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.

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 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. e.g if button_a.is_pressed():.
Multiple elif blocks 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

if requires a condition that returns True or False.
The code below checks if the A-button is pressed and displays “A” if it is.
A short sleep pauses the code between presses.

from microbit import *

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

Tasks

Edit the code to scroll your name when the A-button is pressed.
Edit the code to display a happy face when the A-button is pressed.
Edit the code to scroll your age when the B-button is pressed.
Edit the code to display a sad face when the B-button is pressed.

10.5. if - else

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

Edit the code to scroll your name when the A-button is pressed and to show “?” when nothing is pressed.
Edit the code to display a happy face when the A-button is pressed and a sad face when nothing is pressed.
Edit the code to display a sad face when the B-button is pressed and a confused face when nothing is pressed.

10.6. if - elif

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

Edit the code to scroll your name when the A-button is pressed and your tutor group when the B-button is pressed.
Edit the code to display a happy face when the A-button is pressed and a sad face when the B-button is pressed.

10.7. If - elif - else

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

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.
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.

10.8. If - elif - elif - else via logical and

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 with 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

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.
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.