Arithmetic and Operators

Why this matters

Almost every program does some maths: totals, averages, conversions. Python has the usual operators plus two that are surprisingly useful: // and %.

The idea

Operator	Meaning	Example	Result
+ -	add, subtract	7 - 2	5
*	multiply	3 * 4	12
/	divide (always a float)	7 / 2	3.5
//	floor division: divide and drop the remainder	7 // 2	3
%	modulo: the remainder	7 % 2	1
**	power	2 ** 3	8

// and % work as a pair: 7 // 2 is how many whole 2s fit into 7 (three), and 7 % 2 is what is left over (one).

Order of operations is like school maths: ** first, then * / // %, then + -. Use brackets when in doubt: (2 + 3) * 4 is 20, but 2 + 3 * 4 is 14.

A very common pattern is updating a variable using its own value:

score = 10
score = score + 5   # 15
score += 5          # shorthand for the line above: 20

Try it

Predict, then run: print(17 // 5), print(17 % 5), print(2 ** 10), print(2 + 3 * 4), print((2 + 3) * 4).

Your turn

The starter program reads a number of seconds. Convert it into hours, minutes and seconds with // and %, and print them like this (for 5000 seconds):

1 h 23 m 20 s

Hint for thinking: there are 3600 seconds in an hour and 60 in a minute. Type a number in the Input box to try it; Check tries several.

Recap

• / always gives a float; // gives whole-number division and % the remainder.
• ** is power.
• Use brackets to make the order of operations obvious.

Common mistakes

• Expecting 7 / 2 to be 3. It is 3.5. Use // for 3.
• Writing ^ for power. In Python ^ is something else; use **.
• Forgetting that input is text. Convert with int() before calculating.