Weird output Calendar in C - c

Question

I have programmed a very simple calendar in C. It takes the year as input and will then calculate the dates and weekdays for that year.
This assuming that the date 0001-01-01 is a monday.
I've gotten it to work pretty good except in one place. When it's about to print out the line with all the 31st dates.
Basically what's going wrong is, when it's going to check if there is anything to print at april 31st (which there isn't), it bugs out the weekday variable. Which makes the following 31st dates wrong.
Is there anyone who can see if I made something wrong or might wanna help me out? :)
EDIT Forgot to mention that it's a task ment to be solved without any formulas for calculating the daycode and so on. The only reference allowed to be used is that the date 0001-01-01 is a monday.
Heres my code. (Yes it's a bit messy atm, but I'm still learning.)
int isLeapYear(int year){
if(((year%4==0) && (year%100!=0)) || (year%400==0)){
return 1;
}
else
return 0;
}
int getYear(){
int year = 0;
while(year==0 || year < 0){
printf("Enter year: ");
scanf("%d", &year);
if(year > 0){
break;
}
printf("Invalid input. Try again.\n\n");
}
return year;
}
void printWeekday(int w){
switch(w){
case 0:
printf("Sun");
break;
case 1:
printf("Mon");
break;
case 2:
printf("Tue");
break;
case 3:
printf("Wed");
break;
case 4:
printf("Thu");
break;
case 5:
printf("Fri");
break;
case 6:
printf("Sat");
break;
}
}
void printMonth(int m){
switch(m){
case 1:
printf("Jan");
break;
case 2:
printf(" Feb");
break;
case 3:
printf(" Mar");
break;
case 4:
printf(" Apr");
break;
case 5:
printf(" May");
break;
case 6:
printf(" Jun");
break;
case 7:
printf(" Jul");
break;
case 8:
printf(" Aug");
break;
case 9:
printf(" Sep");
break;
case 10:
printf(" Oct");
break;
case 11:
printf(" Nov");
break;
case 12:
printf(" Dec\n");
break;
}
}
void calendar(int year){
int y = 1;
int m = 1;
int d = 1;
int loop = 1;
int day = 1;
int days_in_month[14] = {0,31,28,31,30,31,30,31,31,30,31,30,31,0};
if(isLeapYear(year)){
days_in_month[1] = 29;
}
for(m=1; m<=12; m++){
printMonth(m);
}
while(loop){
int weekday = 1;
for(y=1; y<=year; y++){
for(m=1; m<=12; m++){
for(d=1; d<=31; d++){
if(weekday%7 == 0){
// printf("h");
weekday = 0;
}
if(y==year){
if(day>days_in_month[m]){
printf(" ");
printf("%d", weekday);
break;
}else if(d == day){
//printf("%d", weekday);
if(m == 1){
printf("%02d ", d);
printWeekday(weekday);
}else if(m == 12){
printf(" %02d ", d);
printWeekday(weekday);
printf("\n");
}else{
printf(" %02d ", d);
printWeekday(weekday);
}
}
}
if(d<=days_in_month[m]){
weekday = weekday + 1;
}
}
}
}
day++;
if(day == 32){
break;
}
}
}
int main()
{
int end = 1;
while(end){
int year = getYear();
printf(" %d\n", year);
calendar(year);
printf("\nEnter 0 to quit: ");
scanf("%d", &end);
//system("cls");
}
return 0;
}

Answer 1

Here, inside of calendar() function add this code:
if(day>days_in_month[m] /*Add this code: */ && d>days_in_month[m]){
printf(" ");
printf("%d", weekday);
break;
}else if(d == day){...
Where is problem? Well, did you see the 31st day of march? Its a Saturday:
At the end of iteration of days (I mean for(d=1; d<=31; d++){ loop ) you iterate the weekday. So First April should be Sunday. Which is valid if you check results.
But, you calculate weekday by day of month. So when we print 31 first March, our day variable is equal to 31.
Now, the March is ended (we print 31st day of March), our weekday sets on Sunday. And we go to calculate days of April. But look at your code, inside of for(d=1; d<=31; d++) loop, you have if(day>days_in_month[m]) {break;}.
And when our program checks first day of April, and day is equal to 31, then its stop calculating days for April with break instruction. We go to calculate days of May. But our weekday still remains as Sunday. If you see the valid code, first may should be Tuesday. But when day variable equal to 31, first may starts as Sunday. And starting with 1st May all days calculated wrong.
PROBLEM WITH LEAPS YEARS:
Because you use this:
if(isLeapYear(year)){
days_in_month[2] = 29;
}
It adds 29th day for each year (So you add three 29th days to first three years, and you see the result in 1st january of 4th year :) ).
To solve it, delete those piece of code. And inside of loop for months, add variable that will represent number of days:
for(m=1; m<=12; m++){
int monthDays = days_in_month[m]; /*Add this variable*/
if (m==2 && isLeapYear(y))
monthDays++; // Add 29th day to feb. of leap year
for(d=1; d<=31; d++){...
And inside of function, replace all days_in_month[m] to monthDays

Answer 2

Instead of holding a weekday variable you can call a function which will calculate the day of the week given the date. For this you can use Tomohiko Sakamoto's Day of the week algorithm.
int dayofweek(int d, int m , int y){
static int t[] = { 0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4 };
y -= m < 3;
return (y + y/4 - y/100 + y/400 + t[m-1] + d) % 7;
}
// 0 - Sunday
// 1 - Monday and so on
If you wish to understand this algorithm you can see a detailed explanation here : https://www.quora.com/How-does-Tomohiko-Sakamotos-Algorithm-work
But since you are a beginner I wouldnt really recoment it :) Its a little confusing.
After implementing this you can then simplify your calendar function to be
void calendar(int year) {
int days_in_month[14] = { 0,31,28,31,30,31,30,31,31,30,31,30,31,0 };
if (isLeapYear(year)) {
days_in_month[2] = 29;
}
for (int m = 1; m<=12; m++) {
printMonth(m);
}
for (int d = 1; d<=31; d++) {
for (int m = 1; m<=12; m++) {
if (d <= days_in_month[m]) {
int weekday = dayofweek(d, m, year);
printf("%02d ", d);
printWeekday(weekday);
printf(" ");
}
else {
printf(" ");
}
}
printf("\n");
}
}
This is actually a much more efficient way of generating the weekday rather than looping through the whole year