PHP - Recursive in place replacing of strings to numbers takes too long

Because coercion used to be faster than casting, I ran this code to calculate timings on PHP 7 :

function getTime($start) {
    return round((microtime(true) - $start) * 1000000) / 1000;
}

function mockData($length) {
    $data = [];
    $i = -1;

    while ($i++ < $length) {
        $data[$i] = strval(rand(1, 10000) / 100);
    }

    return $data;
}

$data = mockData(100000);

// Let's check that they are string before
echo gettype($data[0]) . '<br><br>';

$start = microtime(true);
$convertedData = [];
foreach ($data as $key => $value) {
    $convertedData[$key] = (double) $value;
}
echo '(double) cast took ' . getTime($start) . ' ms.<br>';

$start = microtime(true);
$convertedData = [];
foreach ($data as $key => $value) {
    $convertedData[$key] = 0 + $value;
}
echo 'Coercion took ' . getTime($start) . ' ms.<br>';

And my results are :

(double) cast took 27.508 ms.
Coercion took 28.789 ms.

CONCLUSION

Since using floatval (a third way to achieve string to double conversion) is even longer, you can't do better than that with PHP. What you're trying to achieve is a scripting operation, it shouldn't be used as a normal back-end operation for a web application.

But if you still want to do that, you can higher your memory_limit inside your php.ini file, as long as you don't use the -1 workaround.

UPDATE

I forgot one possible optimization, you should pass your variable by reference to at least execute immediate assignation :

$start = microtime(true);
foreach ($data as $key => $value) {
    $data[$key] = (double) $value;
}
echo getTime($start) . ' ms.<br>';

=> 34.018 ms.

$start = microtime(true);
foreach ($data as &$value) {
    $value = (double) $value;
}
echo getTime($start) . ' ms.<br>';

=> 17.081 ms.

And apparently using coercion with by reference gives even better results :

$start = microtime(true);
foreach ($data as &$value) {
    $value = 0 + $value;
}
echo getTime($start) . ' ms.<br>';

=> 13.1 ms.

Here is a nice description of your question: Doing calculations in MySQL vs PHP

In case of the second example the speed issue can be significant. First of all you do not know how big are your comments, so in case of

$x = mysql_query("SELECT * FROM comments");

 while( $res = mysql_fetch_assoc( $x ) ){
   $min_comment = substr( $x['comment'],0,10 ) ;
 }

you ask your server to return you everything (here I mean the whole length of the comment) and this can be significant. Multiplying by the number of rows in the table it can be quite big size of data, which you have to transfer between php and sql. In the second case this SELECT * , SUBSTR(comment, 0, 10) as min_comment FROM comments this will be already done on the server and will not require additional memory.

In case of the first example, I think it is also better to do it on sql side, because you will still need to do additional loop afterwards. Apart from this, people who will be reading your code might be confused why exactly do you need that code.

You can Edit a subitem of the listview (in report mode) using a TEdit, a custom message and handling the OnClick event of the ListView.

Try this sample

Const
  USER_EDITLISTVIEW = WM_USER + 666;

type
  TForm1 = class(TForm)
    ListView1: TListView;
    procedure FormCreate(Sender: TObject);
    procedure ListView1Click(Sender: TObject);
  private
    ListViewEditor: TEdit;
    LItem: TListitem;
    procedure UserEditListView( Var Message: TMessage ); message USER_EDITLISTVIEW;
    procedure ListViewEditorExit(Sender: TObject);
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

uses
  CommCtrl;
const
  EDIT_COLUMN = 2; //Index of the column to Edit

procedure TForm1.FormCreate(Sender: TObject);
Var
  I : Integer;
  Item : TListItem;
begin
  for I := 0 to 9 do
  begin
   Item:=ListView1.Items.Add;
   Item.Caption:=Format('%d.%d',[i,1]);
   Item.SubItems.Add(Format('%d.%d',[i,2]));
   Item.SubItems.Add(Format('%d.%d',[i,3]));
  end;

  //create the TEdit and assign the OnExit event
  ListViewEditor:=TEdit.Create(Self);
  ListViewEditor.Parent:=ListView1;
  ListViewEditor.OnExit:=ListViewEditorExit;
  ListViewEditor.Visible:=False;
end;

procedure TForm1.ListView1Click(Sender: TObject);
var
  LPoint: TPoint;
  LVHitTestInfo: TLVHitTestInfo;
begin
  LPoint:= listview1.ScreenToClient(Mouse.CursorPos);
  ZeroMemory( @LVHitTestInfo, SizeOf(LVHitTestInfo));
  LVHitTestInfo.pt := LPoint;
  //Check if the click was made in the column to edit
  If (ListView1.perform( LVM_SUBITEMHITTEST, 0, LPARAM(@LVHitTestInfo))<>-1) and ( LVHitTestInfo.iSubItem = EDIT_COLUMN ) Then
    PostMessage( self.Handle, USER_EDITLISTVIEW, LVHitTestInfo.iItem, 0 )
  else
    ListViewEditor.Visible:=False; //hide the TEdit 
end;

procedure TForm1.ListViewEditorExit(Sender: TObject);
begin
  If Assigned(LItem) Then
  Begin
    //assign the vslue of the TEdit to the Subitem
    LItem.SubItems[ EDIT_COLUMN-1 ] := ListViewEditor.Text;
    LItem := nil;
  End;
end;

procedure TForm1.UserEditListView(var Message: TMessage);
var
  LRect: TRect;
begin
  LRect.Top := EDIT_COLUMN;
  LRect.Left:= LVIR_BOUNDS;
  listview1.Perform( LVM_GETSUBITEMRECT, Message.wparam,  LPARAM(@LRect) );
  MapWindowPoints( listview1.Handle, ListViewEditor.Parent.Handle, LRect, 2 );
  //get the current Item to edit
  LItem := listview1.Items[ Message.wparam ];
  //set the text of the Edit 
  ListViewEditor.Text := LItem.Subitems[ EDIT_COLUMN-1];
  //set the bounds of the TEdit
  ListViewEditor.BoundsRect := LRect; 
  //Show the TEdit
  ListViewEditor.Visible:=True;
end;