[Tutor] A question about None

[Dennis Lee Bieber]

On Sun, 22 Aug 2021 17:25:10 +1000, Phil <phillor9 at gmail.com> declaimed the
following:

>The distance sensor outputs "None" if the distance is greater than can 
>be measured and "None", of course cannot be compared. So I came up with 
>the following which is not quite correct:
>
>while True
>     if distance_sensor.get_distance_cm() is not None:
>         if distance_sensor.get_distance_cm() > 20:

	Note that you have a race condition... Between the test for None and
the test for >20 you have TWO "reads" of distance -- the distance could
have changed enough between those reads that the second could return None!

>             motor_pair.set_default_speed(20)
>             motor_pair.start()
>         else:
>             continue
>
>     elif distance_sensor.get_distance_cm() is not None:

	You will only get here is the first read returns None! And the continue
statements are unneeded.

>         if distance_sensor.get_distance_cm() < 20:
>             motor_pair.set_default_speed(-20)
>             motor_pair.start()
>         else:
>             continue
>

	You perform FOUR reads of distance for a loop that should only require
ONE read. Also, what happens if distance is exactly "20".

>The idea is that if "None" returned from the sensor then nothing will 
>happen and the loop will continue until "None" is not returned. Can 
>anyone see the error of my ways?

	Uhm... "lots" <G> 

	Consider...

speed = 0
While True:
	speed_change = True
	distance = distance_sensor.get_distance_cm()
	if distance is not None:	#assumes distance of 0 is a valid result
							#otherwise, just use
							#	if distance:

		if distance >= 20 and speed <> 20:
			speed = 20
		elif distance < 20 and speed <> -20:
			speed = -20
		else:
			speed_change = False

		if speed_change:
			#no sense setting speed and starting motors if they
			#are already configured
			#NOTE: should there be a motor_pair.stop() before
			#reversing speed value?
			motor_pair.set_default_speed(speed)
			motor_pair.start()



		


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	Wulfraed                 Dennis Lee Bieber         AF6VN
	wlfraed at ix.netcom.com    http://wlfraed.microdiversity.freeddns.org/