Printing a Julian Calendar in C using month and year input? - c

Question

I'm trying to output a calendar in C and I think I've got the format down, but I'm having trouble with:
Validating user input (I've commented out the potential solution, but it hasn't compiled correctly yet. It always prints the "invalid year/month selected" and the break statement doesn't work.)
Starting the calendar days on the correct day of the week (May of 2018 starts on Tuesday, not on Sunday)
My current output:
Enter month: 5
Enter year: 2018
Su M Tu W Th F Sa
1 2 3 4 5 6 7
8 9 10 11 12 13 14
15 16 17 18 19 20 21
22 23 24 25 26 27 28
29 30 31
My desired output:
Enter month: 5
Enter year: 2018
Su M Tu W Th F Sa
1 2 3 4 5
6 7 8 9 10 11 12
13 14 15 16 17 18 19
20 21 22 23 24 25 26
27 28 29 30 31
Here's my program:
#include <stdio.h>
#include <stdlib.h>
/* #defines */
#define BEGIN 1900
#define DAYS_IN_WEEK 7
/* function prototypes */
void getMonthYear(int *month, int *year);
int toJulian(int month, int day, int year);
int daysInMonth(int month, int year);
int leapYear(int year);
long yearsToDays(int year);
void printCalendar(int startDay, int numDays);
void printHeader();
/* Calendar.c: Prints monthly calendar. Lab 4 / Week 9 */
void main(void)
{
int month, year;
int startDay; // what day is first day of month. 1/1/1900 was Monday, so...
// day 1 is Mon, day 2 is Tue, ... , day 0 is Sun
getMonthYear(&month, &year);
startDay = (toJulian(month, 1, year) + yearsToDays(year)) % DAYS_IN_WEEK;
printCalendar(startDay, daysInMonth(month, year));
}
void getMonthYear(int *month, int *year) {
printf("Enter month: ");
scanf("%d", month);
/*if (month < 1 || month > 12) {
printf("Invalid month selected");
//break;
}*/
printf("Enter year: ");
scanf("%d", year);
/* if (year < BEGIN) {
printf("Invalid year selected");
//break;
}*/
printf("\n");
}
int toJulian(int month, int day, int year) {
int count;
for(count = 1; count < month; ++count) {
day += daysInMonth(month, year);
}
return day;
}
int daysInMonth(int month, int year) {
int numDays;
switch (month) {
case 1: numDays = 31;
break;
case 2: numDays = 28;
break;
case 3: numDays = 31;
break;
case 4: numDays = 30;
break;
case 5: numDays = 31;
break;
case 6: numDays = 30;
break;
case 7: numDays = 31;
break;
case 8: numDays = 31;
break;
case 9: numDays = 30;
break;
case 10: numDays = 31;
break;
case 11: numDays = 30;
break;
case 12: numDays = 31;
break;
}
return numDays;
}
int leapYear(int year) {
if (year % 400 == 0 && (year % 100 != 0 || year % 400 == 0)) {
return 1;
}
else {
return 0;
}
}
long yearsToDays(int year) {
int count;
long days;
for (count = BEGIN; count < year; ++count) {
days = 365 + leapYear(year);
}
return days;
}
void printCalendar(int startDay, int numDays) {
int dayid;
printHeader();
// Shifts position for the first date... sort of
for ( dayid = 0; dayid < startDay; dayid++ ) {
printf(" ");
}
// Supposedly prints all the dates for one month
for ( dayid = 1; dayid <= numDays; dayid++ ) {
printf("%3d", dayid );
// If the day is not before Saturday, start next line on Sun
if ( ( dayid + startDay ) % DAYS_IN_WEEK > 0 ) {
printf(" ");
}
else {
printf("\n" );
}
}
}
void printHeader() {
printf(" Su M Tu W Th F Sa\n");
}

Answer 1

break is used for exiting loops or switch statements. It can only be used inside a loop inside the current function scope.
So even if getMonthYear() were called inside a loop in main() (which it isn't), a break inside it will not exit that loop.
As it is, you have no loop at all, neither in getMonthYear(), nor in the calling sequence leading to it, so the break would be a no-op.
You have to ask yourself "what do I actually want to do when they make an invalid entry?"
One possibility would be to return the validity from getMonthYear()
bool valid_input = 0;
while (!valid_input) {
valid_input = getMonthYear(&month, &year);
}
To deal with the problem of getting the day correct, you probably need to do a little more debugging yourself: put print statements in all the places where there are calculations, print out the intermediate results, and find out where you have an error that way.

Answer 2

You have errors in the following functions:
toJulian
int toJulian(int month, int day, int year) {
int count;
for(count = 1; count < month; ++count) {
day += daysInMonth(month, year);
}
return day;
}
You are using the month in every call. You need to use count.
You don't need the input argument day. You can have a local variable that is initialized to 0 before the loop starts.
The updated version:
int toJulian(int month, int year) {
int count;
int days = 0;
for(count = 1; count < month; ++count) {
days += daysInMonth(count, year);
}
return days;
}
leapYear.
int leapYear(int year) {
if (year % 400 == 0 && (year % 100 != 0 || year % 400 == 0)) {
return 1;
}
else {
return 0;
}
}
The logic in the if statement is not correct. It needs to be:
(year % 400 == 0 || (year % 100 != 0 && year % 4 == 0))
You can make the function to be more readable and less error prone by using:
int leapYear(int year) {
int ret = 0;
if ( year % 4 != 0 )
{
ret = 0;
}
else
{
if ( year % 100 != 0 )
{
ret = 1;
}
else
{
ret = (year % 400 == 0);
}
}
return ret;
}
yearsToDays
long yearsToDays(int year) {
int count;
long days;
for (count = BEGIN; count < year; ++count) {
days = 365 + leapYear(year);
}
return days;
}
You have not initialized days.
You are not accumulating the number of days. You are just assigning a value. The last returned value gets returned instead of the accumulated number of days.
You are using year in the loop instead of count in the argument to leapYear.
Here's the updated version:
long yearsToDays(int year) {
int count;
long days = 0;
for (count = BEGIN; count < year; ++count) {
days += 365;
days += leapYear(count);
}
return days;
}
printCalendar
void printCalendar(int startDay, int numDays) {
int dayid;
printHeader();
// Shifts position for the first date... sort of
for ( dayid = 0; dayid < startDay; dayid++ ) {
printf(" ");
}
// Supposedly prints all the dates for one month
for ( dayid = 1; dayid <= numDays; dayid++ ) {
printf("%3d", dayid );
// If the day is not before Saturday, start next line on Sun
if ( ( dayid + startDay ) % DAYS_IN_WEEK > 0 ) {
printf(" ");
}
else {
printf("\n" );
}
}
}
You are using up four spaces when printing the days of the month - "%3d" and " ". You need to write four spaces for each day up to startDay in the first for loop. Otherwise, the first row of output won't be properly aligned.
You need to use:
for ( dayid = 0; dayid < startDay; dayid++ ) {
printf(" ");
}
main
The line
startDay = (toJulian(month, 1, year) + yearsToDays(year)) % DAYS_IN_WEEK;
should be
startDay = (1 + toJulian(month, year) + yearsToDays(year)) % DAYS_IN_WEEK;
to account for (1) the fact that 1900-Jan-01 falls on a Monday and (2) change to the interface of toJulian.