# Different screens in a game

[Programming guides](README.md)

A game often has several **screens**: a title screen, the game itself, a winning screen, and a losing screen. These do not have to be separate programs. One program can remember which screen is active.

This article builds on [state](state.md). We will model the changes with text before adding drawings or buttons.

## Choose the next room

Think of the screens as rooms connected by doors. Pressing Start takes you from the title to playing. Winning takes you from playing to the winning room. Some doors should only work from particular rooms.

A change from one state to another is called a **transition**. A transition table helps you decide the rules before you draw anything.

| Current screen | Action | Next screen |
| --- | --- | --- |
| Title, won, or lost | Start | Playing |
| Playing | Win | Won |
| Playing | Lose | Lost |

Our prototype also allows Start during play to restart a round.

## Rehearse a game

The action list is pretend input. It lets us test a whole sequence without clicking buttons.

```pliro
# language: en
let screen = "title"
say screen
for action in ["start", "win", "start", "lose"]:
    if action == "start":
        set screen = "playing"
    else:
        if screen == "playing":
            if action == "win":
                set screen = "won"
            if action == "lose":
                set screen = "lost"
    say screen
```

## Predict, then run

Can the second Start bring us back from the winning screen?

```text
title
playing
won
playing
lost
```

Yes. The current screen is one text value. This prevents combinations such as “won and lost at the same time,” which are easy to create accidentally with several unrelated true/false flags.

## Your challenge

Put `"win"` at the beginning of the action list, before Start. It should leave the title screen unchanged. Explain which condition prevents a player winning before playing.

## A common mistake

Changing a picture without changing the remembered state can make buttons react as if the previous screen were still active. In a graphical version, draw the scene that matches `screen` and handle only the actions allowed there. The strings here are your program's own labels, not special Pliro keywords.

## Where next?

[State](state.md) · [Events](events.md) · [Drawing](graphics.md)