5. Iteration - Event Loop

5.1 While Loop

You may have noticed that many of the example programs have contained a line with:

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while True: 

This is a loop. A loop will run whatever is inside it until it meets a false condition. while True: is the same as saying run forever and will run until you stop the Edison.

Let's look at the syntax (structure) of a while loop – lines 11 and 12:

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#-------------Setup----------------

import Ed

Ed.EdisonVersion = Ed.V3

Ed.DistanceUnits = Ed.TIME
Ed.Tempo = Ed.TEMPO_MEDIUM

# --- Event loop -----
while True:
  Ed.PlayBeep()

If this code gets loaded onto the Edison it will play a beep sound, and keep playing it.

1. The code enters at line 11 and see that the while loop is true
2. It will then run line 12 and play the beep
3. It will then come back up to line 11 and once again see that the loop is true
4. It will then run line 12 and play the beep
5. etc...

We can also make the Edison continually drive:

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#-------------Setup----------------

import Ed

Ed.EdisonVersion = Ed.V3

Ed.DistanceUnits = Ed.CM
Ed.Tempo = Ed.TEMPO_MEDIUM

#--------Your code below-----------
Ed.ReadIRData()

while True:
    Ed.Drive(Ed.FORWARD, Ed.SPEED_3, 20)

5.2 Python Syntax

Notice that the code underneath the event loop (while True:) is indented. This is how Python understands that the code below needs to be repeated. Indentation is used in Python to allow us to attach code to structures such as while loops.

Indentation can either be created using the tab key or spaces, so long as they are consistent.

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#-------------Setup----------------

import Ed

Ed.EdisonVersion = Ed.V3

Ed.DistanceUnits = Ed.CM
Ed.Tempo = Ed.TEMPO_MEDIUM

#--------Your code below-----------
while True:
    Ed.Drive(Ed.FORWARD, Ed.SPEED_3, 10)
Ed.Drive(Ed.BACKWARDS, Ed.SPEED_3, 10)

In the above example we could expect the Edison to move forward by 10cm and then go backwards 10cm. What actually happens is that the Edison will move forwards forever and never go backwards. Why?

Of course, the event loop can be combined with what we've learnt previously (functions and variables):

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#-------------Setup----------------
import Ed

Ed.EdisonVersion = Ed.V3

Ed.DistanceUnits = Ed.CM
Ed.Tempo = Ed.TEMPO_MEDIUM
#--------Your code below-----------

# this function returns a number from one to six
def get_number():
    return Ed.Random(1, 3)

# beeping(num) will beep 'num' times
def beeping(num):
    Ed.PlayBeep()
    Ed.TimeWait(num, Ed.TIME_SECONDS)

while True:
    n = get_number()
    beeping(n)

In the above example the Edison will beep and pause for a random number of seconds between one and three. You can see from above that functions are being called from within the while loop.