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4. BUTTONS

 

 

YOU LEARN HERE...

 

how to use the buttons to develop interactive programs.

 

 

CHECK IF A BUTTON IS PRESSED

 

The micro:bit has two programmable buttons. The left one is referenced with the object variable button_a, the right one with button_b.

The function button_a.is_pressed() returns True if the left button is pressed at the moment of the call (analog button_b_is pressed()).

 

 

 

Your program is waiting in an endless loop until you press button A. Then the middle LED starts flashing with a period of 2s, as long as you keep the button pressed.

To structure your program, define a function blink(x, y). It turns the LED at position x, y on and off.


 



Program:

from microbit import *

def blink(x, y):
    display.set_pixel(x, y, 9)
    sleep(1000)    
    display.set_pixel(x, y, 0)
    sleep(1000)    

while True:
    if button_a.is_pressed():
        blink(2, 2)
    sleep(10)    
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In the main loop the state of the button is "polled" in an endless loop. sleep(10) is important in order to prevent the waste of computer resources when the button is not pressed.

 

 

CHECK IF THE BUTTON WAS HIT

 

In the following program you want to flash the LED with a period of 2 seconds. By clicking button A, the flashing is interrupted and a square is displayed.

If you click the button only for a short time,  the following program does not work correctly. Most of the time it misses the button click
 


Program:

from microbit import *

def blink(x, y):
    display.set_pixel(x, y, 9)
    sleep(1000)    
    display.set_pixel(x, y, 0)
    sleep(1000)    

while True:
    if button_a.is_pressed():
        display.show(Image.SQUARE)
        sleep(1000)
        display.clear()
    blink(2, 2)
    sleep(10)
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The wrong behavior has a simple explanation: Since the program spends most of the time in the blinking function, it is improbable that it just calls button_a.is_pressed() at the time you click the button.

To overcome this failure, you must use button_a.was_pressed(). This function "remembers" that you clicked the button anytime since the last call and returns True, if this is the case. The click is perceived as an event that is registered by the system even if your program is doing something else at the the time the button is down.

The corrected program responds to clicks as desired.

Program:

from microbit import *

def blink(x, y):
    display.set_pixel(x, y, 9)
    sleep(1000)    
    display.set_pixel(x, y, 0)
    sleep(1000)    

while True:
    if button_a.was_pressed():
        display.show(Image.SQUARE)
        sleep(1000)
        display.clear()
    blink(2, 2)
    sleep(10)    
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MEMO

 

You can develop interactive programs which respond to a button that is hold down or a button that was clicked. is_pressed() returns True, if the button is pressed at the moment of the function call. was_pressed() returns True, if the button has been clicked since the start of the program or since the last call.

 

 

EXERCISES

 

 

1.

Program the buttons as follows:

If you press button A, an arrow appears pointing to the left (ARROW_W), if you press the right button, an arrow  appears pointing to the right (ARROW_E)..

 

2.

Program a click counter: Each time you click button A, the number n, which is 0 at the beginning, will be increased by one and the number is shown in the terminal window as a check. When clicking button B, the number appears as scrolling text on the display.

Remark: The command scroll() can only display a text (string). Use str(n) you convert a number to a string.

3.

You simulate the throwing of a dice: Each time you click button A, the image of a random number between 1..6 appears on the display.

Hint: To create dice numbers as images, use the string parameter of Image(), e.g.
img = Image('00000:09090:00000:09090:00000:')

2-1
Professional information:

Images defined in the Image class:

HEART, HEART_SMALL, HAPPY, SMILE, SAD, CONFUSED, ANGRY, ASLEEP, SURPRISED,
SILLY, FABULOUS, MEH, YES, NO, CLOCK12, CLOCK11, CLOCK10, CLOCK9, CLOCK8,
CLOCK7, CLOCK6, CLOCK5, CLOCK4, CLOCK3, CLOCK2, CLOCK1, ARROW_N,
ARROW_NE, ARROW_E, ARROW_SE, ARROW_S, ARROW_SW, ARROW_W, ARROW_NW,
TRIANGLE, TRIANGLE_LEFT, CHESSBOARD, DIAMOND, DIAMOND_SMALL, SQUARE,
SQUARE_SMALL, RABBIT, COW, MUSIC_CROTCHET, MUSIC_QUAVER, MUSIC_QUAVERS,
PITCHFORK, XMAS, PACMAN, TARGET, TSHIRT, ROLLERSKATE, DUCK, HOUSE,
TORTOISE, BUTTERFLY, STICKFIGURE, GHOST, SWORD, GIRAFFE, SKULL, SNAKE