12. EXT: while-loops

See: https://www.w3schools.com/python/python_while_loops.asp
while-loops run a set of statements as long as a test condition is true.
The statements within the loop are indented.
../_images/while_loop.png

12.1. Events as conditions

while-loops have a condition that is tested each time the loop runs.
The condition must be True for the loop to run.
If the condition is False, the loop is exited.
Button pressing can be used as the condition in a while-loop.
In the code below, the code within the while-loop runs only while the A-button is being held down.
from microbit import *

while True:
    while button_a.is_pressed():
        display.show(['I', Image.HEART, 'U'], delay=400)
        sleep(300)
    display.show(Image.ALL_CLOCKS, delay=50)

Tasks

  1. Write a while-loop that displays a happy face while the A-button is pressed, and when it is not, Image.ALL_CLOCKS is used to display a rotating clock hand.

  2. Write a while-loop that displays a sad face while the B-button is pressed, and when it is not, Image.ALL_CLOCKS is used to display a rotating clock hand.

Write a while-loop that displays a happy face while the A-button is pressed, and when it is not, Image.ALL_CLOCKS is used to display a rotating clock hand.

from microbit import *

while True:
    while button_a.is_pressed():
        display.show(Image.HAPPY)
        sleep(300)
    display.show(Image.ALL_CLOCKS, delay=50)

Write a while-loop that displays a sad face while the B-button is pressed, and when it is not, Image.ALL_CLOCKS is used to display a rotating clock hand.

from microbit import *

while True:
    while button_b.is_pressed():
        display.show(Image.SAD)
        sleep(300)
    display.show(Image.ALL_CLOCKS, delay=50)

12.2. Counters

while-loops have a condition that is tested each time the loop runs.
The condition must be True for the loop to run.
If the condition is False, the loop is exited.
A counter can be used in the while-loop condition.
The counter is set before the while-loop.
The counter is incremented (increased by a constant amount) in a while-loop.

12.3. Counting up

../_images/while_counter.png
In the example below, i is the counter.
i starts off at 0 and is increased by 1 in the while-loop line: i += 1.
i += 1 is the same as i = i + 1
While the condition, i < 10, is True the while-loop runs.
i += 1 causes i to increase from 0 to 9.
The while-loop exits when i is 10 since i < 10 will be False when i = 10.
from microbit import *

while True:
    i = 0
    while i < 10:
        display.scroll(i, delay=50)
        i += 1

12.4. Counting down

In the code below, i starts off at 5 and is decreased by 1 in the while-loop line: i -= 1.
i -+= 1 is the same as i = i - 1
The test uses a > sign when counting down.
The loop below stops when i is no longer greater than 1, i.e. when it is 1.
from microbit import *

while True:
    i = 5
    while i > 1:
        display.scroll(i, delay=50)
        i -= 1

12.5. Step size

The code below counts up from 0 to 10 in steps of 2.
i += 2 sets a step size of 2.
from microbit import *

while True:
    i = 0
    while i < 11:
        display.scroll(i, delay=50)
        i += 2

Tasks

  1. Write a while-loop that counts up from 1 to 5, showing the numbers 1, 2, 3, 4, 5.

  2. Write a while-loop that counts up from 3 to 12 in steps of 3, scrolling the numbers 3, 6, 9, 12.

  3. Write a while-loop that counts down from 9 to 1, showing the numbers 9, 8, 7, 6, 5, 4, 3, 2, 1.

  4. Write a while-loop that counts down from 24 to 18 in steps of 2, scrolling the numbers 24, 22, 20, 18.

  5. Write 2 while-loops to scroll 0 to 8 going up in 2s then 9 down to 1 going down in 2s.

Write a while-loop that counts up from 1 to 5, showing the numbers 1, 2, 3, 4, 5.

from microbit import *

while True:
    i = 1
    while i < 6:
        display.scroll(i, delay=50)
        i += 1

Write a while-loop that counts up from 3 to 12 in steps of 3, scrolling the numbers 3, 6, 9, 12.

from microbit import *

while True:
    i = 3
    while i < 13:
        display.scroll(i, delay=50)
        i += 3

Write a while-loop that counts down from 9 to 1, showing the numbers 9, 8, 7, 6, 5, 4, 3, 2, 1.

from microbit import *

while True:
    i = 9
    while i > 0:
        display.scroll(i, delay=50)
        i -= 1

Write a while-loop that counts down from 24 to 18 in steps of 2, scrolling the numbers 24, 22, 20, 18.

from microbit import *

while True:
    i = 24
    while i > 17:
        display.scroll(i, delay=50)
        i -= 2

Write 2 while-loops to scroll 0 to 8 going up in 2s then 9 down to 1 going down in 2s.

from microbit import *

while True:
    i = 0
    while i < 9:
        display.scroll(i, delay=50)
        i += 2
    i = 9
    while i > 0:
        display.scroll(i, delay=50)
        i -= 2
    sleep(1000)