Delphi - What object (multidimensional array, etc) will work? - delphi

Question

I have a need to keep the top ten values in sorted order. My data structure is:
TMyRecord = record
Number: Integer;
Value: Float;
end
I will be calculating a bunch of float values. I need to keep the top 10 float values. Each value has an associated number. I want to add "sets"... If my float Value is higher than one of the top 10, it should add itself to the list, and then the "old" number 10, now 11, gets discarded. I should be able to access the list in (float value) sorted order...
It is almost like a TStringList, which maintains sorted order....
Is there anything like this already built into Delphi 2010?

Answer 1

You can use a combination of the generic list Generics.Collections.TList<TMyRecord> and insertion sort.
Your data structure is like this
TMyRecord = record
Number: Integer;
Value: Float;
end;
var
Top10: TList<TMyRecord>;
You'll need to use Generics.Collections to get the generic list.
Instantiate it like this:
Top10 := TList<TMyRecord>.Create;
Use this function to add to the list:
procedure Add(const Item: TMyRecord);
var
i: Integer;
begin
for i := 0 to Top10.Count-1 do
if Item.Value>Top10[i].Value then
begin
Top10.Insert(i, Item);
Top10.Count := Min(10, Top10.Count);
exit;
end;
if Top10.Count<10 then
Top10.Add(Item);
end;
This is a simple implementation of insertion sort. The key to making this algorithm work is to make sure the list is always ordered.

Answer 2

David's answer is great, but I think as you progress through the data, you'll fill the list pretty fast, and the odds of having a value greater than what's already in the list probably decreases over time.
So, for performance, I think you could add this line before the for loop to quickly discard values that don't make it into the top 10:
if (Item.Value <= Top10[Top10.Count - 1].Value) and (Top10.Count = 10) then
Exit;
If the floats are always going to be above a certain threshold, it might make sense to initialize the array with 10 place-holding records with values below the threshold just so you could change the first line to this:
if (Item.Value <= Top10[9].Value) then
Exit;
And improve the method to this:
procedure Add(const Item: TMyRecord);
var
i: Integer;
begin
// Throw it out if it's not bigger than our smallest top10
if (Item.Value <= Top10[9].Value) then
Exit;
// Start at the bottom, since it's more likely
for i := 9 downto 1 do
if Item.Value <= Top10[i - 1].Value then
begin
// We found our spot
Top10.Insert(i, Item);
// We're always setting it to 10 now
Top10.Count := 10;
// We're done
Exit;
end;
// Welcome, leader!
Top10.Insert(0, Item);
// We're always setting it to 10 now
Top10.Count := 10;
end;

Answer 3

Since you are working with a fixed number of items, you could use a plain TMyRecord array, eg:
type
TMyRecord = record
Number: Integer;
Value: Float;
end;
const
MaxRecordsInTopTen = 10;
var
TopTen: array[0..MaxRecordsInTopTen-1] of TMyRecord;
NumRecordsInTopTen: Integer = 0;
procedure CheckValueForTopTen(Value: Float; Number: Integer);
var
I, J, NumToMove: Integer;
begin
// see if the new Value is higher than an value already in the list
for I := 0 to (NumRecordsInTopTen-1) do
begin
if Value > TopTen[I].Value then
begin
// new Value is higher then this value, insert before
// it, moving the following values down a slot, and
// discarding the last value if the list is full
if NumRecordsInTopTen < MaxRecordsInTopTen then
NumToMove := NumRecordsInTopTen - I
else
NumToMove := MaxRecordsInTopTen - I - 1;
for J := 1 to NumToMove do
Move(TopTen[NumRecordsInTopTen-J], TopTen[NumRecordsInTopTen-J-1], SizeOf(TMyRecord));
// insert the new value now
TopTen[I].Number := Number;
TopTen[I].Value := Value;
NumRecordsInTopTen := Min(NumRecordsInTopTen+1, MaxRecordsInTopTen);
// all done
Exit;
end;
end;
// new value is lower then existing values,
// insert at the end of the list if room
if NumRecordsInTopTen < MaxRecordsInTopTen then
begin
TopTen[NumRecordsInTopTen].Number := Number;
TopTen[NumRecordsInTopTen].Value := Value;
Inc(NumRecordsInTopTen);
end;
end;

Answer 4

I wouldn't bother with anything other than straight Object Pascal.
{$APPTYPE CONSOLE}
program test2; uses sysutils, windows;
const
MAX_VALUE = $7FFF;
RANDNUMCOUNT = 1000;
var
topten: array[1..10] of Longint;
i, j: integer;
Value: Longint;
begin
randomize;
FillChar(topten, Sizeof(topten), 0);
for i := 1 to RANDNUMCOUNT do
begin
Value := Random(MAX_VALUE);
j := 1;
while j <= 10 do
begin
if Value > topten[j] then
begin
Move(topten[j], topten[j+1], SizeOf(Longint) * (10-j));
topten[j] := Value;
break;
end;
inc(j);
end;
end;
writeln('Top ten numbers generated were: ');
for j := 1 to 10 do
writeln(j:2, ': ', topten[j]);
readln;
end.