How to append to a given string in a Tstringlist

I'm creating lists of email addresses implemented as csv strings in a TStringlist by appending each new email to an existing csv string. If the csv string in MyStringlist[0] already contains the email I'm about to add then I'll append it to MyStringlist[1]. If that string also already contains it then I'll append it to MyStringlist[2] and so on until I find a string that doesn't contain it.

The problem occurs if, for example, MyStringlist[0] and MyStringlist[1] both contain the email I want to append but MyStringlist[2] doesn't yet exist so it can't be appended to.

e.g. I might need to add 'a.co.uk' to MyStringlist that already contains the strings below.

MyStringlist[0] -> a.co.uk, f.co.uk, h.co.uk, k.co.uk
MyStringlist[1] -> d.co.uk, a.co.uk, g.co.uk

I created a procedure to append an email to a string at a given index or create a new string in the stringlist if necessary.

Is this code the correct way to do it or are there functions I should use to do this in a simpler way - or maybe in a more robust way?

Procedure created

procedure AppendToStringListItem(TheStringList: Tstringlist;
                                 Index : integer;
                                 StringToAppend : string);
var i : integer;
begin
if Index >= 0 then
  begin
  if TheStringList.count -1 < index then //not enough strings
    begin
    for i := 1 to index - TheStringList.Count  do  //add a blank string
       TheStringList.add('');
    TheStringList.add( StringToAppend); //add new string at position 'index'
    end
  else //just append
     TheStringList[Index] :=  TheStringList[Index] + ',' + StringToAppend
  end;
end;

Solution

There is no built-in function to ensure a TStrings/TStringList has a minimum number of strings. So yes, you will have to add strings manually one at a time.

I would suggest something more like this:

procedure AppendToStringListItem(TheStringList: TStrings;
                                 Index : integer;
                                 StringToAppend : string);
var
  s: string;
begin
  if Index < 0 then Exit;
  TheStringList.BeginUpdate;
  try
    while Index >= TheStringList.Count do begin
      TheStringList.Append('');
    end;
    s := TheStringList[Index];
    if s = '' then 
      s := StringToAppend
    else
      s := s + ',' + StringToAppend;
    TheStringList[Index] := s;
  finally
    TheStringList.EndUpdate;
  end;
end;

However, this does require you to know the desired index ahead of time, which means searching the TStringList beforehand, eg:

function AlreadyExistsInStringListItem(const S, StringToAppend: string): Boolean;
begin
  Result := ...;
end;

function FindIndexToAppendToStringListItem(TheStringList: TStrings;
                                           StringToAppend : string);
begin
  for Result := 0 to TheStringList.Count-1 do
  begin
    if not AlreadyExistsInStringListItem(TheStringList[Result], StringToAppend) then
      Exit;
  end;
  Result := TheStringList.Count;
end;

procedure AppendToStringListItem(TheStringList: TStrings;
                                 Index : integer;
                                 StringToAppend : string);
var
  s: string;
begin
  if Index < 0 then Exit;
  TheStringList.BeginUpdate;
  try
    while Index >= TheStringList.Count do begin
      TheStringList.Append('');
    end;
    s := TheStringList[Index];
    if s = '' then 
      s := StringToAppend
    else
      s := s + ',' + StringToAppend;
    TheStringList[Index] := s;
  finally
    TheStringList.EndUpdate;
  end;
end;

Index := FindIndexToAppendToStringListItem(TheStringList, 'a.co.uk');
AppendToStringListItem(TheStringList, Index, 'a.co.uk');

Depending on your code's design, that could be wasted overhead. You could simplify that logic by merging the searching and inserting together, eg:

function AlreadyExistsInStringListItem(const S, StringToAppend: string): Boolean;
begin
  Result := ...;
end;

procedure AppendToStringListItem(TheStringList: TStrings;
                                 StringToAppend : string);
var
  i : integer;
  s: string;
begin
  TheStringList.BeginUpdate;
  try
    for i := 0 to TheStringList.Count-1 do
    begin
      s := TheStringList[i];
      if not AlreadyExistsInStringListItem(s, StringToAppend) then
      begin
        TheStringList[i] := s + ',' + StringToAppend;
        Exit;
      end;
    end;
    TheStringList.Append(StringToAppend);
  finally
    TheStringList.EndUpdate;
  end;
end;

AppendToStringListItem(TheStringList, 'a.co.uk');