Creating Your Program¶

On this page we will learn

the structure every program in this course uses: imports, setup and main loop
how to organise a program into input, process and output
how to plan, build and test a program step by step

Terminology

import – a line that brings code someone else has already written into our program so we can use it.
setup – the part of a program that runs once when it starts, for creating devices and giving variables their starting values.
main loop – the part of a program, inside while True:, that repeats forever to read inputs, make decisions and control outputs.
variable – a named place in memory that stores a value our program can use and change.
indentation – spaces at the start of a line that show which code belongs inside a loop or if statement.
input – information our program reads, such as a button press, a sensor reading or a radio message.
process – the step where our program works out what to do with its inputs, using calculations and decisions.
output – the result our program shows or does, such as using the display, sounds, LEDs or motors.
if statement – code that makes a decision by running some lines only when a condition is True.
comment – a note in our code, starting with #, that explains the code and is ignored when the program runs.

Every example in this course uses the same structure. We will use it for our own programs too, because it keeps our code organised and makes problems easier to find and fix.

The structure¶

A micro:bit program has three parts, always in this order:

Part	Runs	What goes here
Imports	once, first	the libraries and drivers your program uses
`# Setup`	once, before the loop	creating sensors and devices, and variables with starting values
`# Main loop`	over and over, forever	the code that reads inputs, makes decisions and controls outputs

Imports¶

Imports bring in code that someone else has already written, so we can use it in our program.

Every program starts with from microbit import *. This gives you the display, buttons, sensors and sleep().
Add other modules your program needs, such as import music, import radio or a PiicoDev driver.

`# Setup`¶

Code under # Setup runs once, when the program starts. We use it for things that only need to happen once:

creating PiicoDev devices, for example sensor = PiicoDev_VL53L1X()
setting up variables with their starting values, for example score = 0
settings that don't change, for example radio.config(group=7)

If your program doesn't need any setup, you can leave this part empty.

`# Main loop`¶

while True: starts a loop that repeats forever. A micro:bit program usually never finishes: it keeps checking its inputs and updating its outputs until it is turned off.

Everything indented under while True: is inside the loop.

Input, process, output¶

Inside the main loop, we organise our code into three steps:

Input → read the information your program needs, such as a button, a sensor or a radio message.
Process → work out what to do with it, using calculations and decisions (if statements).
Output → show the result, using the display, sounds, LEDs or motors.

Short examples in this course often combine these steps. In our own programs, we use # Input, # Process and # Output comments to keep them separate.

We end the loop with a short sleep(), so the micro:bit isn't checking faster than it needs to.

Template¶

New program, new folder

Every program needs its own folder with its own main.py. Before you start a new program, create a new folder for it. See Create a new program for how.

Copy this into a new main.py to start a program. It is also in the micropython/template folder of your tutorial files.

from microbit import *

# Setup

# Main loop
while True:
    # Input

    # Process

    # Output

    sleep(100)

Worked example: Too Hot Alarm¶

Let's build a program that shows a happy face when the room is comfortable, and a sad face and a beep when it gets too hot.

Step 1: Plan¶

Before we write any code, we list the inputs, processing and outputs:

Step	What happens
Input	read the temperature
Process	check if the temperature is above the limit
Output	happy face if it isn't; sad face and a beep if it is

Step 2: Imports¶

The temperature sensor and display are in the microbit library. The beep needs the music module.

from microbit import *
import music

Step 3: Setup¶

The temperature limit only needs to be set once, so it goes in setup. Storing it in a variable means we only need to change one line to adjust the alarm.

# Setup
limit = 25

Step 4: Main loop¶

Now we add the loop, then fill in the input, process and output steps from our plan.

# Main loop
while True:
    # Input
    temp = temperature()

    # Process
    too_hot = temp > limit

    # Output
    if too_hot:
        display.show(Image.SAD)
        music.pitch(880, 200)
    else:
        display.show(Image.HAPPY)
    sleep(1000)

The complete program¶

from microbit import *
import music

# Setup
limit = 25

# Main loop
while True:
    # Input
    temp = temperature()

    # Process
    too_hot = temp > limit

    # Output
    if too_hot:
        display.show(Image.SAD)
        music.pitch(880, 200)
    else:
        display.show(Image.HAPPY)
    sleep(1000)

PRIMM

Predict what you think will happen. Be specific.
Run the program.
Time to investigate the code. What does each line do?

Code explanation

line 1 → imports all the commands from the microbit library.
line 2 → imports the music module.
line 5 → stores the temperature limit, 25 °C, in limit.
line 8 → starts an endless loop.
line 10 → input: reads the temperature and stores it in temp.
line 13 → process: checks if temp is greater than limit, and stores True or False in too_hot.
line 16 → output: if too_hot is True…
line 17 → …shows a sad face.
line 18 → …and beeps.
line 19 → otherwise…
line 20 → …shows a happy face.
line 21 → waits 1 second before checking again.

Step 5: Test¶

Run the program. You should see a happy face.
Hold the micro:bit's processor (the back, near the top) or breathe on it to warm it up.
If the temperature doesn't go over the limit, change limit on line 5 to a number a little lower than the current temperature and run it again.
When the temperature passes the limit, you should see a sad face and hear a beep.