Unit 6: Loops That Watch

Keep going until something changes.

Unit 6 of 8 in Robotics coding for kids. Its 4 lessons are While, Until You Get There, Round the Edge and The Maze — below is everything each one explains, and a question or two from it to try.

Every program on this page was driven round the simulated room before it shipped, and prints exactly what it says it prints.

This unit opens with a fortnight’s trial of everything — no card needed — or with a family plan, bought in the iPhone app. The first two units of every track are free for ever. Try it in the app.

⏳ While

Keep going while…

for needs to know how many times before it starts. while does not — it checks something before every pass and keeps going as long as that stays true.

So this creeps forward in 5 cm nudges and stops as soon as the wall is close, without anyone measuring the room first.

Python

while robot.distance() > 20:
    robot.forward(5)

robot.say(robot.distance())

It prints

17.0

Try it yourself

Why does it stop at 17 rather than exactly 20?

It checks before each nudge, and the last 5 cm nudge took it from 22 to 17
The sensor is wrong by 3
while always overshoots by 3
It stopped because it bumped

What happens if the thing a while is waiting for never becomes false?

It keeps going until the robot runs out of power, and the robot says so
Python skips the loop
It runs exactly 100 times
The robot bumps and stops

Answer them in the app

🎯 Until You Get There

Waiting for the flag

robot.at_goal() is True once the robot is standing in the green circle. Put not in front and you have "keep going until you arrive".

The robot has no idea how far away the flag is. It does not need to.

Python

while not robot.at_goal():
    robot.forward(10)

robot.say("Made it")

It prints

Made it

Try it yourself

The flag is 140 cm up the room and its circle has a 16 cm radius. How many nudges does this take?

Python

steps = 0

while not robot.at_goal():
    robot.forward(10)
    steps = steps + 1

robot.say(steps)

What does not do in while not robot.at_goal():?

It flips it: keep looping while the robot is not there yet
It stops the loop immediately
It makes the robot drive backwards
Nothing — it is optional

Answer them in the app

🧱 Round the Edge

A loop inside a loop

Put a while inside a for and you get "do this four times, and each time keep going until…".

Here the inner loop runs the robot up to a wall, and the outer one turns it and does the whole thing again. Four walls, one lap of the room — and it works in a room of any size.

Python

for wall in range(4):
    while robot.distance() > 12:
        robot.forward(5)
    robot.right(90)

robot.say(robot.x(), robot.y())

It prints

20.0 180.0

Try it yourself

Which line belongs to the outer loop, not the inner one?

robot.right(90) — it runs once per wall, after the inner loop has finished
robot.forward(5)
while robot.distance() > 12:
All of them belong to both

Answer it in the app

🏆 The Maze

One rule, any maze

A maze looks like it needs a route. It does not — it needs a rule, applied over and over:

If something is close in front, turn towards whichever side has more room. Then edge forward.

That is four lines, and it never mentions this maze. Watch it find its own way out.

Python

while not robot.at_goal():
    if robot.distance() < 20:
        if robot.distance_left() > robot.distance_right():
            robot.left(90)
        else:
            robot.right(90)
    robot.forward(5)

robot.say("Out!")

It prints

Out!

Try it yourself

What can a while loop do that a for loop cannot?

Stop when something happens, without knowing beforehand how many turns that takes
Run more than ten times
Move the robot
Contain an if

Answer it in the app