for (int i = 0; i < m - 1; ++i) {
  r_day += m_days[i];
 }
 r_day += d;
 return r_day;
}

//計算某日是星期幾
int getWeek(int y, int m, int d) {
 //先計算出來距離1970年1月1日周四有多少天
 int days = 0; //當日距離1970.1.1的天數
 int res;   //存放結果，周幾

 if (y == 1970) {
  days = day_in_year(y, m, d);
 }
 else {
  for (int i = 1970; i < y; ++i) {
   days = days + 365 + (i % 400 == 0 || i % 4 == 0 && i % 100 != 0);
  }
  days += (day_in_year(y, m, d) - 1);//因為算的是距離多少天，所以需要減一
 }

 //然後計算當日是周幾
 res = (days + 4) % 7;
 return res;
}

//打印日歷
void displayCalendar(int y) {
 int m_days[] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };//每個月的天數
 m_days[1] = 28 + (y % 400 == 0 || y % 4 == 0 && y % 100 != 0);
 int week_l;//左邊月份的第一天是周幾
 int week_r;//右邊月份的第一天是周幾
 for (int m = 0; m < 6; ++m) {//m控制月份,比實際月份小1
  //打印日歷的表頭
  printf("%-12s                    %-12s\n", month[m], month[m + 6]);
  printf("---------------------------     --------------------------- \n");
  printf("Sun Mon Tue Wed Thu Fri Sat     Sun Mon Tue Wed Thu Fri Sat \n");
  week_l = getWeek(y, m + 1, 1);//求出左邊月份的第一天是星期幾
  week_r = getWeek(y, m + 7, 1);//求出右邊列月份的第一天是星期幾
  int date_l = 1;
  int date_r = 1;
  while (date_l <= week_l + m_days[m] || date_r <= week_r + m_days[m + 6]) {
   //在范圍內，打印
   int tag[15] = { 0 };//用來標記應該打印什麼，整數表示打印日期，-1表示打印空格
   tag[7] = -1;//左右界線
   for (int p = 0; p < 7; ++p, ++date_l, ++date_r) {//p為tag的指針
    //根據指針p給tag賦值
    if (date_l <= week_l + m_days[m]) {
     if (date_l < week_l) {
      //指針還沒有到左邊月份的第一天，應該打印空格
      tag[p] = -1;
     }
     else {
      //記錄應該打印的日期
      tag[p] = date_l - week_l;
     }
    }
    else {//該打印的日期標記完瞭，其他的用空格補上
     tag[p] = -1;
    }
    //標記右邊的情況，邏輯與左邊的一樣
    if (date_r <= week_r + m_days[m + 6]) {
     if (date_r < week_r) {
      tag[p + 8] = -1;
     }
     else {
      tag[p + 8] = date_r - week_r;
     }
    }
    else {
     tag[p + 8] = -1;
    }
   }
   //一行一行打印
   for (int i = 0; i < 15; ++i) {
    if (tag[i] == -1||tag[i] == 0) {
     //沒有日期要打印，打印四個空格
     printf("    ");
    }
    else {
     //有日期打印，打印日期
     printf("%-4d", tag[i]);
    }
   }
   printf("\n");
  }
  printf("\n");
 }
}

int main() {

 int year;//輸入年份，打印該年的日歷
 scanf("%d", &year);
 displayCalendar(year);
 return 0;
}