Location Sensor

The location sensor, or GPS, gives the position of the robot in metres. On the starter robot it is named gps. You need it to report wall tokens, and it helps a lot to explore systematically.

getValues() returns [x, y, z]. X and Z are horizontal, and Y is vertical, so use X and Z for the position in the maze:

Python

from controller import Robot

robot = Robot()
timeStep = int(robot.getBasicTimeStep())

gps = robot.getDevice("gps")
gps.enable(timeStep)

while robot.step(timeStep) != -1:
    x, y, z = gps.getValues()  # In meters. X and Z are horizontal, Y is vertical.
    print("x:", round(x, 3), "z:", round(z, 3))

C/C++

#include <iostream>
#include <webots/Robot.hpp>
#include <webots/GPS.hpp>

using namespace webots;
using namespace std;

int main() {
  Robot *robot = new Robot();
  int timeStep = (int)robot->getBasicTimeStep();

  GPS *gps = robot->getGPS("gps");
  gps->enable(timeStep);

  while (robot->step(timeStep) != -1) {
    const double *position = gps->getValues(); // In meters. X and Z are horizontal, Y is vertical.
    cout << "x: " << position[0] << " z: " << position[2] << endl;
  }

  delete robot;
  return 0;
}

The GPS has noise of a few millimetres, so the values change slightly even when the robot stands still. Never compare positions for exact equality.

Example: distance from the start

This program drives forward until the robot is half a tile from where it started:

Python

from controller import Robot
import math

robot = Robot()
timeStep = int(robot.getBasicTimeStep())

gps = robot.getDevice("gps")
gps.enable(timeStep)
leftWheel = robot.getDevice("left wheel motor")
rightWheel = robot.getDevice("right wheel motor")
leftWheel.setPosition(float("inf"))
rightWheel.setPosition(float("inf"))

robot.step(timeStep)  # Step once, so that the GPS has a value
startX, _, startZ = gps.getValues()

leftWheel.setVelocity(3.0)
rightWheel.setVelocity(3.0)

while robot.step(timeStep) != -1:
    x, _, z = gps.getValues()
    distance = math.sqrt((x - startX) ** 2 + (z - startZ) ** 2)
    print("Distance from the start:", round(distance, 3), "m")
    if distance > 0.06:  # Half a tile
        leftWheel.setVelocity(0)
        rightWheel.setVelocity(0)
        break

C/C++

#include <iostream>
#include <cmath>
#include <webots/Robot.hpp>
#include <webots/GPS.hpp>
#include <webots/Motor.hpp>

using namespace webots;
using namespace std;

int main() {
  Robot *robot = new Robot();
  int timeStep = (int)robot->getBasicTimeStep();

  GPS *gps = robot->getGPS("gps");
  gps->enable(timeStep);
  Motor *leftWheel = robot->getMotor("left wheel motor");
  Motor *rightWheel = robot->getMotor("right wheel motor");
  leftWheel->setPosition(INFINITY);
  rightWheel->setPosition(INFINITY);

  robot->step(timeStep); // Step once, so that the GPS has a value
  double startX = gps->getValues()[0], startZ = gps->getValues()[2];

  leftWheel->setVelocity(3.0);
  rightWheel->setVelocity(3.0);

  while (robot->step(timeStep) != -1) {
    double x = gps->getValues()[0], z = gps->getValues()[2];
    double distance = sqrt(pow(x - startX, 2) + pow(z - startZ, 2));
    cout << "Distance from the start: " << distance << " m" << endl;
    if (distance > 0.06) { // Half a tile
      leftWheel->setVelocity(0);
      rightWheel->setVelocity(0);
      break;
    }
  }

  delete robot;
  return 0;
}