How to sort an array of dates (TDate) descending? - arrays

Question

I have an array[0..99] of TDate and need to sort it, in descending order. I can't find an example anywhere, not even here on StackOverflow...
Btw, not all array items have a value. Most of the time, the array is filled with 10 to 30 values. I do not need a "super fast" way to do this, but the simplest / easier way...

Answer 1

I don't know why you have an array of TDates, rather than using a TList that Delphi provides for you, but I'll assume you are making some sort of test project, and you just need a quick and dirty way to sort your array. Here's a way to do it that will be more than quick enough for such a small array
program Project1;
{$APPTYPE CONSOLE}
{$R *.res}
uses
System.SysUtils, DateUtils;
var
A : array[0..99] of TDateTime;
T : TDateTime; //Used to exchange dates
I,J : integer;
begin
//Initialize with some test data:
for I := 0 to 99 do
A[I] := DateOf(Now) - 365 + random(365);
//Output the unsorted list:
for I := 0 to 99 do
writeln(DateToStr(A[I]));
readln;
//Here's our sorting algorithm (Bubble Sort, probably the easiest sorting algorithm to
//understand, and the quickes to implement - but the worst sorting algorithm in actual use
for I := 0 to 98 do //One less than the max index, but you really should use some constants
for J := I+1 to 99 do //Up to the max index
if A[I] < A[J] then //Change < to > to sort in ascending order
begin
T := A[I]; //We'll be overwriting A[I], so we should store the value
A[I] := A[J]; //Here we overwrite A[I] with A[J]
A[J] := T; //And here we put T back in the new position
end;
//Output the sorted list:
for I := 0 to 99 do
writeln(DateToStr(A[I]));
readln;
end.
I would really recommend using some other data structure (TList being the most obvious choice). Here's a way to do that:
program Project1;
{$APPTYPE CONSOLE}
{$R *.res}
uses
System.SysUtils, DateUtils, generics.collections;
var
A : TList<TDate>;
I : integer;
T : TDate;
begin
A := TList<TDate>.Create;
try
for I := 0 to 99 do
A.Add(DateOf(Now) - 365 + random(365));
A.Sort; //Sorts in ascending order by default
A.Reverse; //But you wanted descending order, so we'll reverse the list
for T in A do
writeln(DateToStr(T));
readln;
finally
A.Free;
end;
end.