import time
from machine import Pin
time.sleep(0.1) # Wait for USB to become ready

# initialize the counter to 0
counter = 0

# initialize the input button
btn1 = Pin(28, Pin.IN, Pin.PULL_DOWN)

# initialize the output leds as per the array of pins assigned
led_pins = [21, 20, 19, 18]
# using for loop comprehension to create a list of output objects
leds = [Pin(i, Pin.OUT) for i in led_pins]

# the forever loop
while True:
    if btn1.value():
        counter += 1
        # reset counter if reached max value of 4 digit binary number
        if counter >= 16:
            counter = 0
        # convert the number to a string which represents its binary form
        binary_value = bin(counter)[2:]
        # padding with leading zeros to ensure the string is 4 char long
        binary_value = "0"*(4-len(binary_value)) + binary_value
        for i, state in enumerate(binary_value):
            print(binary_value)
            s = int(state)
            leds[i].value(s)
        # to prevent unstable rapid input registers
        time.sleep(0.25)