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  // function to return the day of the week given the date
// (01/01/1800 was supposed to be a Wednesday)
// original Turbo C, modified for Pelles C by  vegaseat    8oct2004
// Pelles C free at:  http://smorgasbordet.com/pellesc/index.htm
 
#include <stdio.h>  // for printf(), scanf(), getchar()
 
// years ending with 00 have to be divisible by 400 to leap
// note the "&&" is a DaniWeb problem and should be a double & for AND
#define isleapyear(year) ((!(year % 4) && (year % 100)) || (!(year % 400) && (year % 1000)))
 
int isdatevalid(int month, int day, int year);
int weekday(int month, int day, int year);
 
char week[7][10] = {
  "Monday","Tuesday","Wednesday","Thursday",
  "Friday","Saturday","Sunday"
};
 
int main()
{
  int  month, day, year;
  
  printf("Return the day of the week given the date.");
  printf("Enter date in the form mm/dd/yyyy : ");
  scanf("%d/%d/%d",&month,&day,&year);
  if (isdatevalid(month,day,year))
  {
    printf("The day of the week for this date is %s",
      week[weekday(month,day,year)]);
  }
  else
    printf("%d/%d/%d not a valid date!",
    month,day,year);
  
  getchar();   // wait 
  getchar();   // 2nd wait needed
  return 0;
}
 
int isdatevalid(int month, int day, int year)
{
  if (day <= 0) return 0 ;
  switch( month )
     {
       case 1  :
       case 3  :
       case 5  :
       case 7  :
       case 8  :
        case 10 :
       case 12 : if (day > 31) return 0 ; else return 1 ;
       case 4  :
       case 6  :
       case 9  :
       case 11 : if (day > 30) return 0 ; else return 1 ;
       case 2  : 
         if ( day > 29 ) return 0 ;
      if ( day < 29 ) return 1 ;
      if (isleapyear(year)) return 1 ;   // leap year
    else return 0 ;
     }
  return 0 ;
}
 
//
// given month, day, year, returns day of week, eg. Monday = 0 etc.
// tested for 1901 to 2099 (seems to work from 1800 on too)
// 
int weekday(int month, int day, int year)
{     
  int ix, tx, vx;
 
  switch (month) {
    case 2  :
     case 6  : vx = 0; break;
     case 8  : vx = 4; break;
     case 10 : vx = 8; break;
     case 9  :
     case 12 : vx = 12; break;
     case 3  :
     case 11 : vx = 16; break;
     case 1  :
     case 5  : vx = 20; break;
     case 4  :
     case 7  : vx = 24; break;
  }
  if (year > 1900)  // 1900 was not a leap year
    year -= 1900;
  ix = ((year - 21) % 28) + vx + (month > 2);  // take care of February 
  tx = (ix + (ix / 4)) % 7 + day;              // take care of leap year
  return (tx % 7);
}