Variables¶

Variables are used to store data that may change, such as the number of points you have in a game. Variables can have different data types.

A variable can be thought of as a box that the computer can use to store a value. The value held in that box can change or ‘vary’. All variables are made up of three parts: a name, a type and a value. In the figure below there are three variables of different types:

Image from: <https://developer.mozilla.org/en-US/docs/Learn/JavaScript/First_steps/Variables>

The variable name contains the string Bob, the variable winner contains the value True and the variable score contains the value 35.

In Python we must give the variables we want to use a name and once we have done that we can start to use them, assigning and manipulating values.

Let’s look at an example using arithmetic operators. Imagine that you want to convert the temperature you read from the microbit in Celsius to Fahrenheit, you could use code like this

celsiusTemp = temperature()
fahrenheitTemp = celsiusTemp * 9 / 5 + 32

Assigning number variables¶

Set variables to their initial values at the start of a program using the = sign:

>>> count = 0

Changing number variables¶

You can modify number variables by adding, subtracting, multiplying and dividing:

increment¶

score = 2
score += 5
display.scroll(score)

decrement¶

score = 7
score -= 5
display.scroll(score)

multiply¶

score = 2
score *= 5
display.scroll(score)

divide¶

score = 10
score /= 2
display.scroll(score)

Assigning strings¶

A group of words or symbols is called a string. Put quotation marks around the contents of a string:

name = 'micro:bit'
display.scroll(name)

You can use single ' or double quotation marks " around strings, but you should be consistent. We use single quotation marks in examples because they are easier to type and double quotation marks can be confused for two single quotation marks.

Boolean variables¶

Boolean variables only have two values: True or False. They can be useful for adding control to your program:

# Imports go at the top
from microbit import *

running = False
while True:
        if button_a.was_pressed():
                running = not running
        if running:
                display.show(1)
        else:
                display.show(0)

This example uses a Boolean variable running to create a toggle button. The program has two states: ‘running’ (showing 1 on the display) and ‘not running’ (showing 0). Pressing button A toggles the state of the running variable between False and True every time you press it.

not is a logical operator that flips the state of the variable. If its value was True, it becomes False. If it was False, it becomes True.

Note

True and False must start with capital letters.

Variable names¶

should describe what they contain so your code is easier to understand
must start with a letter, not a number
cannot contain any spaces
can’t be the same as Python keywords like True, if or while
are case-sensitive: score would be a different variable to Score

Exercises¶

Try to output a variable through display.show() by assigning a different number to the variable number and Number. For example 20 for number and 10 for Number.
Try to use increment these variables by incrementing them by 10 and show the result on the LED screen.