Headings for 13 months on Array - arrays

Question

I was wondering if someone can assist me, why the headers of the below code is not creating them correctly for 13 months. I would like to add Jan(LY) Feb(LY)......until Jan(TY).
Please see below Heading code only,
//Headings
J:=0;
for I:=12 downto 0 do
begin
J:=J+1;
Amonth:=IntToStr(J);
If basemonth-12 <=0 then Harray[J]:=Basemonth-I+13;
If basemonth-12 > 0 then Harray[J]:=Basemonth-I;
If Harray[J] >13 then Harray[J]:=Harray[J]-13;
//showmessage(INTTOSTR(Harray[J]));
end;
Heading1:=Monthcalc(Harray[1]);
Heading2:=Monthcalc(Harray[2]);
Heading3:=Monthcalc(Harray[3]);
Heading4:=Monthcalc(Harray[4]);
Heading5:=Monthcalc(Harray[5]);
Heading6:=Monthcalc(Harray[6]);
Heading7:=Monthcalc(Harray[7]);
Heading8:=Monthcalc(Harray[8]);
Heading9:=Monthcalc(Harray[9]);
Heading10:=Monthcalc(Harray[10]);
Heading11:=Monthcalc(Harray[11]);
Heading12:=Monthcalc(Harray[12]);
Heading13:=Monthcalc(Harray[13]);
// showmessage(DateToStr(startdate));
// showmessage(DateToStr(enddate));
// Showmassage('test');
end;
Function Monthcalc(Amonth:integer):String;
begin
Monthname[1]:='Jan';
Monthname[2]:='Feb';
Monthname[3]:='Mar';
Monthname[4]:='Apr';
Monthname[5]:='May';
Monthname[6]:='Jun';
Monthname[7]:='Jul';
Monthname[8]:='Aug';
Monthname[9]:='Sep';
Monthname[10]:='Oct';
Monthname[11]:='Nov';
Monthname[12]:='Dec';
Monthname[13]:='LY';
Result:=Monthname[Amonth];
// showmessage(DateToStr(startdate));
// showmessage(DateToStr(enddate));
// Showmassage('test');
end;

Answer 1

May I first suggest that you change month name array to a constant:
const
MonthNames: array[1..12] of string = ('Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec');
Next I suggest some changes to avoid a hardcoded time span of 13 months, because as you are now experiencing, requirements do change and hardcoded stuff is always harder to change than variable stuff.
Declare a variable e.g. MonthSpan: integer to indicate how many months should be included (in the future a new requirement of alternative timespans of 3 months, 6 months etc are possible). Also replace all Heading1, Heading2 ... variables with a dynamic array, named Headings:
Monthspan: integer;
Headings: array of string;
For now I just initialize these at the beginning
Monthspan := 13;
SetLength(Headings, Monthspan);
You have already previously calculated StartDate: TDate so using that and the existing variable M we can now write a simple loop for getting the headings:
M := MonthOf(StartDate);
for i := 0 to MonthSpan-1 do
begin
Headings[i] := MonthNames[(M + i) mod 12];
Memo1.Lines.Add(Headings[i]);
end;
MonthOf() is a function in unit System.DateUtils.
The above replaces Function MonthCalc(), and all of your code below the //Headings comment. Elsewhere in your code, where you used e.g. Heading1 you now use Headings[0] (dynamic arrays are always indexed from 0 up) etc.
I'm not sure what the purpose of HArray[] is, but it is not needed for the determination of the headings.