1.4 Expressions and Operators

An expression combines values, variables, and operators. Programs use expressions to calculate, join, compare, and update data.

Arithmetic operators

Python supports common arithmetic operations, plus a few operators that are especially useful in programming.

The floor-division operator // returns the integer part of a division result.

print(21 // 4)   # 5

The modulo operator % returns the remainder after integer division.

print(22 % 3)    # 1
print(4 % 7)     # 4

Reversing a three-digit integer

Floor division and modulo are often used together to split an integer into digits.

num = int(input("Enter a 3-digit integer: "))
a = num // 100
b = num // 10 % 10
c = num % 10
print("Reversed:", c * 100 + b * 10 + a)

One possible run:

Enter a 3-digit integer: 520
Reversed: 25

The result is displayed as 25 because the reversed number is 025, and leading zeros are not shown when printing an integer.

Compound operators

Compound operators make variable updates shorter.

• a += b means a = a + b
• a -= b means a = a - b
• a *= b means a = a * b
• a /= b means a = a / b
• a %= b means a = a % b

String concatenation

+ is not only for numbers. It can also join strings.

s = "Hello" + "World"
s += "!"
print(s)

Output:

HelloWorld!