trjhtr

Here's the problem:

How many Sundays fell on the first of the month during the twentieth
century (1 Jan 1901 to 31 Dec 2000)?

And here's my code:

#include <stdio.h>

int main(void)
 int months[12] = 31 , 28 , 31 , 30 , 31 , 30 , 31 , 31 , 30 , 31 , 30 , 31 ;
 int year=1901;
 int dayoftheweek=2; //put a 2 because the 1 jan 1901 is tuesday. 0-sunday 6-Saturday
 int sunday=0;
 int firstsundays=0;

 while(year<=2000)
 if(year%4==0) //checking if it's a leap year
 months[1]=29;

 for (int i = 0; i < 12; ++i) 
 for (int d = 1; d <= months[i]; d++)
 if(dayoftheweek==7) //reset the week
 dayoftheweek=0;

 if(dayoftheweek==sunday && d==1)
 firstsundays++;

 dayoftheweek++;
 
 

 months[1]=28;
 year++;
 

 printf("There are %d Sundays that fell on the first of the monthn", firstsundays);

 return 0;

What could I improve?

The loops could be clearer:

• The outermost loop is actually spread across three lines: int year=1901, while(year<=2000), and year++. It should be written as a for loop.


• 
  i should be renamed to month or m.


• The exceptional case for leap years is awkward: you sometimes clobber the February entry at the top of the loop, then reset it at the end. I'd consider such mutations to be a hack that makes the code harder to understand. It also makes the code harder to maintain, since you have hard-coded 29 and 28 in scattered places.

As for the algorithm itself… you should be able to do better than advancing a day at a time through an entire century. Why not advance a month at a time?

#include <stdio.h>

static const int *month_lengths(int year) year % 400 == 0)) ?
 LEAP_YEAR : NON_LEAP_YEAR;


int main(void) 
 int first_sundays = 0;
 int day_of_the_week = 2; // 0 = Sun, 6 = Sat. 1 Jan 1901 was a Tuesday.
 for (int year = 1901; year <= 2000; ++year) 
 const int *lengths = month_lengths(year);
 for (int month = 0; month < 12; month++) 
 if (day_of_the_week == 0) 
 first_sundays++;
 
 day_of_the_week = (day_of_the_week + lengths[month]) % 7;
 
 
 printf("There are %d Sundays that fell on the first of the monthn",
 first_sundays);