Road Crossing II

Review — the first crossing 5 min

• One hidden Car sprite makes every car as a clone.
• A clone is born as a copy of its parent: same place, same costume.
• A hop is a hat block: when [up arrow v] key pressed.
• touching [Car v]? sends the frog back to the start.

Quick-fire

You want lane 2 to drive left. But every car runs the same clone stack. What does a car need?

Today’s Topic 3 min

• “For this sprite only” — a variable each clone keeps for itself
• (my speed) and (my lane), set at birth
• The bug you get with “for all sprites”
• A lane that drives left
• Three lives, reset on every flag, and a game over

Learning outcome

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

1. Give every clone its own speed with a “for this sprite only” variable.
2. Make one sprite drive lanes at different speeds and in two directions.
3. Count lives, reset them on the flag, and end the game at zero.

Learnt — every car its own speed 28 min

One stack, one speed

Last lesson every clone ran this block: change x by (3). Every car, in every lane, went right at 3. A real road is not like that.

A variable for this sprite only

You met this in Lesson 2-10. Every zombie had its own (hp). When you make a variable, Scratch asks a question:

• For all sprites — one box. Everything shares it.
• For this sprite only — every clone gets its own copy.

1. Click the Car sprite. Open Variables.
2. Click Make a Variable. Type my speed.
3. Tick For this sprite only. Click OK.
4. Do it again for my lane.
5. Untick both boxes in the palette. Clones do not need a monitor each.

Set it, then clone

The lane stack sets the two numbers just before it makes the clone. The clone is born holding a copy of both.

when flag clicked
forever
  go to x: (-230) y: (-120)
  set [my lane v] to (1)
  set [my speed v] to (2)
  create clone of [myself v]
  wait (2.5) seconds
end
              

when flag clicked
forever
  go to x: (230) y: (-80)
  set [my lane v] to (2)
  set [my speed v] to (-1.5)
  create clone of [myself v]
  wait (4) seconds
end
              

when flag clicked
forever
  go to x: (-230) y: (-40)
  set [my lane v] to (3)
  set [my speed v] to (3.5)
  create clone of [myself v]
  wait (2) seconds
end
              

A second later the parent sets different numbers for the next lane. The clone does not care. Its copy is its own.

The clone reads its own numbers

when I start as a clone
switch costume to (my lane)
if <(my speed) < (0)> then
  point in direction (-90)
end
show
repeat until <<(x position) > (235)> or <(x position) < (-235)>>
  change x by (my speed)
end
delete this clone
            

One more block goes in the setup stack: set rotation style [left-right v]. Without it, a truck pointing left drives upside down.

The bug, on purpose

Now try it wrong, so you can see why the tick box matters. Here (my speed) is “for all sprites”.

Three lives

(lives) is the opposite kind. There is one frog and one game, so there is one number. Make it for all sprites.

A hit now costs a life:

when flag clicked
go to x: (0) y: (-160)
forever
  if <touching [Car v]?> then
    change [lives v] by (-1)
    go to x: (0) y: (-160)
  end
end
            

And one stack resets the lives, then watches them:

when flag clicked
set [lives v] to (3)
wait until <(lives) < (1)>
stop [other scripts in sprite v]
say [Game over!] for (2) seconds
stop [all v]
            

1. set [lives v] to (3) — every new game starts full. Put it here, above the wait, so the wait never sees last game’s 0.
2. wait until <(lives) < (1)> — sits quietly all game.
3. stop [other scripts in sprite v] — the frog stops checking for cars. No more lives are lost.
4. say [Game over!] for (2) seconds — the player reads it.
5. stop [all v] — then everything stops.

Basic — turn the draft into a game 24 min

Open your crossing from last lesson. Test after every step.

Step 1 — the two variables

1. On the Car, make my speed and my lane.
2. Tick For this sprite only for both.

Step 2 — set them in each lane

1. In each lane stack, add the two set blocks between the go to and the clone.
2. Lane 1: lane 1, speed 2. Lane 2: lane 2, speed −1.5. Lane 3: lane 3, speed 3.5.
3. Change lane 2’s go to x from −230 to 230.

Step 3 — the clone uses them

1. Drop (my lane) into the costume block.
2. Drop (my speed) into change x by (3).
3. Add the if-block that points left, and the “or” test at the top of the repeat.
4. Add set rotation style [left-right v] to the setup stack.

Step 4 — check the costumes

Lane 2 must wear the truck. If it wears a car, drag the truck costume into second place.

Step 5 — lives

1. Make lives, For all sprites. Leave its box ticked.
2. Add the change block to the frog’s hit check.
3. Add the game-over stack to the frog.

Step 6 — lose on purpose

Hop into three cars. Then click the flag again. Lives must be back to 3.

Three speeds

• Still one Car sprite.
• Each lane moves at its own speed: one slow, one medium, one fast.
• No car ever changes speed once it is on the road.

One lane the other way

• The middle lane drives from right to left.
• Its vehicles face the way they drive, and stay the right way up.
• They disappear off the left edge, not the right.

Three lives

• Lives are shown on the Stage.
• A car costs one life and sends the frog back.
• At zero the player is told, and the game stops.
• The next green flag starts a fresh game with 3.

when flag clicked
go to x: (0) y: (-160)
forever
  if <touching [Car v]?> then
    change [lives v] by (-1)
    go to x: (0) y: (-160)
  end
end

when flag clicked
set [lives v] to (3)
wait until <(lives) < (1)>
stop [other scripts in sprite v]
say [Game over!] for (2) seconds
stop [all v]
            

Hearts, not numbers

A number in a grey box is fine. Hearts are better. Show the lives as hearts in the corner.

• Three hearts at the start.
• One goes each time a car hits the frog.
• The lives monitor is hidden.

when flag clicked
hide
forever
  if <(lives) > (0)> then
    go to x: (-205) y: (150)
    create clone of [myself v]
  end
  if <(lives) > (1)> then
    go to x: (-172) y: (150)
    create clone of [myself v]
  end
  if <(lives) > (2)> then
    go to x: (-139) y: (150)
    create clone of [myself v]
  end
end

when I start as a clone
show
wait (0.03) seconds
delete this clone
            

Summary 5 min

• For this sprite only gives every clone its own copy of a variable.
• Set the numbers, then make the clone. It is born holding a copy.
• A speed below zero drives left.
• For all sprites is one box. Use it for things the whole game shares, like lives.
• Reset a variable in the same stack that watches it.

For this sprite only

A variable each clone owns separately.

For all sprites

One variable, shared by every sprite and clone.

Set at birth

A number given to a clone as it is made, which it keeps.

Lives

How many more mistakes you can make. Zero ends the game.

Hand in

Share your game and send the link to your teacher. Next lesson the park floods. You will need logs.