Numbers and names

Part of the free LogicWiz Python course, Module 1: Start coding. The examples and practice run in the app.

Aarav says 2 + 3 * 4 is 14. Meher says 20. Python agrees with Aarav, every time. Why?

You'll learn to

• Use + - * / // % ** like a calculator
• Predict which operator goes first (precedence), including right-to-left `**`
• Store values in names and update them
• Read `x = x + 1` the way Python does

Learn

Python as a calculator

print(7 + 3)
print(7 - 3)
print(7 * 3)
print(7 / 2)
print(7 // 2)
print(7 % 2)
print(2 ** 10)

/ always gives a decimal (3.5). // divides and drops the fraction (3). % gives the remainder (1). ** is power: 2 to the 10th is 1024.

The seven arithmetic operators

Who goes first: precedence

Python follows a fixed order, like BODMAS:

1. Brackets ( )
2. ** power, and it goes right to left. It even beats a minus sign in front: -2 ** 2 is -(2 ** 2) = -4 (a board-exam favourite)
3. * / // %, left to right
4. + -, left to right

So 2 + 3 * 4 is 2 + 12 = 14. 2 ** 3 ** 2 is 2 ** 9 = 512, not 8 ** 2. When in doubt, add brackets: they cost nothing and make code clear.

Precedence in action

print(2 + 3 * 4)
print((2 + 3) * 4)
print(10 - 4 - 3)
print(2 ** 3 ** 2)
print((2 ** 3) ** 2)
print(-2 ** 2)

10 - 4 - 3 goes left to right (3). ** goes right to left. -2 ** 2 is -(2 ** 2) = -4, because ** binds tighter than the minus sign.

Names point to values

price = 15 does not mean price equals 15 like in maths. It means make the name price point to the value 15.

Think of a name as a sticky label on a value. Later you can move the label: price = 18 peels it off 15 and sticks it on 18.

A name must exist before you use it. Using a name you never set gives NameError.

Watch the labels move

samosa = 15
chai = 10
total = samosa * 2 + chai
print(total)
samosa = 18
print(total)
total = total + 5
print(total)

Changing samosa later did not change total. total got the value 40 when its line ran, and it keeps 40 until you assign it again.

x = x + 1 is not a broken equation. Python works out the right side first (take x, add 1), then points x at the result. Shortcut: x += 1. Same for -=, *=, //=. And you can set two names at once: a, b = 5, 10, or swap them with a, b = b, a.

CBSE loves "Evaluate: 2 ** 3 ** 2" and "Evaluate: 15 // 4 * 2 % 3". Apply the order rule one step at a time and write each step. Also expected: augmented assignment (+=) and multiple assignment (a, b = b, a).

In one line

// drops the fraction, % gives the remainder, ** runs first and right to left; = points a name at a value, it is not an equation.