Be on the lookout

It sometimes happens that students connect a sensor to ports 2 or 3, but forget to change it in the program.

See note in the “Be on the lookout.” 

It is important that students understand that a stack attached to a sensor watcher will be carried out each time the specified event becomes true. In the Move example, the stack is carried out when the sensor is pressed. Continuing to hold the sensor pressed will not trigger the stack again. The sensor must be first released and then pressed again for the stack to retrigger.

What you can do: Explain that when a watcher monitors the sensor, it looks for “something interesting” for the program. The first time the event becomes true is always “interesting.” After the first time, the watcher will be looking for a change before it will consider the event to be true again.


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