Update

Last chapter you drew one frame. The computer drew it twenty times a second — but it was the same picture every time, so nothing appeared to move.

Movement needs a second function:

Now the loop is a real loop:

1. call update — the numbers change
2. call draw — a picture is made from those numbers
3. put it on screen
4. do it again, twenty times a second

update changes numbers. draw turns numbers into a picture. Keeping those two jobs apart is the single most useful habit in this milestone.

What is state?

state is a dictionary that survives between frames. Everything the animation needs to remember lives in it.

You already have some keys for free:

draw reads them. update changes them. That is the whole conversation.

The rule that will catch you

update must return state. Just like draw returns the grid:

Miss it and you'll see "update() didn't return anything."

Ignore keys for now. It's how the keyboard gets in, and you'll use it in Chapter 9 when you build something you can actually play.

Rows count downward

Worth burning in now, because it feels backwards at first:

• state["y"] bigger → further down the screen
• state["y"] smaller → further up

That is just how grid[row][col] works — row 0 is the top. A dot that "falls" has a y that grows.