TStack.Peek Method

Look at top stack item.

Pascal

function Peek: T;

C++

__fastcall T Peek();

Description

Peek returns the item from the top of the stack without removing it. If Count is 0, an error is raised. Count is unchanged.

Pop functions similarly to Peek except that Pop removes an element from the stack.

This is a O(1) operation.

Delphi Examples:

Copy Code

{
This example demonstrates the usage of the generic TStack class.
}
procedure TForm3.Button1Click(Sender: TObject);
var
  Stack: TStack<String>;
begin
  { Create a new stack }
  Stack := TStack<String>.Create();

  { Register a notification call-back }
  Stack.OnNotify := StackChanged;

  { Push some items up the stack }
  Stack.Push('John');
  Stack.Push('Mary');
  Stack.Push('Bob');
  Stack.Push('Anna');
  Stack.Push('Erica');

  { Show the last pushed element without modifying the stack }
  MessageDlg('Last pushed element is: "' + Stack.Peek() + '".', mtInformation, [mbOK], 0);

  { Extract the top element: "John" }
  Stack.Extract();

  { Reduce the capacity }
  Stack.TrimExcess();

  { The remaining count of elements }
  MessageDlg('The stack contains ' + IntToStr(Stack.Count) + ' elements.', mtInformation, [mbOK], 0);

  { Show the last pushed element by modifying the stack }
  MessageDlg('Last pushed element is: "' + Stack.Pop() + '".', mtInformation, [mbOK], 0);

  { Clear the stack }
  Stack.Clear();

  { Destroy the stack completely }
  Stack.Free;
end;

procedure TForm3.StackChanged(Sender: TObject; const Item: String; Action: TCollectionNotification);
begin
  { This method is called by the Stack everytime a change occurs }
  if Action = cnAdded then
     MessageDlg('Element added: ' + Item, mtInformation, [mbOK], 0)
  else if Action = cnRemoved then
     MessageDlg('Element removed: ' + Item, mtInformation, [mbOK], 0)
  else if Action = cnExtracted then
     MessageDlg('Element extracted: ' + Item, mtInformation, [mbOK], 0)
end;