[Spring4D] Remove collection elements

[Jacek Laskowski 57]

I have a spring collection, like a IList<T>.

I need to remove all items from it that meet the condition:

 

type
  TFoo = interface
    function Value : Integer;
  end;

var
  List : IList<TFoo>;
begin
  i := 0;
  while i < List.Count do
  begin
    if List[i].Value < 100 then 
      List.Delete(i)
    else
      Inc(i);
  end;
end;

Can it be done faster, better?

 

And how can I do that for IDictionary<K, V>?

Edited December 19, 2019 by Jacek Laskowski

[Der schöne Günther 316]

I would do it like this: 

procedure p();
var
	itemsToDelete: IEnumerable<TFoo>;
begin
	itemsToDelete := List.Where(valueIsSmallerThan100);
	List.RemoveRange(itemsToDelete);
end;

function valueIsSmallerThan100(const item: TFoo): boolean;
begin
	Result := (item.getValue < 100);
end;

 

[pietrt 1]

You can also use the RemoveAll method:

  list.RemoveAll(
    function(const foo: TFoo): boolean
    begin
      result := foo.Value < 100;
    end);
  dict.RemoveAll(
    function(const aPair: TPair<string,TFoo>): boolean
    begin
      result := aPair.Value.Value < 100;
    end);

 

Edited December 19, 2019 by pietrt

[Der schöne Günther 316]

Personally, I like to store those things in a local variable as it makes it easier to debug, but that's probably just personal taste.

[Stefan Glienke 2002]

RemoveAll is the way to go - it is optimized internally for lists (at least since 2.0/develop) - not yet for dictionaries but there it more depends on what is the criteria for deleting items as remove by key is already O(1)

 

The approach with Where/Remove does not work because the result of Where is lazy evaluated and will raise an exception once the source gets modified (after the first deleted item). Even when calling ToArray to manifest the collection of to removed items the Remove will be a waste of time because items would be searched all over.

Edited December 19, 2019 by Stefan Glienke