Variables & values

Your phone knows how many steps you've walked today. Your favourite game remembers your score. For a program to remember something, it needs a named place where the value lives. That's exactly what a variable is, and here you'll learn how it works.

A drawer with a label

Picture a variable as a labelled drawer. The label shows the name, inside the drawer lies the value. With x = 5 you put the number 5 into the drawer named x, which is called assigning. The equals sign is a command here, not a statement about equality: put the value on the right into the drawer on the left. If you ask for x later, the program looks into the drawer and finds the 5. If you then assign x = 9, the 5 is taken out and the 9 is put in: a drawer only ever holds the newest value.

Calculating with variables

The best thing about variables: you can calculate with them without knowing the value. The expression x + 3 means: take whatever is in x right now and add 3. If x holds a 5, you get 8. If it holds a 10, you get 13. The calculation stays the same, only the drawer's contents change. Even score = score + 1 is allowed: first the old value is read, then 1 is added, and the result goes back into the drawer. It works the same with minus: balance = balance - 8 reads the old amount, subtracts 8 and puts the result back in.

Good names pay off

The computer doesn't care about the name, but you do. A program full of variables like x, y, and z is like a cabinet full of drawers without labels: after a week you won't remember what's where. Names like score, age, or balance tell you immediately what's inside the drawer. Good names make programs easier to read and mistakes easier to find.