Laby obiektowka - nic nie rozumiem

mwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "QDebug"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}

MainWindow::~MainWindow()
{
    delete ui;
}



void MainWindow::on_calculateButton_clicked()
{
    QString A = ui -> lineEdit -> text();  // podanie 1 wartosci do LE
    QString B = ui -> lineEdit_2 -> text(); // podanie 2 wartosci do LE
    QString r;
    switch(index){
    case 0:
    r = QString::number(A.toFloat()+B.toFloat()); break; // obliczenie r z rzutowaniem na float
    case 1:
    QString odej;
    odej = QString::number(A.toFloat()-B.toFloat());
    ui -> lineEdit_3 -> setText(odej);
    QString dziel;
    dziel = QString::number(A.toFloat()/B.toFloat());
    ui -> lineEdit_3 -> setText(dziel);
    QString mno;
    mno = QString::number(A.toFloat()*B.toFloat());
    ui -> lineEdit_3 -> setText(mno);
    }

    ui -> lineEdit_3 -> setText(r);// przeslanie r do trzeciego LE
   // int i = ui -> comboBox -> currentData();
}