#define SWITCH_PIN 13

void setup() {
  pinMode(SWITCH_PIN, INPUT_PULLUP);
  Serial.begin(115200);
}

void loop() {
  digitalRead(SWITCH_PIN) ? doModeOneStuff() : doModeTwoStuff();
}

void doModeOneStuff() {
  Serial.println("Doing Mode One Stuff");
}
void doModeTwoStuff() {
  Serial.println("Doing Mode Two Stuff");
}