Deciding What to Do

Try It — Sabotage Your Own Robot

Load your Project 12 program — the blue pom run. Before you press start, move the first pom about four inches to one side. Now run it and watch closely.

Something goes slightly wrong in a real match — a cube got bumped, your robot drifted an inch. How much of your run is ruined?

You do this all day

You never follow a plan blindly. You check things and pick. Fill in what you would do:

That last one is exactly what your robot needs — a way to notice that it is carrying nothing.

Learn It — Ask Once, Then Choose

The orange Control shelf has one more pair of jaws: the if-then block. It asks a hexagon question once, and only runs what is in its jaws when the answer is true.

The hexagon is written exactly the way your wait until and repeat until hexagons are — same shapes, same comparisons, same not.

The one difference that matters

Three shapes

How does a robot know it grabbed nothing?

Here is the check that fixes your sabotaged run — and it costs no extra sensor at all.

Think about what “closed” means. Your claw closed number is where the claw stops because the cube is in the way. Close the claw on empty air and nothing stops it — it swings past that number.

For that trick to work, the close itself has to change. A run-for block always swings the exact degrees you asked for — cube or no cube, it lands on the same count, and the count tells you nothing. So the grab check uses a timed squeeze instead: switch the claw motor on toward closed, give it a second, switch it off. A cube stops the squeeze early. Empty air lets it swing right on past.

The claw motor’s own degree count, which the brick has been keeping since Project 3, just became a sensor. It starts at 0 at claw-open — that is the Start Pose Rule earning its keep. Whether your empty claw ends up at a bigger or smaller number than your grip point depends on which way your claw closes — check yours by hand and flip the hexagon if needed.

Do It — Choose a Path

Step 1 — Your first decision

New project called Decisions. Build: if (1 is pressed?) then — play beep for 0.2 seconds. Run it twice — once holding the sensor pressed, once not.

What happened each time — and how fast did the program end when you were not pressing?

Step 2 — Swap if for wait-until and watch what breaks

Replace the if with wait until (1 is pressed?), beep after it. Run it without pressing.

What is the robot doing now, and how is it different from the if version? Use the words “once” and “keeps”.

Swap it back.

Step 3 — Pick a side with a press

Now a real two-way choice. Build: at the start of a program, wait 3 seconds — your window to press or not — then:

Hold the sensor during the wait → robot turns right. Do not → it turns left. One program, two behaviours, chosen at run time.

• My robot picks a different turn depending on whether I pressed

Step 4 — Did I actually grab it?

Open your grab block from Project 12. Swap its claw-close for the timed squeeze from Learn It, then add the empty-claw check right after it: if the claw count is past claw closed by more than about 10, show the sad face and stop; else, carry on.

Test it honestly: run with a cube in place, then run with the cube moved away.

• With the cube there, the run carries on normally
• With the cube missing, my robot notices and says so

Your robot just told you something went wrong instead of carrying air across the field. What should it do about it — try again, skip to the next mission, or stop? Who decides?

Step 5 — A three-way choice

Build a chain: press within the first 3 seconds → the full run; press and hold → just the grab test; no press → a beep and stop. Use an if-then-else with a second if-then-else inside the else mouth.

• Three different behaviours from one program, and I can trigger each on purpose

Why does the order of the questions in your chain matter? What would go wrong if you asked them the other way round?

Step 6 — Mission 3 Advanced — two opposite stacks13 pts

Where the decisions go. This run has four grabs and four placements in a row. If grab two fails and the robot does not notice, everything after it is wasted. Put a check after each grab — the pattern from step 4 — so a miss costs you one cube instead of the whole mission.

Plan it out. Which cube goes where, and in what order?

• I have two mixed-colour stacks
• The two stacks are arranged opposite ways round
• No cube is shared between the two stacks

Step 7 — Run it five times

Score It — Checkpoint

My score

if or until?

Which tool fits each job — an if, a wait until, or a repeat until?

Spot the bug

Can you do it again?

• I can explain the difference between if and until using “once” and “keeps”
• I know exactly one mouth of an if-then-else always runs
• I can build a three-way chain and say why its order matters
• My robot can notice an empty claw using the claw motor’s own degree count
• One of my programs behaves differently depending on what it finds

Think about it

In Try It your robot carried nothing across the field and placed it carefully in a basket. Would you call that a mistake by the robot, or a mistake by the programmer?

A robot that checks its own work looks smarter than one that does not. Is it actually smarter, or is it something else?

Every decision you made today came from a switch — pressed or not pressed. What kinds of choices could your robot make if it could tell how much of something there is, instead of just yes or no?

Next

Everything your robot senses right now is on or off. The field is not like that — a black line and a white mat are not two states of a switch, they are two brightnesses.

In Project 14 — Seeing Light and Dark, your robot gets a sensor that answers with a number instead of a yes.