Variables and Assignment

A program needs named boxes because it works with values that change or that get reused. Writing the number 5 directly into your code means you're stuck with 5 forever. Storing it in a variable called score or total means the same line of code can work no matter what value the variable currently holds. The name also makes code readable: total makes sense to a human in a way that a bare 5 scattered through the program never does.

Every assignment statement works the same way, in two steps, in order. First the computer evaluates the entire right-hand side using whatever values the variables currently hold. Only after that result is fully computed does it get copied into the variable on the left. This matters because the variable being assigned can appear on both sides. When a is set to a plus 3, the computer reads a's old value, adds 3, and only then overwrites a with the new total. The old value is gone the instant the new one is stored; a variable holds exactly one value at a time.

This two-step rule is why copying one variable into another does not create a lasting link between them. It also sets up everything that comes next in the unit: loops repeat an assignment statement many times, building up a running total or counter one step at a time, and procedures use variables to pass values in and out. Get comfortable with assignment now, because every loop trace and procedure trace afterward is really just this same copy-after-evaluate rule happening over and over.