EV3 Robotics›Level 4 · WRO Prep›Lesson 36
Level 4 · Lesson 36 · EV3-L04-3660 minutes · Block 5 of 6 · Mat: WRO 2025 — The Future of Robots
More missions than the screen has lines — without capping the menu at what fits.
Yesterday’s menu had three entries and they all fitted. Your real set has more: three pads, the satellite row, combinations, and probably a test entry or two. The screen does not grow.
The fix is to stop treating the screen as the list. The list can be any length. The screen shows a window onto it, and the window moves when it has to. Every scrolling menu you have ever used works exactly this way.
Write out the full menu now. For most teams on this mat it is something like:
Six entries. More than fits comfortably, which is the point — and adding a seventh should now cost you nothing but a line in the list.
Yesterday you had one variable. Today you need two, and they mean different things.
The buttons only change the selection. The window start is then adjusted if — and only if — the selection has moved outside what is visible.
Say the screen shows three rows. The visible entries run from the window start to the window start plus two. So after every button press:
That last line is the one teams miss. If the window moves on every press, the list scrolls under a cursor that never moves, which is disorienting and makes it very easy to confirm the wrong entry. The window should sit still until the selection is about to leave it.
Draw it on paper before building it. Six entries, a three-row window, and step the selection from 1 to 6 and back, writing down the window start at each step. You should see the window stay at 1 for the first three presses, then follow.
Each redraw paints one row at a time, from the window start:
Clear the screen once, on the first line. Clear on every line and each one wipes the last, so only the bottom row ever appears. Clear on none and yesterday’s longer names show through underneath today’s shorter ones. This costs teams twenty minutes and is a two-second setting.
Getting the name for a given entry number is a Switch on the number with a Display in each case — clumsy but honest, and it is what the EV3 gives you. Keep the names short: the screen is narrow and it truncates without telling you.
Yesterday’s loop redrew every pass. That was fine for three entries and a stationary display. With a window it strobes, because the screen is being repainted continuously whether or not anything moved.
The fix is to restructure the loop so the redraw follows the change:
Now the screen is painted exactly once per press, which is both steady to look at and faster.
Decide the ends again, now that there is a window. If you wrap from the last entry to the first, the window start has to jump back to 1 at the same moment — otherwise the selection is at the top of the list while the screen is still showing the bottom, and the cursor disappears entirely. Clamping avoids that, at the cost of more presses to reach the end.
Hand the Brick to somebody from another team who has not seen your menu, and ask them to select the satellite run. Do not help.
What you are watching for:
Then shorten the names until every one is distinct on the screen rather than on paper.
Step from the first entry to the last, one press at a time, watching the window. It should sit still, then follow, and the selection should never leave the visible rows.
Then choose an entry from the far end of the list — not the first — and press centre, and let the robot run it. Choosing the sixth entry and having the robot do the sixth thing is the actual test; a menu that scrolls beautifully and then runs entry one has a selection the mission Switch never saw.
Finally, add a seventh entry to the list. It should cost you one line and one Switch case, and nothing else. If it costs more than that, the menu is not really general yet.
Today’s entry:
The menu is now general. It does not care how many missions there are, and adding one is a line in a list. That is worth saying to a judge: the design does not have to be revisited when the mission set changes, which on this mat it does.
This model drives, so its challenges are run on a mat. Mats differ between branches — check you are looking at the one in your room.
WRO 2025 RoboMission Elementary — The Future of Robots · official WRO game mat, 2362 × 1143 mm
The challenges name these places rather than distances, so the same challenge works on any mat:
Switch mats above and every route below is redrawn on the mat you chose.
Work through the challenges in order — each is harder than the last. The mission comes after all three, and it is meant to make you plan before you build.
Split one variable into two: the SELECTION, which entry the person is on, and the WINDOW START, which entry is drawn on the top line. Trace it on paper first — six entries, a three-row window, selection 1 to 6 and back, writing the window start at each step.
The window moves only when the selection would leave it: above the window, set the start to the selection; below it, set the start to the selection minus two; otherwise leave it alone. Move the window on every press instead and the list scrolls under a stationary cursor, which is how people confirm the wrong entry.
Draw the window with CLEAR SCREEN on the first line only — clear on every line and each wipes the last, clear on none and old names show through. Then hand the Brick to somebody from another team and ask them to select the satellite run, without helping. Shorten every name that truncates or looks like another.
Prove the menu no longer depends on what fits: choose an entry from the FAR END of the list and confirm the robot runs that mission and not entry one. Then add a seventh entry — it should cost you one line in the list and one Switch case, and nothing else. If it costs more, the menu is not general yet.