11. EXT: Buttons and selection

11.1. if - else with break

break is used to stop the while true loop.

The code below counts up in steps of 1 from 0 to a target number, but does not display the number till the target number is reached after which program breaks out of the loop and displays the message “target reached”.

from microbit import *

num = 0
target = 5
while True:
    if num == target:
        display.scroll(num)
        break
    else:
        num = num + 1
    sleep(200)
display.scroll("target reached", delay=50)

The code below counts up in steps of 1 from 0 to a target number, displaying the number till the target number is reached after which program breaks out of the loop and displays the message “target reached”.

Notice that the condition uses “less than” rather than “equal”.

from microbit import *

num = 0
target = 5
while True:
    if num < target:
        display.scroll(num, delay=50)
        num = num + 1
    else:
        break
    sleep(200)
display.scroll(target, delay=50)
display.scroll("target reached", delay=50)

11.2. String slices

string[start_value: stop_value]: returns character at index (position) start_value to the character at the index before the stop_value character.

“Hello”[0:3] returns characters 0 to 2, “Hel”, with “H” being index 0.

11.3. Converting a Boolean to a string initial

The Booleans True and False are not strings.
The code below, str(button_a.is_pressed()), coverts the boolean values of True or False to a string. True becomes “True” and False becomes “False”.
Then [0:1] takes a string slice giving the first letter only.
So True becomes “T” and False becomes “F”.

from microbit import *

while True:
    true_or_false = str(button_a.is_pressed())
    t_or_f = true_or_false[0:1]
    display.show(t_or_f)
    sleep(500)

11.4. Changing values with A and B-buttons

Use the A-button to increase a variable.
Use the B-button to decrease a variable.
In the code below, the delay_time variable is increased in steps of 10 by the A-button admonition decreased in steps of 10 by the B-button.
Finally, text is scrolled with a delay of delay_time.

from microbit import *

delay_time = 80

while True:
    if button_a.is_pressed():
        delay_time += 10
    elif button_b.is_pressed():
        delay_time -= 10
    else:
        sleep(100)
    display.scroll("ABC", delay=delay_time)

The code above fails when the delay_time goes below 0.
The variable, delay_time, can be restricted to a set range of values using the min and max functions.
delay_time = min(400, delay_time + 10) prevents the delay_time from going above 400.
delay_time = max(50, delay_time - 10) prevents the delay_time from going below 50.

from microbit import *

delay_time = 80

while True:
    if button_a.is_pressed():
        delay_time = min(400, delay_time + 10)
    elif button_b.is_pressed():
        delay_time = max(50, delay_time - 10)
    else:
        sleep(100)
    display.scroll("ABC", delay=delay_time)

Tasks

Edit the code to adjust the scroll delay in steps of 25.
Write code to alter a guess_number variable in steps of 1 by the buttons. Use both buttons to set the number and show it. Start the number at 5 and limit it to a minimum of 1 and a maximum of 9.