10. Buttons and selection

The A and B-buttons can be used to carry out various actions on the microbit.
See https://microbit-micropython.readthedocs.io/en/latest/tutorials/buttons.html

10.1. Button is pressed

The .is_pressed method is used to detect button pressing at the moment the running code gets to that point.
button_a.is_pressed()
returns True if the A-button is being pressed or False if not.
button_b.is_pressed()
returns True if the B-button is being pressed or False if not.
The code below scrolls True or False to indicate whether the A-button is being pressed or not.
from microbit import *

while True:
    display.scroll(button_a.is_pressed(), delay=60)
    sleep(1000)

Tasks

  1. Edit the code to indicate whether the B-button is being pressed or not.

Edit the code to indicate whether the B-button is being pressed or not.

from microbit import *

while True:
    display.scroll(button_b.is_pressed(), delay=60)
    sleep(1000)

10.2. Button was pressed

The .was_pressed method is used to detect button pressing since the last check or since the device started.
The .was_pressed method will clear the press state so that the button must be pressed again before this method will return True again.
button_a.was_pressed()
returns True if the A-button was pressed since the last check, or False if not.
button_b.was_pressed()
returns True if the B-button was pressed since the last check, or False if not.
The code below scrolls True or False.
from microbit import *

while True:
    display.scroll(button_b.was_pressed(), delay=60)
    sleep(1000)

Tasks

  1. Edit the code to indicate whether the A-button was pressed or not.

Edit the code to indicate whether the A-button was pressed or not.

from microbit import *

while True:
    display.scroll(button_a.was_pressed(), delay=60)
    sleep(1000)
The code examples below use is_pressed rather than was_pressed since is_pressed works better when the button is held down.
In contrast, was_pressed requires separate pressing for it to return True.

10.3. Selection

Button pressing can be tested and used as conditions in if and elif statements.
if, elif and else provide choices or branches in the code.
They all are used in lines of code which end with a colon, :.
Both if and elif test a condition that returns True or False. Their indented code block runs if the condition is True.
Multiple elif can be used to provide more choices.
The else block does not have a condition.
The else block only runs if all the previous conditions were False.

10.4. if

../_images/if.png
if requires a condition that returns True or False.
In the code below, num is set to 0 then 1 is added to it each time through the while loop.
The if-line checks the variable, num, and scrolls it if it is 5.
from microbit import *

num = 0
while True:
    if num == 5:
        display.scroll(num)
    num = num + 1
    sleep(200)
The code below checks if the A-button is pressed and displays “A” if it is.
A short sleep pauses the code between checks by the if-line.
from microbit import *

while True:
    if button_a.is_pressed() == True:
        display.scroll("A")
    sleep(200)
Since button_a.is_pressed() returns a boolean, there is no need to use == True.
if button_a.is_pressed(): becomes if True: when the A-button is pressed.
if button_a.is_pressed(): becomes if False: when the A-button is not pressed.
from microbit import *

while True:
    if button_a.is_pressed():
        display.scroll("A")
    sleep(200)

Tasks

  1. Edit the code to scroll your name when the A-button is pressed.

  2. Edit the code to display a happy face when the A-button is pressed.

  3. Edit the code to scroll your age when the B-button is pressed.

  4. Edit the code to display a sad face when the B-button is pressed.

Edit the code to scroll your name when the A-button is pressed.

from microbit import *

while True:
    if button_a.is_pressed():
        display.scroll("name")
    sleep(200)

Edit the code to display a happy face when the A-button is pressed.

from microbit import *

while True:
    if button_a.is_pressed():
        display.show(Image.HAPPY)
    sleep(200)

Edit the code to scroll your age when the B-button is pressed.

from microbit import *

while True:
    if button_b.is_pressed():
        display.scroll(12)
    sleep(200)

Edit the code to display a sad face when the B-button is pressed.

from microbit import *

while True:
    if button_b.is_pressed():
        display.show(Image.SAD)
    sleep(200)

10.5. if - else

../_images/if_else.png
The else block does not have a condition.
The else block only runs if all the previous conditions were False.
The code below checks if the A-button is pressed and displays “A” if it is or “X” if not.
from microbit import *

while True:
    if button_a.is_pressed():
        display.show("A")
    else:
        display.show("X")
    sleep(200)

Tasks

  1. Edit the code to scroll your name when the A-button is pressed and to show “?” when nothing is pressed.

  2. Edit the code to display a happy face when the A-button is pressed and a sad face when nothing is pressed.

  3. Edit the code to display a sad face when the B-button is pressed and a confused face when nothing is pressed.

Edit the code to scroll your name when the A-button is pressed and to show “?” when nothing is pressed.

from microbit import *

while True:
    if button_a.is_pressed():
        display.scroll("name")
    else:
        display.show("?")
    sleep(200)

Edit the code to display a happy face when the A-button is pressed and a sad face when nothing is pressed.

from microbit import *

while True:
    if button_a.is_pressed():
        display.show(Image.HAPPY)
    else:
        display.show(Image.SAD)
    sleep(200)

Edit the code to display a sad face when the B-button is pressed and a confused face when nothing is pressed.

from microbit import *

while True:
    if button_b.is_pressed():
        display.show(Image.SAD)
    else:
        display.show(Image.CONFUSED)
    sleep(200)

10.6. if - elif

../_images/if_elif.png
elif can be used to provide another choice by testing to see if its condition is True.
The code below checks if the A-button is pressed and displays “A” if it is.
If A is not pressed, the code then checks if the B-button is pressed and displays “B” if it is.
from microbit import *

while True:
    if button_a.is_pressed():
        display.show("A")
    elif button_b.is_pressed():
        display.show("B")
    sleep(200)

Tasks

  1. Edit the code to scroll your name when the A-button is pressed and your tutor group when the B-button is pressed.

  2. Edit the code to display a happy face when the A-button is pressed and a sad face when the B-button is pressed.

Edit the code to scroll your name when the A-button is pressed and your tutor group when the B-button is pressed.

from microbit import *

while True:
    if button_a.is_pressed():
        display.scroll("name")
    elif button_b.is_pressed():
        display.scroll("TG")
    sleep(200)

Edit the code to display a happy face when the A-button is pressed and a sad face when the B-button is pressed.

from microbit import *

while True:
    if button_a.is_pressed():
        display.show(Image.HAPPY)
    elif button_b.is_pressed():
        display.show(Image.SAD)
    sleep(200)

10.7. If - elif - else

../_images/if_elif_else.png
Using if, elif and else together provides 3 branches in the code.
The code below checks if the A-button is pressed and displays “A” if it is.
If A is not pressed, the code then checks if the B-button is pressed and displays “B” if it is.
If neither A nor B is pressed, “X” is displayed.
from microbit import *

while True:
    if button_a.is_pressed():
        display.show("A")
    elif button_b.is_pressed():
        display.show("B")
    else:
        display.show("X")
    sleep(200)

Tasks

  1. Edit the code to scroll your name when the A-button is pressed and your school house when the B-button is pressed and your Tutor group when nothing is pressed.

  2. Edit the code to display a happy face when the A-button is pressed and a sad face when the B-button is pressed and a confused face when nothing is pressed.

Edit the code to scroll your name when the A-button is pressed and your school house when the B-button is pressed and your Tutor group when nothing is pressed.

from microbit import *

while True:
    if button_a.is_pressed():
        display.scroll("name")
    elif button_b.is_pressed():
        display.scroll("house")
    else:
        display.scroll("TG")
    sleep(200)

Edit the code to display a happy face when the A-button is pressed and a sad face when the B-button is pressed and a confused face when nothing is pressed.

from microbit import *

while True:
    if button_a.is_pressed():
        display.show(Image.HAPPY)
    elif button_b.is_pressed():
        display.show(Image.SAD)
    else:
        display.show(Image.CONFUSED)
    sleep(200)

10.8. If - elif - elif - else

../_images/if_elif_elif_else.png
Using if, two elif and else together provides 4 branches in the code.
The code below first checks whether both buttons are pressed.
The logical keyword and requires both conditions to be True for the combined condition to be True.
If either button is not pressed the combined condition, button_a.is_pressed() and button_b.is_pressed(), will be False.
from microbit import *

while True:
    if button_a.is_pressed() and button_b.is_pressed():
        display.show(Image.ARROW_N)
    elif button_a.is_pressed():
        display.show(Image.ARROW_W)
    elif button_b.is_pressed():
        display.show(Image.ARROW_E)
    else:
        display.show(Image.ARROW_S)
    sleep(100)

Tasks

  1. Edit the code to scroll your favourite subject when both buttons are pressed together, your best subject when the A-button is pressed, your favourite sport when the B-button is pressed, and your favourite food when nothing is pressed.

  2. Edit the code to display a giraffe when both buttons are pressed, a duck when the A-button is pressed, a rabbit when the B-button is pressed and a snake when nothing is pressed.

Edit the code to scroll your favourite subject when both buttons are pressed together, your best subject when the A-button is pressed, your favourite sport when the B-button is pressed, and your favourite food when nothing is pressed.

from microbit import *

while True:
    if button_a.is_pressed() and button_b.is_pressed():
        display.scroll("digistem")
    elif button_a.is_pressed():
        display.scroll("maths")
    elif button_b.is_pressed():
        display.scroll("table tennis")
    else:
        display.scroll("chicken 5 spice")
    sleep(100)

Edit the code to display a giraffe when both buttons are pressed, a duck when the A-button is pressed, a rabbit when the B-button is pressed and a snake when nothing is pressed.

from microbit import *

while True:
    if button_a.is_pressed() and button_b.is_pressed():
        display.show(Image.GIRAFFE)
    elif button_a.is_pressed():
        display.show(Image.DUCK)
    elif button_b.is_pressed():
        display.show(Image.RABBIT)
    else:
        display.show(Image.SNAKE)
    sleep(100)

10.9. 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 and 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

  1. Edit the code to adjust the scroll delay in steps of 25.

  2. Write code to alter a guess_number variable in steps of 1 by the buttons. Start the number at 5 and limit it to a minimum of 1 and a maximum of 9. Use was_pressed.

  3. 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.

Edit the code to adjust the scroll delay in steps of 25.

from microbit import *

delay_time = 80

while True:
    if button_a.is_pressed():
        delay_time += 25
    elif button_b.is_pressed():
        delay_time -= 25
    else:
        sleep(100)
    display.scroll("ABC", delay=delay_time)

Write code to alter a guess_number variable in steps of 1 by the buttons. Start the number at 5 and limit it to a minimum of 1 and a maximum of 9. Use was_pressed.

from microbit import *

guess_number = 5
step = 1

while True:
    if button_a.was_pressed():
        # limit max to 9
        guess_number = min(9, guess_number + step)
    elif button_b.was_pressed():
        # limit minimum to 1
        guess_number = max(1, guess_number - step)
    else:
        sleep(100)
    display.show(guess_number)

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

from microbit import *

guess_number = 5
while True:
    if button_a.is_pressed() and button_b.is_pressed():
        display.show(guess_number, delay=80)
        # now start again
        guess_number = 5
    if button_a.was_pressed():
        guess_number = min(9, guess_number + 1)
    elif button_b.was_pressed():
        guess_number = max(1, guess_number - 1)
    else:
        sleep(100)
    display.show(guess_number, delay=80)
    sleep(200)