5. Modulo operator

Let’s go back a step. When I wrote and executed:

var r = new UsedRobot();
view_source(RUR.control.turn_left);

I saw the following:

function (robot, no_frame){
    "use strict";
    robot._prev_orientation = robot.orientation;
    robot._prev_x = robot.x;
    robot._prev_y = robot.y;
    robot.orientation += 1;  // could have used "++" instead of "+= 1"
    robot.orientation %= 4;
    if (no_frame) return;
    RUR.control.sound_id = "#turn-sound";
    RUR.rec.record_frame();
}

Notice the highlighted line with the % symbol; this symbol represents the modulo operator, in both Python and Javascript, as well as many other languages. Before I explain what it does try the following Python code; the result will appear in Reeborg’s Diary. (Make sure to have Python selected as your programming language.)

for i in range(10):
    print(i, i%4)

You should see two columns of integers. On the left, the integers increase steadily from 0 to 9. On the right, the integers cycle from 0 to 3 in a repeating pattern.

The modulo operator calculates the remainder from division by an integer. Remember when you first learn about divisions, before learning about fractions. You first learned that 8 divided by 4 gave 2, but that 7 could not be divided by 4. Then, you learned that you could say 7 divided by 4 gives 1 with a remainder of 3. Yet, later, you saw that 7 divided by 4 was 1 and 3/4, etc.

So, the modulo operator calculates for you that remainder from division by an integer. In the above code for the RUR.control.turn_left function, it is used to ensure that orientation cycles between the values from 0 to 3, incrementing by 1 (modulo 4) each time a left turn is done. Since a right turn is equivalent to 3 left turns, this suggest that we increase the orientation by 3 instead of by 1 when attempting to implement a turn_right method.

Try this!

Try the following code to see if it works:

class RepairedRobot(UsedRobot):
    def turn_right(self):
        self.body.orientation += 3
        self.body.orientation %= 4
        RUR.rec.record_frame()

reeborg = RepairedRobot(3, 3)  # away from walls
for i in range(4):
    reeborg.move()
    reeborg.turn_right()

You will notice that the “oil leak” does not look right; this is because it is drawn from some assumed prior position and orientation. Although we will eventually “fix” the robot by removing the oil leak, it still might be nice to have this look better while the oil leak is present. To do so, we can pretend that we did two left turns first, calculate what the position and orientation should be at that point, use these as “previous values” which are used to draw a trace from the previous position to the current one after the move. Here’s the code to do this:

class RepairedRobot(UsedRobot):
    def turn_right(self):

        # save previous values to know from where to start drawing
        self.body._prev_orientation = self.body.orientation
        self.body._prev_x = self.body.x
        self.body._prev_y = self.body.y

        # mimic two previous left turns for prior orientation
        self.body._prev_orientation += 2
        self.body._prev_orientation %= 4

        # do right turn
        self.body.orientation += 3
        self.body.orientation %= 4
        RUR.rec.record_frame()

reeborg = RepairedRobot(3, 3)  # away from walls
for i in range(4):
    reeborg.move()
    reeborg.turn_right()

Try it!

Make sure you try to run the above code and try to understand what each line does.

Your turn!

Add a turn_around method, which is equivalent to having Reeborg do two left turns in one single step. Test your method by having Reeborg move around in its world and make sure that traces left by the “oil leak” are straight lines.