Expressions

An expression is defined as a combination of constants, variables, and operators . An expression always evaluates to a value . A value or a standalone variable is also considered as an expression but a standalone operator is not an expression.

Examples of expressions in python:

• 125
• n
• n - 25
• 25 + 67
• “abc” + “def”

Precedence of Operators

• Evaluation of the expression is based on precedence of operators. 
• When an expression contains different kinds of operators, precedence determines which operator should be applied first. 
• Higher precedence operator is evaluated before the lower precedence operator.
• Unary operators need only one operand, and they have a higher precedence than the binary operators.
• Parenthesis can be used to override the precedence of operators. The expression within () is evaluated first. 
• For operators with equal precedence, the expression is evaluated from left to right.

Examples:

1. 30 + 40 * 2

Step 1: 30 + (40*2)

Step 2: 30 + 80

Answer: 110

2. 20 - 30  + 50

Step 1: (20-30) + 50

Step 2: -10 + 50

Answer: 40