EV3 Robotics›Level 4 · WRO Prep›Lesson 37
Level 4 · Lesson 37 · EV3-L04-3760 minutes · Block 5 of 6 · Mat: WRO 2025 — The Future of Robots
Make the robot remember what it did, and show the total when it finishes.
After a bad round, nobody agrees on what happened. One person saw it miss the pad, another is sure it stopped early, and the score sheet just says a number. A log written by the robot as it went is the only account that was not reconstructed afterwards.
A variable holds one value. A list holds many. That is the whole new idea today — and it is what lets the robot record a sequence of things rather than only the last one.
One entry per scoring stop, and nothing else. On this mat that means an entry when the robot completes its job at a launch pad, at the gantry at D, or at the satellite row at E.
Two things per entry:
Resist logging more. Every extra entry is more list handling on a Brick that is not good at it, and a log nobody reads is worse than no log because it takes time to write.
An EV3 list holds one type of thing, so a log of station-and-points is two lists side by side: a stations list and a points list, with matching positions. Entry three of one goes with entry three of the other.
Do not try to squeeze both into a single number. Teams attempt it, and unpacking it afterwards costs more than the second list ever would.
Appending on an EV3 is three steps, every time:
Wrap those three in a My Block called Log Stop with two inputs, station and points, and you never have to think about it again. That is lesson 13’s rule doing exactly what it was for.
Write the log as you go, never at the end. The run you most want a log for is the run that failed early — and that run never reaches an end-of-program write. Logging at the end is tidier, and it throws away the only case that mattered.
And empty both lists at the top of the program, beside the menu variable. Lists persist between runs exactly as variables do, so without it, round two’s log has round one’s stops in it and the total is nonsense.
Build the My Block first, test it on the bench, then place the calls.
Log Stop takes a station number and a points value, appends each to its list, and returns nothing. Test it by calling it three times with made-up values and displaying the length of the stations list. It should read 3.
Then place a call immediately after each scoring action in the mission sequences — after the job is done, not before the robot sets off. A log entry written on departure records an intention. You want a record of what actually happened.
Keep the calls out of the follow loops. A Log Stop inside a line follower would fire hundreds of times and also slow the loop, which after lesson 28 you know changes what the D term means.
At the end of every mission, before the program stops, show what happened.
The total. Walk the points list with a loop: a running total variable, set to zero first, adding each entry in turn. The loop runs as many times as the list is long — read that with Array Operations set to Length rather than assuming a number.
The summary screen. Three lines is enough: how many stops, the total points, and the stations in order. Clear the screen on the first line only, exactly as in lesson 36.
Then wait. Put a Wait for Brick Button at the very end, so the summary stays up until somebody has read it. Without it the program ends, the screen returns to the menu, and the numbers you just worked for are gone.
Now the robot’s count can be compared against the scorer’s. If they disagree, that is worth finding out while the mat is still set up and the round is still fresh — either the robot thinks it scored something it did not, or you are owed points.
This is the test that proves the log is worth having, and it is the one teams skip.
Start a multi-stop mission. Part-way through — after one or two stops — stop the robot by hand, or press the Brick’s back button to end the program.
Then check the log. It should contain exactly the stops that genuinely happened, and nothing after them.
From HOME, choose a pad run from the menu, and let it go all the way: out to the pad, deliver to the gantry at D, back to HOME.
Read the summary off the screen and write it in the journal beside what you watched happen. They should match.
Then run a second mission from the menu without reloading, and check the summary describes only the second run. That is the emptying-at-the-top rule, tested where it actually bites.
Today’s entry:
The log is evidence, and it is also a judging point. A team that can say “the robot recorded four stops and eighteen points, and here is where it stopped short” is describing its own performance from data. That is a different conversation from “something went wrong”.
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.
Build a Log Stop My Block taking a station number and a points value. An EV3 list holds one type, so the log is TWO lists side by side with matching positions — do not try to squeeze both into one number. Test it by calling it three times and displaying the length.
Place a Log Stop call after each scoring action — after the job is done, not before the robot sets off — and keep them out of the follow loops. Then walk the points list with a loop to total it, and show stops, total and stations on a summary screen with a wait at the end so somebody can read it.
The test that proves the log is worth having: start a multi-stop mission, stop the robot part-way, and check the log holds exactly the stops that happened. Empty log means you are writing at the end. Extra stops mean the calls are before the action. Last run's stops mean the lists are not emptied at the top.
Run a full mission from the menu with logging, read the summary off the screen and write it beside what you watched happen — they should match. Then run a second mission without reloading and confirm the summary describes only the second run. The run you most want a log for is the one that failed early, which is exactly the run that never reaches an end-of-program write.