The following simple Ada program prints a month in the form:

 SUN MON TUE WED THU FRI SAT
           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

It uses a simple process which asks the user the number of days in the month and the starting day of the week. The program demonstrates the overloading of packages in Ada, enumerations, types, subtypes, and output.

Here is the algorithm for displaying the month:

Output the heading with the names of the days of the week.
Output the initial blanks.
Set day = the first day input by the user.
loop date in 1 to number_of_days_in_the_month
   output(date)
   if (day = Saturday) then
      insert a new line
      day = Sunday
   else
      day = next day
   end
end loop
   

Here is the program written in Ada:

with text_IO; use text_io;

procedure month is

   subtype month_days is positive range 1..31;
   type week_days is (Sun,Mon,Tue,Wed,Thu,Fri,Sat);
   package positive_io is new integer_io(positive);
   use positive_io;
   package day_io is new enumeration_io(week_days);
   use day_io;
   answer: character;
   firstday: week_days;
   numberofdays: month_days;

   procedure get_number_of_days(nd: out month_days) is
   begin
      put("Enter the number of days in the month: ");
      new_line;
      get(nd);
      skip_line;
   end get_number_of_days;

   procedure get_first_day(fd: out week_days) is
   begin
      put("Enter the first day of the new month: ");
      put("e.g. Sun, Mon, etc.");
      new_line;
      get(fd);
      skip_line;
   end get_first_day;

   procedure display_month(nd: in month_days; fd: in week_days) is
      day: week_days := fd;
      four_blanks: constant string := "    ";
      width : constant integer := 4;
   begin
      for day in week_days loop
         put(' ');
         put(day);
      end loop;
      new_line;
      for blank_days in Mon..fd loop
         put(four_blanks);
      end loop;
      for date in 1..nd loop
         put(date,width);
         if day = Sat then
            day := Sun;
            new_line;
         else
            day := week_days'succ(day);
         end if;
      end loop;
      new_line;
   end display_month;

begin
   loop
      get_number_of_days(numberofdays);
      get_first_day(firstday);
      display_month(numberofdays,firstday);
      put("Do you wish to see another month? "); new_line;
      put("yes(y) or no(n): "); new_line;
      get(answer); skip_line;
      exit when answer = 'n' or answer = 'N';
   end loop;
end month;

You will notice on Line 9, the redefinition of the package enumeration_io to permit the output of the headings, and the input of the first day of the month in an intuitive manner. Also a subtype, month_days, is created on Line 5 of the program which helps ensure proper input by constraining the values allowed in month_days.

The program repeatedly prompts the user for information, and prints the associated month. Here is the programming running:

Enter the number of days in the month:
31
Enter the first day of the new month: e.g. Sun, Mon, etc.
Wed
 SUN MON TUE WED THU FRI SAT
               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
Do you wish to see another month?
yes(y) or no(n):
n

What happens if the number of days entered by the user is outside the constraints set? The program will raise a constraint error of the form:

raised CONSTRAINT_ERROR : month.adb:19 range check failed

But this can be fixed by modifying the user input routines. Here is how the procedure get_number_of_days() has been fixed.

   procedure get_number_of_days(nd: out month_days) is
      monthlen : month_days;
   begin
      loop
         begin
            put("Enter the number of days in the month: ");
            new_line;
            get(monthlen);
            skip_line;
            if monthlen in 28..31 then
               exit;
            else
               raise constraint_error;
            end if;
         exception
            when others =>
               skip_line;
               put("Bad input. Enter a value between 28-31.");
               new_line;
         end;
      end loop;
      nd := monthlen;
   end get_number_of_days;

On Lines 10-14 there is an if statement which determines if the input is valid. Input outside of the values 28-31 raises a constraint_error, but this time it is dealt with by the procedure on Lines 15-19 (instead of the system). If an exception is raised, then a message is output to the user, and in this case the loop iterates again. If the input is valid, the input loop is exited. This is a good example of how easy it is to deal with various exceptions in Ada. Here is a sample of the program running with invalid input:

Enter the number of days in the month:
42
Bad input. Enter a value between 28-31.
Enter the number of days in the month: