1.2 Expressions

An expression is a written combination of data with operations to perform on that data. That sounds quite formal, so imagine a mathematical formula like:

15 × 6² + 3 × 6 – 4 .

This expression contains six pieces of data: 15, 6, 2, 3, 6, and 4. There are five operations: multiplication, exponentiation, addition, multiplication, and subtraction. If I rewrite this using symbols often used in programming languages, it is:

15 * 6**2 + 3 * 6 - 4

See that repeated 6? Suppose I want to consider different numbers in its place. I can write the formula:

15 × x² + 3 × x – 4

and the corresponding code expression:

15 * x**2 + 3 * x - 4

If I give x the value 6, I get the original expression. If I give it the value 11, I calculate:

15 * 11**2 + 3 * 11 - 4

We call x a variable and the process of giving it a value assignment. The expression...