ButtonMatrix for Arduino without resistors or multiplexer chip

ButtonMatrix.ino

const int ROWS=3;
const int COLS=5;
bool buttons[ROWS][COLS];
int rowPins[] = {2,3,4};
int colPins[] = {5,6,7,8,9};

void setup() {
  Serial.begin(9600);
  
  for (int i=0; i<ROWS; i++) {
    pinMode(rowPins[i],OUTPUT); //row pins are OUTPUT
  }
  
  for (int i=0; i<COLS; i++) {
    pinMode(colPins[i],INPUT); //col pins are INPUT
    digitalWrite(colPins[i],HIGH); //use internal pull-up
  }
  
}

void loop() {
  for (int row=0; row<ROWS; row++) {
    for (int i=0; i<ROWS; i++) {
      digitalWrite(rowPins[i],i!=row);  //active row LOW, others HIGH
    }
    for (int col=0; col<COLS; col++) {
      Serial.print(digitalRead(colPins[col]));
      Serial.print(" ");
    }
    Serial.print("- ");
  }
  Serial.println();
}