Variables, constants & data types

A variable is a named box

• A variable is a name that stores a value.
• You put a value in with = — this is called assignment.
• Use a clear name so the program is easy to read.

age = 15
price = 2.50
name = "Ada"
print(age, price, name)

The five data types

• IGCSE uses five basic data types:
• integer (whole number) → Python int; real (decimal) → float.
• char (one letter), string (text) → str; Boolean (True/False) → bool.

score = 42          # integer
height = 1.75       # real
grade = "A"         # char
town = "Cairo"      # string
passed = True       # Boolean

Constants stay the same

• A constant is a value that never changes while the program runs.
• Python has no special keyword, so we write constants in CAPITALS.
• This warns other readers: do not change me.

PI = 3.14159
VAT_RATE = 0.20
print(PI)

Changing between types

• input() gives a string, so convert it when you need a number.
• int(x) makes a whole number; float(x) a decimal; str(x) makes text.

n = int("5")
print(n + 1)                 # 6
print("Total: " + str(n))    # Total: 5

In the exam: DECLARE and CONSTANT

• Pseudocode names a type with DECLARE, and fixes a value with CONSTANT.

DECLARE Age : INTEGER
Age ← 15
CONSTANT Pi = 3.14

Common mistakes

• Give a variable a value before you use it.
• A constant is a value you do not change — name it clearly.
• Keep the right type: numbers for maths, strings for text.

Now you try

• Make variables and do a little maths with them.
• Press Check answer to test your code.