Making Robots React

Loops: repeat without rewriting

In Week 3 you wrote every step out in a row. That works, but if you want the robot to check its sensor a hundred times, you don't want to write the same step a hundred times. A loop is a block that repeats steps for you.

A plain repeat loop runs its steps a set number of times, like 'repeat 4 times: drive forward, turn right' to make a square. The loop does the counting so your program stays short.

For example: Repeat 4 times to drive a square; Repeat 3 times to knock on a door; Repeat 10 times to inch forward and check

Forever loops and repeat-until loops

A forever loop repeats its steps over and over and never stops on its own - it keeps going until you stop the program. Robots use forever loops to keep watching the world, like 'forever: read the sensor and react'.

A repeat-until loop is different: it repeats its steps until a condition becomes true, then it stops and moves on. 'Repeat until the wall is close: drive forward' means keep driving forward, but the moment the wall is close, stop looping.

For example: Forever: keep checking the distance sensor; Repeat until line is found: drive forward; Repeat until button is pressed: wait

Conditions are true or false (boolean)

A condition is a question about the world that can only be answered yes or no - in programming we say true or false. A value that is only ever true or false is called a boolean.

'Is the wall closer than 10 cm?' is a condition. Right now it might be true; a second later, after the robot moves, it might be false. Conditions are how a robot turns a sensor reading into a clear yes-or-no it can act on.

For example: Is the wall close? -> true or false; Is the sensor over the dark line? -> true or false; Is the button pressed? -> true or false

If and if/else: choosing what to do

An if block runs its steps only when its condition is true. 'If the wall is close, turn right' means the robot turns only when a wall is actually close; otherwise it skips the turn.

An if/else block picks between two actions: it runs one set of steps when the condition is true and a different set when it is false. 'If the wall is close, turn right; else drive forward' means the robot always does one or the other, depending on what it senses.

For example: If line lost, turn to find it; If/else: wall close -> turn, else -> go forward; If item is red, turn to the red bin

Obstacle avoidance = loop + sensor + if/else

Now put it all together. Obstacle avoidance is a forever loop that reads the distance sensor and uses if/else to decide: if something is close, turn away; else keep driving forward. Because the loop repeats many times a second, the robot reacts the instant an obstacle appears.

This is the pattern behind self-parking cars and robot vacuums: sense, decide, act - over and over, forever.

For example: Forever: read distance; if close turn, else forward; A robot vacuum bumping around a room; A rover steering around a rock

Line following: keep checking and steer

Line following uses a light or color sensor pointed at the floor. A dark line reflects less light than a light floor, so the sensor can tell 'on the line' from 'off the line'. The robot keeps checking, again and again, and steers to stay on the line.

One simple rule: if the sensor sees the line, curve one way; else curve back the other way. The robot never drives perfectly straight - it wiggles along the edge of the line, correcting all the time. That constant checking and steering is responsive behavior.

For example: Warehouse robots following floor tape; A factory cart on a painted line; Repeat-until the end mark: keep following the line

Obstacle-avoidance reliability test

Place the robot in front of an obstacle and run the program. Record whether it turned away in time. Repeat three times from the same start.

Measure: How many of the three tries the robot successfully avoided the obstacle