How to call defined slots from keypress event in qt / How to connect keypressed event with QPushbutton on gui?

I am beginner to qt. I was working on calculator gui application, I have already defined slots like numPressed() when any of the number pushbutton is pressed on the calculator that will be displayed on the lineEdit.

void Calculator::numPressed(){
    QPushButton *button = (QPushButton *)sender();
    QString buttonValue = button->text();
    Qstring display = ui->lineEdit->text();
    Qdouble ProcessedValue = previousValue + buttonValue.toDouble();
    .
    .
    .
    ui->lineEdit->setText(QString::number(ProcessedValue));
}

connect(ui->add_button,SIGNAL(released),this,SLOT(numPressed));

Here add_button is a QPushbutton name on the UI.

What i want to achieve is?

When a number key is pressed on the key board i want to connect the number pressed and number pushbutton on the calculator which fills the lineEdit with the number that i have pressed on the keyboard?

PS: I want to connect number pressed on the keyboard to the number on the pushbutton.

If QPushButton is used for the calculator buttons then the solution might be very easy:

Qt provides a concept for short cuts. Actions as well as certain widgets (QPushButton included) are already prepared for this.

QPushButton::shortcut (inherited from base class QAbstractButton):

This property holds the mnemonic associated with the button

Access functions:
QKeySequence shortcut() const
void setShortcut(const QKeySequence &key)

Example:

// a button with label '0'
QPushButton qBtn0("0");
// set shortcut for key 0
qBtn0.setShortcut(QKeySequence("0"));

To try this out, I used my (old) pocket calculator example and added the support of shortcut keys.

The one-and-only source code qCalc.cc:

#include <QtWidgets>

class Calculator: public QWidget {
  private:
    const char *const _error;
    int _accuSum, _accuProd;
    char _op;
    bool _clr;
    QGridLayout _qGrid;
    QLineEdit _qView;
    QPushButton _qBtnClr;
    QPushButton _qBtn7, _qBtn8, _qBtn9, _qBtnDiv;
    QPushButton _qBtn4, _qBtn5, _qBtn6, _qBtnMul;
    QPushButton _qBtn1, _qBtn2, _qBtn3, _qBtnSub;
    QPushButton _qBtnNeg, _qBtn0, _qBtnEqu, _qBtnAdd;
  public:
    explicit Calculator(QWidget *pQParent = nullptr);
    virtual ~Calculator() = default;
    Calculator(const Calculator&) = delete;;
    Calculator operator=(const Calculator&) = delete;
  private:
    void clear(bool);
    void negate(bool);
    void addDigit(char digit);
    void eval(char op); 
};

Calculator::Calculator(QWidget *pQParent):
  QWidget(pQParent),
  _error("ERROR"), _accuSum(0), _accuProd(0), _op(' '), _clr(true),
  _qBtnClr("C"),
  _qBtn7("7"), _qBtn8("8"), _qBtn9("9"), _qBtnDiv("/"),
  _qBtn4("4"), _qBtn5("5"), _qBtn6("6"), _qBtnMul("*"),
  _qBtn1("1"), _qBtn2("2"), _qBtn3("3"), _qBtnSub("-"),
  _qBtnNeg("-"), _qBtn0("0"), _qBtnEqu("="), _qBtnAdd("+")
{
  // setup GUI
  _qView.setReadOnly(true);            
  _qGrid.addWidget(&_qView, 0, 0, 1, 3);
  _qBtnClr.setShortcut(QKeySequence("C"));
  _qGrid.addWidget(&_qBtnClr, 0, 3);
  _qBtn7.setShortcut(QKeySequence("7"));
  _qGrid.addWidget(&_qBtn7, 1, 0);
  _qBtn8.setShortcut(QKeySequence("8"));
  _qGrid.addWidget(&_qBtn8, 1, 1);
  _qBtn9.setShortcut(QKeySequence("9"));
  _qGrid.addWidget(&_qBtn9, 1, 2);
  _qBtnDiv.setShortcut(QKeySequence("/"));
  _qGrid.addWidget(&_qBtnDiv, 1, 3);
  _qBtn4.setShortcut(QKeySequence("4"));
  _qGrid.addWidget(&_qBtn4, 2, 0);
  _qBtn5.setShortcut(QKeySequence("5"));
  _qGrid.addWidget(&_qBtn5, 2, 1);
  _qBtn6.setShortcut(QKeySequence("6"));
  _qGrid.addWidget(&_qBtn6, 2, 2);
  _qBtnMul.setShortcut(QKeySequence("*"));
  _qGrid.addWidget(&_qBtnMul, 2, 3);
  _qBtn1.setShortcut(QKeySequence("1"));
  _qGrid.addWidget(&_qBtn1, 3, 0);
  _qBtn2.setShortcut(QKeySequence("2"));
  _qGrid.addWidget(&_qBtn2, 3, 1);
  _qBtn3.setShortcut(QKeySequence("3"));
  _qGrid.addWidget(&_qBtn3, 3, 2);
  _qBtnSub.setShortcut(QKeySequence("-"));
  _qGrid.addWidget(&_qBtnSub, 3, 3);
  /// @todo _qBtnNeg.setShortcut(QKeySequence("???"));
  _qGrid.addWidget(&_qBtnNeg, 4, 0);
  _qBtn0.setShortcut(QKeySequence("0"));
  _qGrid.addWidget(&_qBtn0, 4, 1);
  _qBtnEqu.setShortcut(QKeySequence(Qt::Key_Enter));
  _qGrid.addWidget(&_qBtnEqu, 4, 2);
  _qBtnAdd.setShortcut(QKeySequence("+"));
  _qGrid.addWidget(&_qBtnAdd, 4, 3);
  setLayout(&_qGrid);
  // connect signal handlers
  connect(&_qBtnClr, &QPushButton::clicked,
    this, &Calculator::clear);
  connect(&_qBtn7, &QPushButton::clicked,
    [this](bool) { addDigit('7'); });
  connect(&_qBtn8, &QPushButton::clicked,
    [this](bool) { addDigit('8'); });
  connect(&_qBtn9, &QPushButton::clicked,
    [this](bool) { addDigit('9'); });
  connect(&_qBtnDiv, &QPushButton::clicked,
    [this](bool) { eval('/'); });
  connect(&_qBtn4, &QPushButton::clicked,
    [this](bool) { addDigit('4'); });
  connect(&_qBtn5, &QPushButton::clicked,
    [this](bool) { addDigit('5'); });
  connect(&_qBtn6, &QPushButton::clicked,
    [this](bool) { addDigit('6'); });
  connect(&_qBtnMul, &QPushButton::clicked,
    [this](bool) { eval('*'); });
  connect(&_qBtn1, &QPushButton::clicked,
    [this](bool) { addDigit('1'); });
  connect(&_qBtn2, &QPushButton::clicked,
    [this](bool) { addDigit('2'); });
  connect(&_qBtn3, &QPushButton::clicked,
    [this](bool) { addDigit('3'); });
  connect(&_qBtnSub, &QPushButton::clicked,
    [this](bool) { eval('-'); });
  connect(&_qBtnNeg, &QPushButton::clicked,
    this, &Calculator::negate);
  connect(&_qBtnClr, &QPushButton::clicked,
    this, &Calculator::clear);
  connect(&_qBtn0, &QPushButton::clicked,
    [this](bool) { addDigit('0'); });
  connect(&_qBtnEqu, &QPushButton::clicked,
    [this](bool) { eval('='); });
  connect(&_qBtnAdd, &QPushButton::clicked,
    [this](bool) { eval('+'); });
  // init
  clear(false);
}

void Calculator::clear(bool)
{
  _qView.setText("+0");
  _accuSum = _accuProd = 0; _op = ' ';
  _clr = false;
}

void Calculator::negate(bool)
{
  QString text = _qView.text();
  if (text == _error) return;
  text[0] = text[0] == '-' ? '+' : '-'; 
  _qView.setText(text);
}

void Calculator::addDigit(char c)
{
  QString text = _qView.text();
  if (text == _error) return;
  if (_clr) text = "+";
  else if (text == "+0" || text == "-0") text.chop(1);
  if (text.length() < 10) text += c;
  _clr = false;
  _qView.setText(text);
}

void Calculator::eval(char op)
{
  QString text = _qView.text();
  if (text == _error) return;
  int value = text.toInt();
  switch (_op) {
    case '+':
      _accuSum += _accuProd;
      _accuProd = value;
      break;
    case '-':
      _accuSum += _accuProd;
      _accuProd = -value;
      break;
    case '*':
      _accuProd *= value;
      break;
    case '/':
      if (value == 0) {
        _qView.setText("ERROR"); return;
      }
      _accuProd /= value;
      break;
    default:
     _accuProd = value;
  }
  switch (op) {
    case '=':
    case '+': case '-':
      _accuProd += _accuSum; _accuSum = 0;
  }
  text = QString::number(_accuProd);
  if (text[0] != '-') text.insert(0, '+');
  _qView.setText(text);
  _op = op; _clr = true;
}

int main(int argc, char **argv)
{
  qDebug() << "Qt Version:" << QT_VERSION_STR;
  QApplication app(argc, argv);
  Calculator qCalc;
  qCalc.show();
  return app.exec();
}

A Qt project file to build qCalc.pro:

SOURCES = qCalc.cc

QT = widgets

Built and tested in VS2017 with Qt 5.13: