Calendar - C Program

Calendar

How to create a Calendar ?

Creating a calendar is quite easy. Consider the following facts first.

• A normal year has 365 days.
• A leap year has 366 days.
• Also 365%7=1 while 366%7==2
• So normally if January1 is a Monday for a year x for the next year(x+1) January 1 will be Tuesday. However if year x is a leap year then for the next year January 1 will be a Wednesday.

To create a Calendar Program we need to subdivide our problem:-
1>Finding the day for the Jan 1 of any year.
2>Finding the day for First of the Month for this year.Use a leap correction function so as to correct the first of the month day.(In leap year after feb the day needs to be shifted by one again).
3>Decide on how u wish to display the data.And write Graphics accordingly.

This calendar will work for years greater than 1600. If u wish to find for days less than the year 1601 the diff variable will become negative. Then modulus will become complicated and logic will become hard to understand.

Solution-Part 1 Jan1

1. We firstly need to select a base year for which we know the day for Jan1.
2. Lets select 1600 as base year.
3. Assign values 0 for Monday, 1 for Tuesday and so on. Call this as the day value.
4. Lets consider User gives input year =yearx.
5. Let diff=(yearx-1600)%400(The calendar repeats after every 400 years)
6. day=(diff+day)%7 (We are only interested in finding the day).
7. Now we need to find the correction for the leap years
8. leap=diff/4
9. leap100=diff/100
10. leapcorrection=leap-leap100
11. day=day+leapcorrection+1(since 1600 is leap year)
12. day=day%7
13. This gives day for Jan1 of that year.

Solution-Part 2 Month first day

1. This is pretty simple. Just find out the difference between the first of the month and Jan1.And find difference%7.Add this difference to the day value that was previously calculated and perform a %7 operation.
2. For leap year add an extra day if month is after Feb.
3. This will give you the Month first day.

Solution part -3 Graphics

1. Just remember how many days are to be displayed.
2. February is a tricky case
3. Decide on the interface to be used.(I have used text based graphics, this was one of my earliest programs ,:-) ).

Code

1. This code and compiled .exe file works perfectly on Windows 32 bit Operating System. For 64 bit  u might have to recompile the file or try compability mode.
2. This code uses text based Graphics. The weird numbers in the code are ascii value for characters. 
3. This code works with directional keys. Directional keys can be used to change the year or month and update the display immediately.
4. You can also download this code in .c format along with compiled .exe file at the following mediafire link:-http://www.mediafire.com/?g71862zc76c0w

#include<stdio.h>
#include<conio.h>
#include<string.h>
int getkey();void fmonthname(int m);
char *monthname[12];
int day=0,leap,fstart,fend;
void main()
{
int scankey,year,month,jan1=0,i,j,k;
int montharray[42];
clrscr();
puts("Calendar_v1.0-The portable lite .exe calendar");
puts("A prg by Abhishek Munagekar");
puts("This is a free version and supports 1700-8000 yrs only.");
puts("For bug reports contact avm.abhishek@gmail.com");
puts("Press any key to continue");
scankey=getkey();
clrscr();
yearmonth:
puts("type the year and month as shown below");
puts("yyyy<space>mm");
scanf("%d%d",&year,&month);
clrscr();
actualc:
if(year<1700||year>8000||month<1||month>12)
{
printf("Syntax error, fatal error re-enter all values");
goto yearmonth;
}
/* core function begins*/
for(i=0;i<=41;i++)
montharray[i]=0;


/*leap chkr begins*/
leap=0;
if((year%4)==0)
leap=1;
if((year%100)==0)
leap=0;
if((year%400)==0)
leap=1;
/*leap chkr ends*/
/*sets month name and day count*/
day=0;
fmonthname(month);
/*sets month name and day count above*/
/*JAn1day seterbelow*/
jan1=0;
jan1=((year-1601)+((year-1601)/4)-((year-1601)
/100)+((year-1601)/400))%7;
/*leap corrector below*/
if(leap==1&&month>2)
day=day+1;
/*final start day decider below*/
fstart=(day+jan1)%7;
/*array data feeder starts*/
for(i=0;i<fend;i++)
{
montharray[i+fstart]=i+1;
}
/*array data feeder ends*/
/*basic info display below*/
printf("\nPrg has captured the following month:-\n");
printf("%d%d/%d\n",month/10,month%10,year);
printf("%c%c%c%c%c%c%c\n",196,196,255,196,196,
196,196);
printf("Displaying %s,%d\n",*monthname,year);
/*display of calendar begins-1st line below*/
printf("\n%c",218);
for(i=1;i<=20;i++)
printf("%c",196);
printf("%c",191);
/*2 line now*/
printf("\n%cMo Tu Wd Th Fr Sa Su%c",179,179);
/*core display lines now*/
j=0;
for(i=1;i<=6;i++)
{
printf("\n%c",179);
for(k=1;k<=6;k++)
{
if(montharray[j]==0)
printf("%c%c%c",255,255,255);
else
printf("%d%d%c",montharray[j]/10,montharray[j]
%10,255);
j=j+1;
}
if(montharray[j]!=0)
printf("%d%d%c",montharray[j]/10,montharray[j]
%10,179);
else
printf("%c%c%c",255,255,179);
j=j+1;
}
/*finish display lines below*/
printf("\n%c",192);
for(i=1;i<=20;i++)
printf("%c",196);
printf("%c\n",217);
/*display of calendar ends*/
/*new control structure begins*/
puts("\npress any control 2 continue");
puts("escape key :- termination");
puts("left/right arrow key:- previous/next month");
puts("up/down arrow key:- next/previous yr same month");
puts("D letter:- To renter date");
scankey=getkey();
if(scankey==1)
{
clrscr();
printf("Termination sequence requested by user");
goto terminator;
}
else if(scankey==75)
{
clrscr();
month=month-1;
if(month==0)
{
month=12;
year=year-1;
}
goto actualc;
}
else if(scankey==77)
{
clrscr();
if(month==12)
{
month=1;
year=year+1;
}
else
month=month+1;
goto actualc;
}
else if(scankey==72)
{
clrscr();
year=year+1;
goto actualc;
}
else if(scankey==80)
{
clrscr();
year=year-1;
goto actualc;
}
else if(scankey==32)
{
clrscr();
printf("\nDirecting to reeenter values...");
goto yearmonth;
}
else
{
clrscr();
goto actualc;
}
/*terminator*/
terminator:
puts("Hope u liked the program");
puts("For bug reports mail me:-avm.abhishek@gmail.com");
getch();
}
#include"dos.h"
getkey()
{
union REGS i,o;
while(!kbhit())
;
i.h.ah=0;
int86 (22,&i,&o);
return(o.h.ah);
}
void fmonthname(int m)
{
switch (m)
{
/*switch bracket above*/
case 1:
*monthname="January";
fend=31;
break;
case 2:
*monthname="February";
day=day+31;
fend=28;
if(leap==1)
fend=29;
break;
case 3:
*monthname="March";
day=day+59;
fend=31;
break;
case 4:
*monthname="April";
day=day+90;
fend=30;
break;
case 5:
*monthname="May";
day=day+120;
fend=31;
break;
case 6:
*monthname="June";
day=day+151;
fend=30;
break;
case 7:
*monthname="July";
day=day+181;
fend=31;
break;
case 8:
*monthname="August";
day=day+212;
fend=31;
break;
case 9:
*monthname="September";
day=day+243;
fend=30;
break;
case 10:
*monthname="October";
day=day+273;
fend=31;
break;
case 11:
*monthname="November";
day=day+304;
fend=30;
break;
case 12:
*monthname="December";
day=day+334;
fend=31;
break;
}
/*month name is now set*/
}
/*fmonthname function ends here*/