void MainWindow::onClick4()
{
    numb = ui->lineEdit_2->text().toInt();
       ui->lineEdit->clear();
       ui->lineEdit_2->clear();
       ui->textEdit_2->clear();
       str.clear();
       if(numb>vec.size())
       {
            QMessageBox::information(this,tr("Error"),tr("please enter correct number"),
                                        QMessageBox::Ok);
       }
       else
       {
            QVector<QString>::iterator iter = vec.begin();
            for (; iter != vec.begin() + numb; ++iter)
            {
            QString a = *iter;
            str.push_back(a);
            QVector<QString>::iterator itet = str.begin();
            for (; itet != str.end(); ++itet)
               {
                   ui->textEdit_2->insertPlainText(*itet);
               }
           }
       }
}