Ada - Days of the week - ada

Question

I'm supposed to make a subprogram that has exactly one parameter of type Integer. And this Integer represents a day of the week where 1 is monday, 2 is tuesday etc...
The subprogram should return how many days there's left until Saturday.
This is my approach (I'm not supposed to use arrays):
with Ada.Text_IO; use Ada.Text_IO;
with Ada.Integer_Text_IO; use Ada.Integer_Text_IO;
procedure Test3 is
function Week (Days : in Integer) return Integer is
begin
if Days = 1 then
return 5;
elsif Days = 2 then
return 4;
elsif Days = 3 then
return 3;
elsif Days = 4 then
return 2;
elsif Days = 5 then
return 1;
elsif Days = 6 then
return 7;
elsif Days = 7 then
return 6;
end if;
end Week;
Days : Integer;
begin
Put("What day is it: ");
Get(Days);
if Days < 0 or Days > 7 then
Put_Line("Wrong Value");
end if;
Put("It's ");
Put(Week(Days), Width => 1);
Put(" days until Saturday.");
end Test3;
Even though my program works I have two problems. Do you see the amount of "elsifs", can this not be solved in any other way? I don't like how my program looks.
Secondly,
How am I supposed to make an if statement where if Days < 0 or Days > 7 will raise an error. As you can see I have attempted to this in my main program but it won't work since I have
"Get(Days);" before this statement and my subprogram won't recognize my if statement? Should I put my if statement in my subprogram instead?
Any help is appreciated.

Answer 1

With respect to raising error, how about
if Days not in 1 .. 7 then
Put_Line("Wrong Value");
raise Contrain_Error with "Days is out of range, " & Days'Image;
else
-- normal processing
end if;

Answer 2

Regarding the question of what to do with invalid Days, that is for you to decide, and then you code the subprogram accordingly. I think the most natural, for Ada, would be to put the check in the subprogram and raise Constraint_Error if it fails.
As Simon Wright said, you must decide and specify (and describe in comments) how the week-days are numbered. Otherwise no-one can check your code.
Once you have clearly decided and described those things, for yourself and for any reader of the program, you can have a closer look at your if-then-else nest and see how systematically the return value depends on the input value. Maybe there is a simple formula that can handle most cases, or perhaps all cases with valid input? That would let you reduce the number of if-then-elses.

Answer 3

It appears you allow day numbers in 1 .. 7. You can state this directly
subtype Day_Number is Integer range 1 .. 7;
function Days_Till_Saturday (Day : in Day_Number) return Day_Number;
You can then omit the check on the input value; passing an invalid value to the function will raise Constraint_Error, which you can handle to report the error.
Regarding the long chain of elsifs, you can use a case statement:
case Day is
when 1 =>
return 5;
but for this problem it might be clearer to use arithmetic:
Result : constant Integer := 6 - Day;
begin
if Result not in Day_Number then
return Result + Day_Number'Last;
end if;
return Result;