For loop does NOT go to the max

[Joe Sansalone 6]

Hi,

I've encountered something weird.  Delphi 10.4.2

I loop through the items in a list.

But also may add to that list in the loop.

Thus, extend the loop iterations.

 

var

list: TStringList;

 

for i := 0 to list.count-1 do 

begin

  // whatever processing

  list.append('another item');

end

 

PROBLEM:  the loop does not continue past the initial count.

 

I'm guessing this is a BUG?

 

Help,

Joe

 

 

 

 

[Fr0sT.Brutal 900]

Ranges of for loop counter are calculated once loop starts and won't change later. You'll either have to add another for loop inside or use another kind of loop (while, until + manual counter check and increment)

[luebbe 26]

13 minutes ago, Joe Sansalone said:

Hi,

I've encountered something weird.  Delphi 10.4.2

I loop through the items in a list.

But also may add to that list in the loop.

Thus, extend the loop iterations.

Ouch. Are you sure you want to do this? How are you going to ensure that your loop terminates when you keep adding items inside the loop?

 

Consider

list.count-1

to be a constant while the loop is executed. This ensures termination.
If you really want to change the list content while iterating over the list, use a while loop and ensure termination yourself.

[timfrost 78]

"For purposes of controlling the execution of the loop, the expressions initialValue and finalValue are evaluated only once, before the loop begins."

 

You can use a 'while' statement to force the evaluation of the final value on every iteration.

i := 0
while i < list.count do

 

[Joe Sansalone 6]

I didn't realize For loop evaluates only once.

Thanks for everyone's comments.

 

I'll use a While loop.

[Pat Foley 51]

Quote

PROBLEM:  the loop does not continue past the initial count.

 

I'm guessing this is a BUG?

Most people consider this a feature. 

 

Append list elsewhere; Keep track of List count and start count at a lastProcessed count to reduce processing.

 

 

     

 

   

 

 

 

[Joe Sansalone 6]

I see the benefit of For loop only evaluating once at initial.

 

 

[Pat Foley 51]

3 hours ago, Joe Sansalone said:

I see the benefit of For loop only evaluating once at initial.

 

 

This shows way mentioned to extend for..loop functionality.   

procedure TForm19.Button1Click(Sender: TObject);
var
  needsProcessed, startProcess: integer;
  LList: TList<integer>;
  s,r: string;
begin
  LList := TList<integer>.Create;
  try
    LList.AddRange([0,1,2,3,4,5]);
    needsProcessed := LList.Count; //process everything first time.
    
    Repeat
      startProcess := LList.Count - Needsprocessed;
      needsProcessed := 0;

      for var ii := startProcess to pred(LList.count) do
        begin                      //   ^  LList.Last would increase here!
           if ((LList.List[ii] mod 2) = 0) then
           begin
             inc(needsProcessed);
             r := ' even  ';
             with LList do add(Last + 1);
           end else r := ' odd  ';
           s := s + LList.List[ii].ToString + r + #13#10;
        end;
     until  (LList.count > 150) or (needsProcessed = 0);
     showMessage(s);
  finally
    LList.Free;
  end;
end;