Foundations

Hello, Numbers!

You're helping a friend split a restaurant bill. The total is $47.50 and there are 4 people. You need to calculate each person's share: 47.50 / 4 = 11.875, round to $11.88.

Print a single number

The simplest thing we can do - tell the computer to show us a number.

number.py
Replay: real traced execution (multi-file project)
print(42)
print(100)
print(-7)

  1. print(42)

    1print(42)2print(100)3print(-7)
    output42
100
-7

The computer echoes back exactly what we ask for.

print In Python, `print(...)` displays the value to the screen.

Basic arithmetic

Now let's do some math. We can add, subtract, multiply, and divide.

arithmetic.py
Replay: real traced execution (multi-file project)
print(10 + 5)
print(10 - 5)
print(10 * 5)
print(10 / 5)
print(10 % 3)

  1. print(10 + 5)

    1print(10 + 5)2print(10 - 5)3print(10 * 5)4print(10 / 5)5print(10 % 3)
    output15
5
50
2.0
1
operators Arithmetic operators: `+` add, `-` subtract, `*` multiply, `/` divide, `%` remainder, `//` integer division.

Grouping with parentheses

What if we want to control the order of operations? Use parentheses.

grouping.py
Replay: real traced execution (multi-file project)
print(2 + 3 * 4)
print((2 + 3) * 4)
print(100 / (5 + 5))

  1. print(2 + 3 * 4)

    1print(2 + 3 * 4)2print((2 + 3) * 4)3print(100 / (5 + 5))
    output14
20
10.0

Without parentheses: 2 + 3 * 4 gives 14 (multiply first). With parentheses: (2 + 3) * 4 gives 20 (add first).

precedence Multiplication and division happen before addition and subtraction, unless you use parentheses.

A real calculation

Let's calculate the area of a rectangle with sides 7 and 5.

example
area.py
Replay: real traced execution (multi-file project)
width = 7
height = 5
print(width * height)

width = 3
height = 5
print(width * height)

width = 10
height = 5
print(width * height)

width = 7
height = 4
print(width * height)

width = 7
height = 8
print(width * height)

  1. width ← 7, height ← 5

    1width→ 7 = 7  #@width=3, 102height→ 5 = 5  #@height=4, 83print(width7 * height5)
    output35

  1. width ← 3, height ← 5

    1width→ 3 = 32height→ 5 = 53print(width3 * height5)
    output15

  1. width ← 10, height ← 5

    1width→ 10 = 102height→ 5 = 53print(width10 * height5)
    output50

  1. width ← 7, height ← 4

    1width→ 7 = 72height→ 4 = 43print(width7 * height4)
    output28

  1. width ← 7, height ← 8

    1width→ 7 = 72height→ 8 = 83print(width7 * height8)
    output56

Complex expression

Combine everything: multiple operations, nested parentheses.

complex.py
Replay: real traced execution (multi-file project)
print((10 + 20) * (30 - 15) / 5)
print(((2 + 3) * (4 + 5)) - 10)
print(100 / 10 / 2)

  1. print((10 + 20) * (30 - 15) / 5)

    1print((10 + 20) * (30 - 15) / 5)2print(((2 + 3) * (4 + 5)) - 10)3print(100 / 10 / 2)
    output90.0
35
5.0

The computer follows the same math rules you learned in school.