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An easy way to create an inverted TDictionary <TKey, TValue> key list?

Is there an easy way to take a key list TDictionary<TKey,TValue>

and reverse its order?

I could use the classic one for the I: = ... loop, but as I see it is there TList<T>.Reverse

in Generics.Collections, maybe there is better / shorter one.

Update: a for loop I: = downto will have the same useless order, so the best way is to use a separate TList<TKey>

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2 answers


Keys in TDictionary are unordered. The class makes no guarantees over its order, and the actual order that is used is not the order in which they were added. Reversing the order of an unordered dictionary is a meaningless operation.



Delphi does not ship with an ordered dictionary class, although there are third party classes. If your usage is relatively straightforward, you can manage an additional list containing the keys in the order you want.

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In my case, I am using TDictionary <Tkey, TValue>. TKeyCollection .

function compareKeyReverse(const L, R: TKey): Integer;
begin
  // Reverse compare keys
  // if TKey = String, uncomment code below
  // Result := - SysUtils.CompareText(L, R);
end;

function getReverseSortedKeyArray(dictionary: TDictionary<TKey, TValue>): TArray<TKey>;
var
  keyArray: TArray<TKey>;
  keyCollecttion: TDictionary<Tkey, TValue>.TKeyCollection;
begin
  keyCollecttion:= TDictionary<Tkey, TValue>.TKeyCollection.Create(dictionary);
  try
    keyArray:= valueCollecttion.ToArray;
    TArray.Sort<Tkey>(keyArray, TComparer<Tkey>.Construct(compareKeyReverse));
  finally
    keyCollecttion.Free;
  end;

  Result := keyArray;
end;


Usage example:

var
  key: TKey;
  keyArray : TArray<TKey>;
begin
    keyArray  := getReverseSortedKeyArray (dictionary);
    for key in keyArray  do
    begin
      // ...
    end;
end;
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