Variables, data types and arithmetic

You are scoring points in a game: first 0, then 10, then 25. The program has to remember this number somewhere and update it each time. That is what variables are for. Think of a variable as a box with a label: you put a value inside, and later you find it by its name to look at it or change it.

Variables

name = 'Murad'
age = 14
height = 1.62
print(name, age, height)

age = age + 1
print('Next year:', age)

Murad 14 1.62
Next year: 15

Note that in programming = is not the equals sign from maths but assignment. The line age = age + 1 reads: “take the old value of age, add 1 and store the result back in age”. A variable's value can change at any time — that is where the name comes from.

• A name may contain letters, digits and the underscore (_) but cannot start with a digit: score2 is fine, 2score is not.
• Names cannot contain spaces. Join words with underscores: first_name, total_price.
• Upper and lower case matter: age and Age are two different variables.
• You cannot use Python's reserved words (if, for, class, True and others) as names.
• Choose meaningful names: price tells you much more than x.

Basic data types

Every value has a type, and the type decides what you can do with it: numbers can be added up, strings can be joined together. Python works out the type by itself — you do not have to write it.

You can check the type of a value with the type() function.

print(type(14))
print(type(1.62))
print(type('Baku'))
print(type(True))
print(type('42'))

<class 'int'>
<class 'float'>
<class 'str'>
<class 'bool'>
<class 'str'>

Look at the last line: '42' is in quotes, so it is a string, not a number. True and False are the answers “yes” and “no” — you will need them a lot in the lesson on conditions.

Arithmetic operators

Leyla buys 3 notebooks at 2.5 manat each. Let's hand the calculation to Python and try the other operators too.

price = 2.5
count = 3
total = price * count
print('Total:', total)

print(17 // 5, 17 % 5)
print(2 ** 10)
print(2 + 3 * 4, (2 + 3) * 4)

Total: 7.5
3 2
1024
14 20

Python follows the order of operations from maths: first powers, then *, /, //, %, and finally + and -. Use parentheses to change the order: 2 + 3 * 4 gives 14, but (2 + 3) * 4 gives 20.

minutes = 135
hours = minutes // 60
rest = minutes % 60
print(hours, 'h', rest, 'min')

2 h 15 min

Updating a variable based on its own value is so common that Python has a shortcut: score += 10 means the same as score = score + 10. The same works for -=, *= and /=.

score = 0
score += 10
score += 5
score -= 3
score *= 2
print('Score:', score)

Score: 24

Converting between types

You cannot add a number to a string directly: if you write '5' + 5, Python raises a TypeError, because it does not know how to combine text and a number. In such cases, convert the value to the type you need: int() makes an integer, float() a decimal number and str() a string.

text = '25'
number = int(text)
print(number + 5)
print(text + '5')
print(float('3.5') * 2)
print('Age: ' + str(14))
print(int(7.9))

30
255
7.0
Age: 14
7

text + '5' glues two strings together, so we get 255 — not the number two hundred and fifty-five but text made of the characters 2, 5 and 5.

width = 7
height = 4
# area = ...
# perimeter = ...
# print Area: and Perimeter:

Area: 28
Perimeter: 22

candies_text = '47'
children_text = '6'
# 1) convert both strings to int
# 2) print Each child: and the share (use //)
# 3) print Left over: and the rest (use %)

Each child: 7
Left over: 5