Smooth Motion Control II

Review — where we got to 5 min

Last lesson you built controls that feel right. You learnt three things:

• key [right arrow v] pressed? is a Sensing question, and it goes inside an if.
• Four ifs in one forever means two keys can work at the same time.
• if on edge, bounce goes in the forever, outside the ifs.

Quick-fire

You hold space, then let go. What does the cat look like now?

when flag clicked
forever
  if <key [space v] pressed?> then
    switch costume to [costume2 v]
  end
end
            

Today’s Topic 3 min

• Planning a game before you write any blocks
• Coding one game rule at a time
• Adding a second costume by hand
• if / else

Learning outcome

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

1. Turn a list of game rules into a list of jobs, and do them in a sensible order.
2. Paint a second costume and switch to it while a key is held.
3. Say what if / else does that two separate ifs do not.

Learnt — plan first, then code 27 min

The game, and its rules

Today’s game has one character you drive and several you have to clear off the Stage. Here are its rules, written out before a single block is dragged.

A second costume

Rule 3 needs your character to look different while you hold space. That means a second costume, and you make it yourself.

1. Click the Costumes tab.
2. Right-click your costume in the list and choose duplicate.
3. Draw the difference on the copy — a spray, a glow, anything you can see.
4. Rename them No Spray and Spray, so your blocks read like English.

The new block: if / else

A plain if <> then only knows what to do when the answer is yes. When the answer is no it does nothing at all — which is why the cat in the Review got stuck.

if / else has two mouths. The top one runs when the answer is yes, the bottom one when it is no. One of them always runs.

when flag clicked
forever
  if <key [space v] pressed?> then
    switch costume to [Spray v]
  else
    switch costume to [No Spray v]
  end
end
            

The order inside the loop

Everything goes in one forever, and the order matters:

1. The four movement ifs, each one also setting which way to face (rule 5).
2. The if / else for the costume (rule 3).
3. if on edge, bounce — last, and outside everything (rule 4).

Bounce goes last so it has the final word. Put it above the movement ifs and a key can shove the sprite back through the wall in the same round.

Basic — code the player, rule by rule 25 min

Five rules, five steps. Test after every single one — that is the habit this lesson is really teaching.

Step 1 — the cast and the Stage

New project. Pick your two sprites — one you drive, one to clear. Add a backdrop. Set the sizes so both fit comfortably; the one you clear should be small.

Step 2 — rule 2, where it starts

when flag clicked, then go to x: (0) y: (0). Test: drag it away, click the flag, it comes back.

Step 3 — rule 1, the controls

Add set rotation style [left-right v], then the forever with four ifs. Test all four keys and a diagonal.

Step 4 — rule 5, facing

Add point in direction (90 v) inside the right-arrow if, and point in direction (-90 v) inside the left one. Test: it should flip, not somersault.

Step 5 — rule 3, the spray

Add the if / else underneath the four movement ifs, still inside the forever. Test: hold space, let go, hold again.

Step 6 — rule 4, the bounce

Add if on edge, bounce as the last block in the forever. Test: drive into all four walls.

Step 7 — check your work against this

when flag clicked
set rotation style [left-right v]
go to x: (0) y: (0)
point in direction (90 v)
forever
  if <key [right arrow v] pressed?> then
    point in direction (90 v)
    change x by (3)
  end
  if <key [left arrow v] pressed?> then
    point in direction (-90 v)
    change x by (-3)
  end
  if <key [up arrow v] pressed?> then
    change y by (3)
  end
  if <key [down arrow v] pressed?> then
    change y by (-3)
  end
  if <key [space v] pressed?> then
    switch costume to [Spray v]
  else
    switch costume to [No Spray v]
  end
  if on edge, bounce
end
            

Rule 6 — four of them, everywhere

Put four of the other sprite on the Stage, and make each one start somewhere different every time the flag is clicked.

when flag clicked
show
go to [random position v]
            

Rules 7 and 8 — spraying is what counts

Finish the game rules.

• Touched while you hold space: it shouts “No!” and disappears.
• Touched without space: it says “I’m not afraid of you!” and stays right where it is.

when flag clicked
show
go to [random position v]
forever
  if <touching [Cleaner v]?> then
    if <key [space v] pressed?> then
      say [No!] for (1) seconds
      hide
    else
      say [I'm not afraid of you!] for (1) seconds
    end
  end
end
            

Give the game an ending

Right now the game never finishes. When the last one has gone, your character should say something and everything should stop.

Clearing three of the four must not end it. Only the fourth.

They fight back

Standing still is easy. Make them move — slowly, on their own, all the time — and make one of them come after you.

If the one chasing you touches you while you are not spraying, you lose and the game says so.

Summary 5 min

• Write the game rules down before you code. Then build them one at a time and test after each one.
• Duplicate a costume and draw on the copy to give a sprite two looks. Name them properly.
• if <> then handles yes. if / else handles yes and no — one of the two always runs.
• Inside the forever: movement ifs, then the costume if / else, then if on edge, bounce last.

Game rule

One sentence about what the game does. One rule, one piece of code.

if / else

A block with two mouths: one for yes, one for no. Exactly one of them runs.

Duplicate

Right-click to make a copy — of a costume, a sprite or a stack of blocks.

Hand in

Share your project and send the link to your teacher. Missions you finish at home count too — bring the link to the next class and claim your Adcoins.