Checkpoint Saves — don't restart the whole level

Learning Goals 3 min

By the end of this lesson you will be able to:

1. Build a Checkpoint flag sprite that writes the player's current position into two variables — (checkpoint-x) and (checkpoint-y) — when the player touches it.
2. Send the player to the checkpoint on death with go to x: (checkpoint-x) y: (checkpoint-y) instead of the level's start position.
3. Reset the checkpoint variables to the level's start every time the player completes a level, so checkpoints are per-level, not permanent.

Warm-Up — death by a thousand restarts 7 min

After L04-20, level 3 has three chasers. Tough. When the cat dies, your current death script probably looks like this:

when I receive (player-died v)
go to x: (-200) y: (-100)
        

Predict — you've crossed three-quarters of level 3 and just touched a chaser. Where does the cat appear?

Today: a Checkpoint sprite, two new variables, and a tiny edit to the death script. Same death penalty (you still lose a life), but you only re-do half the level.

New Concept — saving a position into variables 15 min

A checkpoint is just two numbers in two variables — the x and y the player should respawn at. Whoever updates those numbers controls where you come back. The Checkpoint sprite writes the numbers when touched. The Player sprite reads them when dying.

The two variables

Make two new variables, For all sprites, so any sprite can read or write them:

• (checkpoint-x) — the x-coordinate to respawn at.
• (checkpoint-y) — the y-coordinate to respawn at.

Setting them at level start

When a level begins, the checkpoint is the level's start. The Stage (or the Player) initialises:

when I receive (level-up v)
set [checkpoint-x v] to (-200)
set [checkpoint-y v] to (-100)
        

The Checkpoint sprite overwrites them

Paint a small flag sprite. Place it halfway through level 1 (around x: 30, y: −100). The flag asks one question forever — am I being touched by the Player? — and the instant the answer is yes, it overwrites the two variables and changes colour as a visible confirmation:

when flag clicked
clear graphic effects
forever
  if <touching (Player v) ?> then
    set [checkpoint-x v] to (x position)
    set [checkpoint-y v] to (y position)
    change [color v] effect by (100)
  end
end
        

The Player reads them on death

Tiny edit to the existing death script — replace the hard-coded coordinates with the variables:

when I receive (player-died v)
go to x: (checkpoint-x) y: (checkpoint-y)
        

Worked Example — one Checkpoint flag, one cat, three levels 12 min

We'll keep it focused — one Checkpoint sprite that you place by hand per level (a more advanced version would clone or auto-position, but that's L04-22+).

Step 1 — Make the two variables

Variables → Make a Variable → checkpoint-x, For all sprites. Same for checkpoint-y. Untick both watcher boxes (they're internal — players don't need to see them).

Step 2 — Paint the Checkpoint sprite

New sprite, paint a small white flag on a brown pole. Rename to Checkpoint. Drag it on the Stage to roughly x: 30, y: −100 for level 1's midpoint.

Step 3 — Checkpoint script (the saver)

when flag clicked
clear graphic effects
forever
  if <touching (Player v) ?> then
    set [checkpoint-x v] to (x position)
    set [checkpoint-y v] to (y position)
    change [color v] effect by (100)
  end
end
        

Step 4 — Initialise checkpoint at level start (on the Player)

Three hats — one per level — so every level resets the checkpoint to its own start position. The cat from L04-19 already starts at different x/y per backdrop:

when backdrop switches to [level1 v]
set [checkpoint-x v] to (-200)
set [checkpoint-y v] to (-100)

when backdrop switches to [level2 v]
set [checkpoint-x v] to (-220)
set [checkpoint-y v] to (-80)

when backdrop switches to [level3 v]
set [checkpoint-x v] to (-210)
set [checkpoint-y v] to (-60)
        

Step 5 — Edit the death script

Find your existing when I receive (player-died v). Replace the hard-coded coordinates:

when I receive (player-died v)
change [lives v] by (-1)
go to x: (checkpoint-x) y: (checkpoint-y)
        

Step 6 — Reset the flag's colour on level-up

The flag turned pink when you touched it last level. For the next level it should look fresh again:

when I receive (level-up v)
clear graphic effects
        

Step 7 — Move the flag per level

For levels 2 and 3 the flag should sit at that level's midpoint. Add three position hats on the Checkpoint:

when backdrop switches to [level1 v]
go to x: (30) y: (-100)
show

when backdrop switches to [level2 v]
go to x: (0) y: (-80)
show

when backdrop switches to [level3 v]
go to x: (20) y: (-40)
show
        

Step 8 — Test the loop

Click the flag. Walk the cat to the midpoint flag — it should turn pink. Now deliberately walk into a Lipan. The cat should respawn at the flag, not at the level start. Beautiful — half the level skipped on death.

The Checkpoint sprite's full assembled stack

when flag clicked
clear graphic effects
forever
  if <touching (Player v) ?> then
    set [checkpoint-x v] to (x position)
    set [checkpoint-y v] to (y position)
    change [color v] effect by (100)
  end
end

when I receive (level-up v)
clear graphic effects
        

What you just built: a save system. Tiny one — two numbers — but the exact same pattern as Super Mario's flag, Sonic's lamppost, every "respawn here" rock in every modern platformer.

Try It Yourself — three checkpoint drills 15 min

when flag clicked
forever
  if <touching (Player v) ?> then
    set [checkpoint-x v] to (x position)
    set [checkpoint-y v] to (y position)
    change [color v] effect by (100)
    play sound (Pop v)
  end
end
          

when backdrop switches to [level1 v]
go to x: (130) y: (-100)
show
          

when flag clicked
forever
  if <touching (Player v) ?> then
    if <(checkpoint-armed) = (1)> then
      set [checkpoint-x v] to (x position)
      set [checkpoint-y v] to (y position)
      set [checkpoint-armed v] to (0)
      hide
    end
  end
end
          

Mini-Challenge — the checkpoint that never moves 5 min

"Aishah's stuck flag"

Aishah's checkpoint works perfectly on level 1. She walks halfway, touches the flag, dies, respawns at the flag. Beautiful. But on level 2, dying sends the cat to level 1's midpoint — totally off the cave floor, sometimes off-screen. She wrote this on the Player:

when backdrop switches to [level1 v]
set [checkpoint-x v] to (-200)
set [checkpoint-y v] to (-100)
        

What did Aishah forget, and what's the smallest fix?

when backdrop switches to [level2 v]
set [checkpoint-x v] to (-220)
set [checkpoint-y v] to (-80)

when backdrop switches to [level3 v]
set [checkpoint-x v] to (-210)
set [checkpoint-y v] to (-60)
          

Recap 3 min

You built a save system. Two variables — (checkpoint-x) and (checkpoint-y) — hold the respawn position. A Checkpoint flag sprite writes them when the Player touches it, with a colour-change for visible confirmation. The death script reads them with go to x: (checkpoint-x) y: (checkpoint-y). Every level-up resets the checkpoint to that level's start so old midpoints don't carry over. Half-level resets instead of full-level. Same number of deaths, far less frustration.

Checkpoint

A position partway through a level where the player respawns on death, instead of going back to the level start. Stored as two variables: x and y.

Respawn

What happens after a death — the player sprite teleports back to a safe position. With checkpoints, that position is the latest checkpoint, not always the level start.

Sticky variable

A variable that keeps its old value across scenes (level changes, deaths, restarts) until something explicitly overwrites it. Helpful for saves; dangerous if you forget to reset.

Per-level reset

The pattern of overwriting state at the start of every level — (checkpoint-x) here, but the same idea applies to enemy positions, coin counts, timers, anything that shouldn't carry over.

Visible confirmation

A small visual change (colour, sound, animation) that tells the player something happened. Without it, the checkpoint feels broken even when it works.

Homework 2 min

The Checkpoint Pass. Continue your platformer project. Add a single Checkpoint flag and prove it works on all three levels.

1. Two variables: checkpoint-x, checkpoint-y, For all sprites, watchers off.
2. A new sprite called Checkpoint — a small flag. Three when backdrop switches to hats positioning it at the midpoint of each level.
3. Checkpoint's main script: forever-if-touching-Player → write its x and y into the variables and change colour by 100.
4. On the Player: three when backdrop switches to hats setting the checkpoint to the level's start position.
5. Edit the existing when I receive (player-died v) to go to x: (checkpoint-x) y: (checkpoint-y).
6. On the Checkpoint: when I receive (level-up v) → clear graphic effects, so each new level shows a fresh-looking flag.

Save as HW-L4-21-Checkpoint-Pass.sb3. Test by dying before reaching the flag (should respawn at level start) and dying after (should respawn at the flag).

Bring back next class:

• The .sb3 file.
• Your answer to: "What if a level has TWO checkpoints? Does your current script handle that correctly? Test by adding a second Checkpoint sprite and walking past both — does respawn use the right one?"

Heads up for next class: SCR-L04-22 builds a Boss sprite for level 3 — a single big enemy with multiple phases (different behaviour as its health drops). It re-uses the level-gate pattern from L04-20 but gates on boss-health instead of level.