Microbit Lesson 2: Conditional execution
Lesson overview
This lesson:
- The idea of conditional execution
- Syntax: The colon and indentation
- if statement
- while statement
- Microbit feature: Buttons
- Microbit feature: Touch pins
Video
Conditional execution: If
Recall last lession we said: statements are executed in order, top down - unless otherwise directed . We will now look at the "unless otherwise directed" bit.
An if statement asks a question. If the answer is True, it will run the intended section that immediately follows. Stop intending to return to normal. If the answer is False, Python will skip over any new intended section and look for the next line that matches the previous level of indenting.
from microbit import *
display.scroll("How awesome is Python?", delay=75)
if button_a.was_pressed():
display.show(Image.HAPPY)
if button_b.was_pressed():
display.show(Image.SAD)
- What happens if you don't press either button?
- What happens if you press both buttons?
Note: The delay=75 parameter instructs the Microbit how fast to scroll text. The default speed is 150, any number smaller than that will scroll faster.
We can extend this program as follows...
from microbit import *
display.scroll("How awesome is Python?", delay=75)
if button_a.was_pressed():
display.show(Image.HAPPY)
sleep(2000)
display.scroll("Glad you think Python is amazing", delay=75)
if button_b.was_pressed():
display.show(Image.SAD)
In this instance, we are instructing Python to run three commands if button A was pressed. We can provide as many commands as we wish, provided we match the same level of indentation.
Conditional execution: While
Consider this one change...
from microbit import *
while True:
display.scroll("How awesome is Python?")
if button_a.was_pressed():
display.show(Image.HAPPY)
sleep(2000)
display.scroll("Glad you think Python is amazing", delay=75)
if button_b.was_pressed():
display.show(Image.SAD)
A while statement works effectively the same as the if statement, in that it poses a question and if the response is True it will execute the indented code, and if the answer is False it will skip the intended code. The key difference is that if it did run the indented code, prior to moving on, Python will cycle back to the while statement and re-ask the question. This has the effect of creating a looping effect so the indented code will repeatedly run until the question responses with False.
One side-effect of the above while loop is that because we have specified the condition response as True, it will never quit the loop. We could modify it....
from microbit import *
while not pin0.is_touched():
display.scroll("How awesome is Python?")
if button_a.was_pressed():
display.show(Image.HAPPY)
sleep(2000)
display.scroll("Glad you think Python is amazing", delay=75)
if button_b.was_pressed():
display.show(Image.SAD)
display.scroll("Bye!")
We have now provided a condition with which to exit the loop. Execute this program and touch both the GND pin and the 0 pin on the Microbit at the same time to terminate the loop.
Nested if's
We can put "if" statements inside other "if" statements. Consider how this would behave?
from microbit import *
display.scroll("Whose your Marvel character?")
display.scroll("Goodie or badie?")
if button_a.was_pressed():
display.scroll("Boy or girl?")
if button_a.was_pressed():
display.scroll("Iron man")
elif button_b.was_pressed():
display.scroll("Captain Marvel")
elif button_b.was_pressed():
display.scroll("Secretly good?")
if button_a.was_pressed():
display.scroll("Nebula")
elif button_b.was_pressed():
display.scroll("Thanos")
Activity: 20 questions
By creating a series of yes/no questions using if statements inside of if statements, create a reduced version of a 20 questions game.