same dayofweek for a given year in delphi-CodePudding

I'm using Delphi 11 with the new DateUtils library but i'm not able to get a date of same day in the past, for a given number of years. i must retirn the same day of week for a given date of n years ago. so if today is sunday i must return the same sunday of n years ago. i added the complete source of a real test application written in delphi with a bug fixed

unit Unit1;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, Vcl.ComCtrls, system.DateUtils, system.Types;

type
  TForm1 = class(TForm)
    dtDataOggi: TDateTimePicker;
    Label1: TLabel;
    edYears: TEdit;
    bnCalcola: TButton;
    procedure bnCalcolaClick(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
    function annobisestile(value : tDate) : integer;
  end;


var
  Form1: TForm1;

implementation

{$R *.dfm}


procedure TForm1.bnCalcolaClick(Sender: TObject);
var
   DataAppoggio : tDate;
   NumberOfYearToSubtract : integer;
   annobis : integer; // is leap year?
   annoappoggio : integer;
   giorno : boolean;
   febb28 :tDate;
   i : integer;
begin
   DataAppoggio := dtDataOggi.date;
   NumberOfYearToSubtract := strToInt(edYears.Text);
   annobis := 0;
   annoappoggio := yearOf(DataAppoggio);
   febb28 := encodedate(annoappoggio ,2,28);

  if CompareDate(febb28,DataAppoggio) = GreaterThanValue then
     Giorno := true
  else
     Giorno := false;

     DataAppoggio := IncYear(DataAppoggio, -NumberOfYearToSubtract);
     // check if adataappoggio is before feb 28  so i must add a day
     if giorno then  begin
        for I := 0 to NumberOfYearToSubtract  do begin
           //annobiststile is a custom functions that returns 1 if the given year is a leap year
           annobis := annobis   annobisestile(encodeDate(annoappoggio,01,01));
           annoappoggio := annoappoggio -1;
        end;
     end
     else begin
        for I := 0 to NumberOfYearToSubtract -1  do begin
           annobis := annobis   annobisestile(encodeDate(annoappoggio,01,01));
           annoappoggio := annoappoggio -1;
        end;
     end;
     label1.Caption  :=  incDay(DataAppoggio, NumberOfYearToSubtract annobis).ToString;
 end;

 function TForm1.annobisestile(value: tDate): integer;
begin
   if IsInLeapYear(value) then
      result := 1
   else
      result := 0;
end;

this functions return the exact day of a given numbers of past years but if this years are 3 or more, it does not mattch the same WeekOfTheYear. any idea? thank you.

CodePudding user response：

You're overcomplicating things. You don't need function annobisestile and you can replace TForm1.bnCalcolaClick with the following method. In a nutshell, this subtracts the specified number of years, then adjusts the date by the number of days that it's off the original day of the week.

(This was done with 10.4, which doesn't have the new TDateTime helpers, so I've used DateToStr() instead of .toString.)

procedure TForm1.bnCalcolaClick(Sender: TObject);
var
   DataAppoggio : tDate;
   NumberOfYearToSubtract : integer;
   DOW : Integer;
begin
   DataAppoggio := dtDataOggi.date;
   NumberOfYearToSubtract := strToInt(edYears.Text);
   DOW := DayOfWeek(DataAppoggio);

   DataAppoggio := IncYear(DataAppoggio,-NumberOfYearToSubtract);
   DataAppoggio := IncDay(DataAppoggio,DOW-DayOfWeek(DataAppoggio));
   label1.Caption  :=  DateToStr(DataAppoggio);
end;

CodePudding user response：

The solution is straight forward if you need the date of same week number and same day of the week n years before (or later). The System.DateUtils unit has all necessary functions.

First use functions YearOf(), WeekOf() and DayOfTheWeek() to get InputYear, InputWeek and InputDOW (day-of-week) from the InputDate.

Then use the EncodeDateWeek() function to convert InputYear YearsToAdd, InputWeek, InputDOW to a TDateTime;

A complete sample code follows:

procedure TForm2.Button3Click(Sender: TObject);
var
  InputDate: TDate;
  InputYear: word;
  InputWeek: word;
  InputDOW:  word;
  YearsToAdd: word;
  dt: TDate;
begin
  InputDate := dtDataOggi.date;
  InputYear := YearOf(InputDate);
  InputWeek := WeekOf(InputDate);
  InputDOW  := DayOfTheWeek(InputDate); // ISO 8601 Monday is first dow,
                                        // use DayOfWeek() for Sunday as first dow
  YearsToAdd := strToInt(edYears.Text); // use '-' in input for subtraction

  dt := EncodeDateWeek(InputYear   YearsToAdd, InputWeek, InputDOW);

  Memo1.Lines.Add(DateToStr(InputDate));
  Memo1.Lines.Add(DateToStr(dt));
end;