TStringList-TStrings Tutorial

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TStringList

The TStringList (or its parent TStrings) is much like a fancy dynamic array or Set of Strings (a set of strings is not possible in FPC). It will come in handy a lot when programming and I'm going to teach you basic TStringList usage!

Simple example

program StrList;
{$mode objfpc}
uses
 Classes, SysUtils;
var
  Str: TStringList;
begin
  Str := TStringList.Create; // This is needed when using this class(or most classes)
  Str.Add('Some String!');
  writeln('The stringlist now has ' + IntToStr(Str.Count) + ' string(s).');
  Readln;
  Str.Free; //Release the memory used by this stringlist instance
end.

This is a simple console program that will create and add one string to a stringlist. Now here's some things you should know:

Create - Will create the string list for modifying. If you use constructor Create, you have to later Free. Calling free causes destructor a Destroy invitation and release the memory it takes. If not, you program will not crash, but it will not release all the memory it occupied: a memory leak.

Count - This property is a counter for the number of strings in the List.

Add - This method allows you to add one string to the stringlist. It is a function that will return the Index of the String. This is where the counter comes in handy.

Delete - Will delete a string from the stringlist. Just know that you do not simply input the string, you have to input the index of the string. Like I said: it's like a fancy Dynamic Array.

IndexOf - Will return the index of the string in the list. If it is not found it returns -1.

Clear - Will clear the list.

Expanded Example

How about a more juicy example, eh?

program StrList2;
{$mode ObjFPC}
uses
Classes, SysUtils;

var
  Str: TStringList;
  S: String;
  Counter: Integer;
begin
  Str := TStringList.Create;
  Writeln('String List Test');
  repeat
    Writeln('Enter a string to add (type EXIT to stop adding strings)');
    Readln(S); 
    if (S = 'EXIT') then 
      Break; // exit the loop
    
    if (S <> '') then
    begin
      Counter := Str.Add(S);
      Writeln('String: ' + S + ' was Added!');
      Writeln('Index is: ' + IntToStr(Counter)); // The counter will always become the index of the last thing added
    end 
    else 
    begin
      Writeln('No data entered...');
    end;
  until (S = 'EXIT');
  writeln('Contents of the TStringList: '+ Str.CommaText);
  Str.Free; //release the memory again
end.

However, to avoid possible memory leaks you should always use a Try - Finally block where possible for this, so you get something like:

var
  slist: TStringList;

...

slist := TStringList.Create;
try
  ...
  // do things with your stringlist  
  ...
finally
  if Assigned(slist) then
    FreeAndNil(slist);
end;

// This works perfect, no double creation of stringlist... comments free to send to edgarrod71@gmail.com
function theStringList: TStringList;
var
 J: integer;
begin
  result := TStringList.Create;
  for J:=0 to 10 do
    result.add(intToStr(J));
end;

procedure Caller;
var
  SL: TStringList;
  K: integer;
begin
  SL := theStringList;
  for K:=0 to pred(SL.Count) do
    writeln(SL[K]);
  if assigned(SL) then
    SL.Free;
end;

To iterate through the list it is better to use the while-do loop instead of the for-to loop. The reason is that the list can be empty, then the for-to loop does not work correctly. In this example, SubscriberDel procedure is called for each ID in the List and the list of the IDs is passed to the procedure as a string.

procedure SubscribersDelete(const Subscriber_ID: string);
var
  List : TStringList; 
  i : Integer;
begin
  List := TStringList.Create;
  List.Text := Subscriber_ID;
  i := 0;
  while i < List.Count do
    begin
      SubscriberDel(List.Strings[i]);
      inc(i);
    end;
  List.Free;
end;

Conversion to and from delimited strings

Code below will result in a stringlist, containing 4 elements ('1', '2', '3' and '4');

procedure Sample;
var
    MyStringList: TStringList=nil;
begin
     MyStringList:= TStringList.create;
     MyStringList.Delimiter := ';';
     MyStringList.DelimitedText:='1;2;3;4';
     MyStringList.free;
end;

Respectively next code will assemble a stringlist into a delimited string ('1;2;3;4'):

function Sample2 : string;
var
    MyStringList: TStringList=nil;
begin
     MyStringList:= TStringList.create;
     MyStringList.Delimiter := ';';
     MyStringList.Add('1');
     MyStringList.Add('2');
     MyStringList.Add('3');
     MyStringList.Add('4');
     Result :=MyStringList.DelimitedText;
     MyStringList.free;
end;

Note that Delimter is a character, not a string! If your separator is a string (for example „\n‟), you could use code below, to get a stringlist, containing 4 elements ('1', '2', '3' and '4'):

procedure Sample;
var
    MyStringList: TStringList=nil;
begin
     MyStringList:= TStringList.create;
     MyStringList.text:=StringReplace('1\n2\n3\n4','\n',Lineending,[rfReplaceAll, rfIgnoreCase]);
     MyStringList.free;
end;

Vice versa, next function will return „1\n2\n3‟:

Function Sample : string;
var
    MyStringList: TStringList=nil;
begin
     MyStringList:= TStringList.create;
     MyStringList.SkipLastLineBreak := True;
     MyStringList.add('1');
     MyStringList.add('2');
     MyStringList.add('3');
     result := StringReplace(MyStringList.Text,Lineending,'\n', [rfReplaceAll, rfIgnoreCase]);
     MyStringList.free;
end;

File Handling

When using the TStringList you have 2 file handling procedures: SaveToFile and LoadFromFile. SavetoFile will save all strings in the list to a file. LoadFromFile will open the file and add the file data to the list string by string.

program StrListFile;
{$mode objfpc}
uses
 Classes, SysUtils;

var
  Str: TStringList;
begin
  Str := TStringList.Create;
  try
    Str.LoadFromFile('SomeFile.txt');
    Str.Add('Hello');
    Str.SaveToFile('SomeFile.txt');
  finally
    Str.Free;
  end;
end.

You just opened a file, edited it and saved it right back to were it was!

Sorting

The Find procedure requires that the list be sorted. The easiest approach is to set its Sorted property to True, then new items added to the list will be inserted in the correct position. This has a few extra side effects, firstly, you cannot use the Insert() procedure, use only Add(). Secondly, it sets the Duplicates property, its default setting is "dupIgnore" which means attempts to Add identical entries will be ignored. However, calls to AddObject() that have an identical string and a new Object, the new Object replaces the existing one.

Dynamic string array comparison

TStringList is simply an object-oriented version of a dynamic string array. Some methods have analogs:

Operation	array of string	TStringList
Variable declaration	StringList: array of string;	StringList: TStringList;
Initialization	implicit constructor	StringList := TStringList.Create
Set size	SetLength(StringList, X);	StringList.Size := X;
Get size	X := Length(StringList);	X := StringList.Count;
Add item	SetLength(StringList, Length(StringList) + 1); StringList[Length(StringList) - 1] := X;	StringList.Add(X);
Delete item	for I := Index to Length(StringList) - 2 do StringList[I] := StringList[I + 1]; SetLength(StringList, Length(StringList) - 1);	StringList.Delete(Index);
Remove all items	SetLength(StringList, 0);	StringList.Clear;
Finalization	implicit destructor	StringList.Free;

However, TStringList offers much more functionality than a basic structure such as a dynamic array.

Keep Learning

TStringList has many other interesting features:

1. It allows you to sort the strings
2. It allows you to limit the list to only unique strings
3. You can get the text of all strings as a single string using the Text property.
4. You can store an object or other data next to the string

You can learn all the different procedures, functions and properties. See TStringList documentation... or the help in Lazarus.

... and you might like to extend this tutorial if you feel like it.

See also

• Character and string types