try/catch umesto if/else

double a, o;
    QString stranica, obim;
    QMessageBox poruka;
    bool ok, prazno;

    stranica = ui->lineEdit->text();
    prazno = stranica.isEmpty();

    a = stranica.toDouble(&ok);

    if(prazno)
    {
        poruka.setText("Niste uneli nikakvu vrednost!");
        poruka.exec();
        ui->lineEdit_2->clear();
    }
    else if(!ok)
    {
        poruka.setText("Niste uneli broj!");
        poruka.exec();
        ui->lineEdit->clear();
        ui->lineEdit_2->clear();
    }
    else if(a <= 0.)
    {
        poruka.setText("Broj mora biti pozitivan!");
        poruka.exec();
        ui->lineEdit->clear();
        ui->lineEdit_2->clear();
    }
    else
    {
        o = 4. * a;
        obim = QString::number(o);
        ui->lineEdit_2->setText(obim);
    }

    ui->lineEdit->setFocus();