Treasure Hunt I

Review — Shooting Game, finished 5 min

The last four lessons built a shooting game.

• Each zombie was a clone with its own (hp).
• Waves got harder, and every kill paid coins.
• The shop checked before it sold anything: <not <(coins) < (cost)>>.

Quick-fire

You have 100 coins. The upgrade costs 100.

Is <(coins) < (cost)> true or false?

Today’s Topic 3 min

• A sprite that follows the mouse
• (distance to [Treasure v]) — how far away something is
• Comparing two numbers with <() < ()> and <() > ()>
• if / then and if / then / else
• <mouse down?> — digging with the mouse button

Learning outcome

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

1. Measure how far one sprite is from another.
2. Ask a yes-or-no question by comparing two numbers.
3. Choose between three answers with an if / else chain.

Learnt — hot, warm or cold 28 min

The game, on paper first

• A chest hides at a random spot in the sand.
• You move a metal detector with the mouse.
• The detector says Hot!, Warm or Cold.
• When it says Hot!, you press the mouse button to dig.

You need two sprites: a Detector and a Treasure.

1 · A detector that follows the mouse

You met go to [mouse-pointer v] in Level 1. Inside a forever, it makes a sprite stick to the mouse.

1. Open Motion. Drag out go to [random position v].
2. Click the little arrow. Pick mouse-pointer.
3. Put it inside a forever, under when flag clicked.
4. Click the green flag. Move the mouse. The detector follows.

Sensing has two more mouse blocks. (mouse x) is how far left or right the mouse is. (mouse y) is how far up or down.

go to [mouse-pointer v]
              

go to x: (mouse x) y: (mouse y)
              

The long way looks like extra work. In Challenge 2 you will see why it is useful.

2 · How far away? distance to

(distance to [Treasure v]) is a round Sensing block. It holds one number: how many steps away the Treasure is.

1. Open Sensing. Find (distance to [mouse-pointer v]).
2. Click its arrow. Pick Treasure.
3. Click the block itself. A little bubble shows the number.
4. Drag the Treasure somewhere else. Click the block again. The number changes.

when flag clicked
forever
  go to [mouse-pointer v]
  say (round (distance to [Treasure v]))
end
            

3 · A question with a yes or no answer

A pointy block is a question. Its answer is only ever true or false. You met some already, like <mouse down?>.

Operators has three that compare two numbers:

<(distance to [Treasure v]) < (60)>
              

<(distance to [Treasure v]) > (150)>
              

The third is <() = ()>, for “exactly the same”. You will use it next lesson.

4 · if / then, or if / then / else

A pointy question fits the pointy slot in an if block.

• if / then does something when the answer is yes. When it is no, it does nothing.
• if / then / else always does one of two things. Yes runs the top part. No runs the bottom part.

when flag clicked
forever
  go to [mouse-pointer v]
  if <(distance to [Treasure v]) < (60)> then
    say [Hot!]
  else
    say [Cold]
  end
end
            

5 · Three answers: the chain

Two words is not enough. We want Warm in the middle. Put a second if / else inside the first one’s else.

when flag clicked
forever
  go to [mouse-pointer v]
  if <(distance to [Treasure v]) < (60)> then
    say [Hot!]
  else
    if <(distance to [Treasure v]) < (150)> then
      say [Warm]
    else
      say [Cold]
    end
  end
end
            

6 · Bury it, then dig

Now the chest has to hide. At the flag it does three things:

1. hide — nobody can see it.
2. switch costume to [chest v] — shut, ready for this game.
3. go to x: (pick random (-180) to (200)) y: (pick random (-150) to (-40)) — a random spot on the sand.

Digging is <mouse down?>. It is true while the mouse button is held. We only dig when Hot!, so that check goes inside the Hot! part.

when flag clicked
hide
switch costume to [chest v]
go to x: (pick random (-180) to (200)) y: (pick random (-150) to (-40))

when I receive [found v]
switch costume to [open v]
show
start sound [Coin v]
              

if <(distance to [Treasure v]) < (60)> then
  say [Hot!]
  if <mouse down?> then
    broadcast [found v]
  end
else
  ... :: grey
end
              

Basic — build the draft hunt 24 min

Start a new project. Build one step at a time, and play after each.

Step 1 — the desert

1. Click the Stage, then the backdrop button.
2. Pick Desert from the library.

Step 2 — the two sprites

1. Paint a sprite called Detector: a grey circle with a stick for a handle. Or pick Magic Wand.
2. Paint a sprite called Treasure with two costumes: chest (shut) and open. A box with a lid is enough.
3. Add the Coin sound to the Treasure.

Step 3 — follow the mouse

1. On the Detector: when flag clicked, then forever.
2. Inside it: go to [mouse-pointer v].
3. Play. The detector sticks to the mouse.

Step 4 — the three words

1. Add an if / else under the go to. Put <(distance to [Treasure v]) < (60)> in its slot.
2. Top part: say [Hot!].
3. Bottom part: a second if / else asking <(distance to [Treasure v]) < (150)>.
4. Its top part says Warm. Its bottom part says Cold.
5. Play with the chest showing. Check all three words.

Step 5 — bury the chest

1. On the Treasure, build the flag stack: hide, costume, random spot.
2. Play. The chest vanishes. The words still work.
3. Press the flag a few times. The Hot! spot moves each time.

Step 6 — dig

1. In the Hot! part, add an if / then. Put <mouse down?> in its slot.
2. Inside it: broadcast [found v]. Make a new message called found.
3. On the Treasure: when I receive [found v], open costume, show, start sound [Coin v].
4. On the Detector: when I receive [found v], then stop [other scripts in sprite v] and say [You found it!].

The whole Detector

when flag clicked
forever
  go to [mouse-pointer v]
  if <(distance to [Treasure v]) < (60)> then
    say [Hot!]
    if <mouse down?> then
      broadcast [found v]
    end
  else
    if <(distance to [Treasure v]) < (150)> then
      say [Warm]
    else
      say [Cold]
    end
  end
end

when I receive [found v]
stop [other scripts in sprite v]
say [You found it!]
            

Boiling!

Add a fourth word for when you are nearly on top of it.

• Under 30 steps, the detector says Boiling!.
• Hot!, Warm and Cold still work.
• You can still dig when it says Boiling!.

if <(distance to [Treasure v]) < (30)> then
  say [Boiling!]
  if <mouse down?> then
    broadcast [found v]
  end
else
  if <(distance to [Treasure v]) < (60)> then
    say [Hot!]
    if <mouse down?> then
      broadcast [found v]
    end
  else
    if <(distance to [Treasure v]) < (150)> then
      say [Warm]
    else
      say [Cold]
    end
  end
end
            

Stay on the sand

A metal detector does not fly. Keep it on the ground.

• While the mouse is on the sand, the detector follows it.
• When the mouse goes up into the sky, the detector stays put.
• The sand ends at about y = -30.

when flag clicked
forever
  if <(mouse y) < (-30)> then
    go to x: (mouse x) y: (mouse y)
  end
  if <(distance to [Treasure v]) < (60)> then
    say [Hot!]
    if <mouse down?> then
      broadcast [found v]
    end
  else
    if <(distance to [Treasure v]) < (150)> then
      say [Warm]
    else
      say [Cold]
    end
  end
end
            

Nothing here!

Right now, digging only works when it is Hot!. Let the player dig anywhere.

• A dig that is Hot! opens the chest, as before.
• A dig anywhere else says Nothing here! for one second.
• While it says so, the detector stays still. Digging takes time!

when flag clicked
forever
  go to [mouse-pointer v]
  if <(distance to [Treasure v]) < (60)> then
    say [Hot!]
  else
    if <(distance to [Treasure v]) < (150)> then
      say [Warm]
    else
      say [Cold]
    end
  end
  if <mouse down?> then
    if <(distance to [Treasure v]) < (60)> then
      broadcast [found v]
    else
      say [Nothing here!] for (1) seconds
    end
  end
end
            

A detector that grows

Words are good. A picture is faster. Make the detector show how close it is.

• Far from the chest, the detector is small.
• The closer it gets, the bigger it grows.
• Right on top of it, it is at its biggest.

when flag clicked
forever
  go to [mouse-pointer v]
  set size to ((120) - ((distance to [Treasure v]) / (5))) %
  if <(distance to [Treasure v]) < (60)> then
    say [Hot!]
    if <mouse down?> then
      broadcast [found v]
    end
  else
    if <(distance to [Treasure v]) < (150)> then
      say [Warm]
    else
      say [Cold]
    end
  end
end
            

Summary 5 min

• go to [mouse-pointer v] in a forever makes a sprite follow the mouse. (mouse x) and (mouse y) give its two numbers.
• (distance to [Treasure v]) is how many steps away a sprite is. It works even when that sprite is hidden.
• <() < ()> and <() > ()> compare two numbers. The answer is true or false.
• if / then may do nothing. if / then / else always does one of two things.
• An if / else inside an else makes a chain. Ask the smallest question first.

Distance

How many steps one sprite is from another.

Boolean

A pointy block whose answer is only true or false.

Compare

Ask if one number is less than, more than, or equal to another.

Chain

An if / else inside an else, so there are more than two answers.

Hand in

Save your hunt. Next lesson you get five digs — and you can lose.