Calendar Project Almost done in C

Question

I am working on a calendar project that was due about 4 days ago but I have been working on it even though the score is going to be grossly diminished because I want to try and understand it. So far I have it to the point where a calendar prints out in debug along with the correct number of days in the calendar. The only things I want to fix are that I want to make the first day of each month start off in the correct position so maybe some sort of loop + " " so that it can loop 4 spaces to the first day of the month or something like that. Also I want it so that when I enter the month, year, and num months the month and year will show the calendar of a month, while nummonths will display the following months. This is a really hard concept for me to understand. Help is Greatly! Appreciated! I have been working on this for WAYYY to long.

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>

int getdaycode(int month, int year);
void printheader(int month, int year);
int getndim(int month, int year);

int main(void) {
    int  day, month, year, nummonths;
    printf("Enter Month, Year, and number of Months: ");
    if (scanf("%d %d %d", &month, &year, &nummonths) != 3
    ||  year < 0 || month < 1 || month > 12) {
        printf("invalid input\n");
        return 1;
    }

    for (int i = 0; i < nummonths; i++) {
        printheader(month, year);
        int numdays = getndim(month, year);
        int daycode = getdaycode(month, year);

        printf("%*s", daycode * 4, "");   /* print 4 spaces for each skipped day */
        for (day = 1; day <= numdays; day++) {
            printf("%3d", day);
            daycode = (daycode + 1) % 7;
            if (daycode != 0)
                printf(" ");
            else
                printf("\n");
        }
        if (daycode != 0)
            printf("\n");
        printf("\n");

        month = month + 1;
        if (month > 12) {
            month -= 12;
            year += 1;
        }
    }
    return 0;
}

int getdaycode(int month, int year)
{
	int numdays;
	{
		numdays = ((year - 1) * 365 + ((year - 1) / 4) - ((year - 1) / 100) + ((year - 1) / 400)); // how many days including exceptions

		if (((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0))		//check if leapyear
		{
			if (month == 1)							// January 
				numdays = numdays;
			if (month == 2)							// February 
				numdays = numdays + 31;
			if (month == 3)							// March 
				numdays = numdays + 28 + 31 + 1;
			if (month == 4)							// April 
				numdays = numdays + 31 + 28 + 31 + 1;
			if (month == 5)							// May 
				numdays = numdays + 30 + 31 + 28 + 31 + 1;
			if (month == 6)							// June 
				numdays = numdays + 31 + 30 + 31 + 28 + 31 + 1;
			if (month == 7)							// July 
				numdays = numdays + 30 + 31 + 30 + 31 + 28 + 31 + 1;
			if (month == 8)							// August 
				numdays = numdays + 31 + 30 + 31 + 30 + 31 + 28 + 31 + 1;
			if (month == 9)							// September 
				numdays = numdays + 31 + 31 + 30 + 31 + 30 + 31 + 28 + 31 + 1;
			if (month == 10)							// October						
				numdays = numdays + 30 + 31 + 31 + 30 + 31 + 30 + 31 + 28 + 31 + 1;
			if (month == 11)							// November
				numdays = numdays + 31 + 30 + 31 + 31 + 30 + 31 + 30 + 31 + 28 + 31 + 1;
			if (month == 12)							// December
				numdays = numdays + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30 + 31 + 28 + 31 + 1;
		}
		else
		{
			if (month == 1)							// January 
				numdays = numdays;
			if (month == 2)							// February 
				numdays = numdays + 31;
			if (month == 3)							// March 
				numdays = numdays + 28 + 31;
			if (month == 4)							// April 
				numdays = numdays + 31 + 28 + 31;
			if (month == 5)							// May 
				numdays = numdays + 30 + 31 + 28 + 31;
			if (month == 6)							// June 
				numdays = numdays + 31 + 30 + 31 + 28 + 31;
			if (month == 7)							// July 
				numdays = numdays + 30 + 31 + 30 + 31 + 28 + 31;
			if (month == 8)							// August 
				numdays = numdays + 31 + 30 + 31 + 30 + 31 + 28 + 31;
			if (month == 9)							// September 
				numdays = numdays + 31 + 31 + 30 + 31 + 30 + 31 + 28 + 31;
			if (month == 10)							// October						
				numdays = numdays + 30 + 31 + 31 + 30 + 31 + 30 + 31 + 28 + 31;
			if (month == 11)							// November
				numdays = numdays + 31 + 30 + 31 + 31 + 30 + 31 + 30 + 31 + 28 + 31;
			if (month == 12)							// December
				numdays = numdays + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30 + 31 + 28 + 31;
		}

	}
	int daycode = (numdays + 1) % 7;
	return daycode;
}

void printheader(int month, int year)
	{
			printf("%14d %1d\n", month, year);
			printf("Sun ");
			printf("Mon ");
			printf("Tue ");
			printf("Wed ");
			printf("Thu ");
			printf("Fri ");
			printf("Sat\n");
		}

int getndim(int month, int year)
{
	int numdays;
	if (((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0))		//check if leapyear
	{
		if (month == 1)							// January 
			numdays = 31;
		if (month == 2)							// February 
			numdays = 29;
		if (month == 3)							// March 
			numdays = 31;
		if (month == 4)							// April 
			numdays = 30;
		if (month == 5)							// May 
			numdays = 31;
		if (month == 6)							// June 
			numdays = 30;
		if (month == 7)							// July 
			numdays = 31;
		if (month == 8)							// August 
			numdays = 31;
		if (month == 9)							// September 
			numdays = 30;
		if (month == 10)							// October						
			numdays = 31;
		if (month == 11)							// November
			numdays = 30;
		if (month == 12)							// December
			numdays = 31;
	}
	else
	{
		if (month == 1)							// January 
			numdays = 31;
		if (month == 2)							// February 
			numdays = 28;
		if (month == 3)							// March 
			numdays = 31;
		if (month == 4)							// April 
			numdays = 30;
		if (month == 5)							// May 
			numdays = 31;
		if (month == 6)							// June 
			numdays = 30;
		if (month == 7)							// July 
			numdays = 31;
		if (month == 8)							// August 
			numdays = 31;
		if (month == 9)							// September 
			numdays = 30;
		if (month == 10)							// October						
			numdays = 31;
		if (month == 11)							// November
			numdays = 30;
		if (month == 12)							// December
			numdays = 31;
	}
	return numdays;
}

Answer 1

Your code has several issues:

• Your calculation for daycode is off by one.
• You do not use daycode, not nummonths in main(), resulting in all months appearing to start on a Sunday.

Here is a suggestion:

int main(void) {
    int  day, month, year, nummonths;
    printf("Enter Month, Year, and number of Months: ");
    if (scanf("%d %d %d", &month, &year, &nummonths) != 3
    ||  year <= 0 || month < 1 || month > 12) {
        printf("invalid input\n");
        return 1;
    }

    for (int i = 0; i < nummonths; i++) {
        printheader(month, year);
        int numdays = getndim(month, year);
        int daycode = getdaycode(month, year);

        printf("%*s", daycode * 4, "");   /* print 4 spaces for each skipped day */
        for (day = 1; day <= numdays; day++) {
            printf("%3d", day);
            daycode = (daycode + 1) % 7;
            if (daycode != 0)
                printf(" ");
            else
                printf("\n");
        }
        if (daycode != 0)
            printf("\n");
        printf("\n");

        month = month + 1;
        if (month > 12) {
            month -= 12;
            year += 1;
        }
    }
    return 0;
}

You also need to fix getdaycode by changing the formula to compute it from numdays at the end of the function:

            ...
        }
        int daycode = (numdays + 1) % 7;
        return daycode;
    }
}

If you have learned arrays already, your code could be simplified a lot:

static int const days[12] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
static int const leapdays[12] = { 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };

int getdaycode(int month, int year) {
    int numdays = 0;
    if (year > 0) {
        // how many days in previous years including exceptions
        numdays = (year - 1) * 365 + (year - 1) / 4 - (year - 1) / 100 + (year - 1) / 400;
    }
    if (month >= 1 && month <= 12) {
        if (month > 2 && (((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0)))
            numdays += 1;  // count february 29
        for (int i = 0; i < month - 1; i++)
            numdays += days[i];
    }
    // need to offset by one so 1/1/1 falls on a Monday.
    // dates before 1753 are computed according to the
    // proleptic Gregorian calendar.
    return (numdays + 1) % 7;
}

int getndim(int month, int year) {
    if (month < 1 || month > 12)
        return 0;
    else
    if ((year % 4 == 0 && year % 100 != 0) || (year % 400 == 0))
        return leapdays[month - 1];
    else
        return days[month - 1];
}

Answer 2

I did something similar recently because I wanted to know the number of Tuesdays in March, April, and May. So I wrote this code

#include <stdio.h>

int main( void )
{
    char *monthNames[] = { "March", "April", "May" };

    printf( "Enter starting day (0-6): " );
    int day;
    if ( scanf( "%d", &day ) != 1 )
        return 1;

    day = -(day % 7);
    for ( int m = 0; m < 3; m++ ) {
        printf( "\n%s\n", monthNames[m] );
        int maxday = (m == 1) ? 30 : 31;

        while ( day < maxday ) {
            for ( int col = 0; col < 7; col++ ) {
                if ( day < 0 )
                    printf( "   " );
                else if ( day < maxday )
                    printf( " %2d", day+1 );
                day++;
            }
            printf( "\n" );
        }

        if ( day > maxday )
            day = day - maxday - 7;
        else
            day = 0;
    }
}

The general idea is the variable day starts as a number between -6 and 0. As long as day is negative, the code prints three spaces. When the day is between 0 and the number of days in the month, the code prints a number. At the end of the month, the starting value for day is computed for the next month, based on where the previous month ended.