Operators and expressions

An expression is anything that produces a value: 3 + 4, price * 2, age >= 18. Operators are the symbols that combine values into expressions.

Arithmetic#

/ always gives a float, even for 8 / 2, which is 4.0. When you want a whole number, use //.

The three that matter most for problems#

// and % together take a number apart. For any whole number n, n % 10 is its last digit and n // 10 is everything else:

n = 4071
print(n % 10)     # 1
print(n // 10)    # 407

% tests divisibility. n % 2 == 0 means even; n % 3 == 0 means divisible by three.

% wraps around. To step through positions 0 to k - 1 in a circle, (i + 1) % k returns to 0 after k - 1.

Comparisons#

These produce True or False:

5 == 5      # equal
5 != 3      # not equal
5 > 3       # greater
5 <= 5      # less or equal

= assigns and == compares. Mixing them up is one of the most common beginner mistakes.

Python lets you chain comparisons the way you would write them in maths:

0 <= x < 10       # is x between 0 and 9?

and, or, not#

age = 20
has_ticket = True

age >= 18 and has_ticket    # True: both are true
age < 13 or has_ticket      # True: at least one is true
not has_ticket              # False

Python stops as soon as it knows the answer. In a and b, if a is false, b is never looked at. That lets you write checks safely:

items = []
len(items) > 0 and items[0] == "x"    # False, and items[0] is never read

Shorthand assignment#

score = 10
score += 5     # score = score + 5  ->  15
score -= 3     # 12
score *= 2     # 24
score //= 5    # 4

There is no ++ in Python. Write count += 1.

Order of operations#

Python follows the usual maths order: brackets, then **, then * / // %, then + -, then comparisons, then not, and, or.

2 + 3 * 4        # 14, not 20
(2 + 3) * 4      # 20
-2 ** 2          # -4: the power happens before the minus

When in doubt, add brackets. They cost nothing and make your intent clear to the next reader.

Key points#

• / gives a float, // gives the whole part, % the remainder.
• n % 10 and n // 10 take a number apart one digit at a time.
• == compares; = assigns.
• and and or stop as soon as the answer is known.
• Use += instead of ++, and brackets whenever the order is not obvious.

Next: working with text, and reading what the user types.