What are variables?
Variables are named storage locations for temporarily storing data while a program is executing.
There are rules about naming variables, 5 types of data you can store inside them and some terminology to know about.
Variable naming rules
- Variable names must start with an underscore or a letter
- Variable names cannot contain spaces
- Variable names should be meaningful i.e. they should give an indication of what data is held in
Variable naming conventions
Naming conventions are used to ensure variable names in a program are standardised. The three most common naming conventions are:
Snake case: everything is lower case and spaces are shown as underscores e.g. snake_case
Camel case: the first word is lower case and all subsequent words start with a capital letter and spaces are ignored e.g. camelCase
Tall camels: this is the same as camel case except the first word also uses a capital letter e.g. CamelCase
Variable data types
There are 5 data types you need to be familiar with at GCSE level as shown below.
| Data type |
Description |
| Character |
An individual character from the character set being used |
| String |
A group of characters |
| Integer |
A whole number |
| Float/Real |
A number with a decimal/fractional part |
| Boolean |
Can hold the value True or False |
Declaring and assigning variables
Declaring a variable means telling the program the name of it and its data type. In many languages this must be done as a separate step on its own. In Python we can declare and assign a value to a variable at the
same time. When you give a suitable variable name and use the assignment operator(=) to assign a new value Python accepts this name as a variable and sets its data type to the data type of the value you assign.
There are some differences in how assignment is carried out in python.
| Data type |
Assignment example |
Details |
| Character |
grade = "A" |
A character must be surrounded by speech marks |
| String |
name = "Bob" |
A string must be surrounded by speech marks |
| Integer |
age = 42 |
Integers must not be in speech marks or they will be treated as strings |
| Float/Real |
length = 1.358 |
Float/real numbers must not be in speech marks or they will be treated as strings |
| Boolean |
member = True |
No speech marks and capital first letter in True or False |
Casting
Casting is when we convert a variable from one data type to another. This will only work when the data in the variable is valid in the new data type. Whenever we get input, it is automatically stored as a string, so it must
be cast if you want it stored as a different data type.
num = int(num) #Takes what is stored in num and converts it to an integer as long as there is a number stored in num
num = float(num) #Takes what is stored in num and converts it to a float as long as there is a number stored in num
num = str(num) #Takes what is stored in num and converts it to a string