Cheese Maze II

Review — last lesson 5 min

• touching color [ ]? asks whether the sprite stands on a colour.
• Move, ask, undo — and undo exactly what you moved.
• Walls must be one flat colour; the sprite must fit the corridor.

Quick-fire

Your mouse has four if blocks, each with its own undo. That is eight blocks doing one job. What would you rather write?

Today’s Topic 3 min

• old x and old y — remembering the last safe place
• Checking x and y separately, so you slide instead of sticking
• Why diagonal movement is faster, with the measurement
• Tunnelling: when a step is bigger than a wall is thick

Learning outcome

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

1. Write one wall check that works for every direction.
2. Make a sprite slide along a wall rather than stop against it.
3. Predict, and then measure, three ways collision goes wrong.

Learnt — a better shape 28 min

1 · Remember where you were

Rather than a matching undo for every direction, record the position before moving and restore it if the move was illegal:

set [old x v] to (x position)
set [old y v] to (y position)
change x by (4)
if <touching color [#1F7A8C]?> then
  go to x: (old x) y: (old y)
end
            

2 · Sliding along a wall

Press up and right together in a corridor that only goes up, and a single combined check stops you dead — both moves are undone because one of them failed. Judge x and y separately and the successful one still happens:

if <key (right arrow v) pressed?> then
  change x by (4)
  if <touching color [#1F7A8C]?> then
    change x by (-4)
  end
end
if <key (up arrow v) pressed?> then
  change y by (4)
  if <touching color [#1F7A8C]?> then
    change y by (-4)
  end
end
            

3 · What the corridor is made of

Three numbers decide whether a maze is playable at all, and they have to agree:

• the corridor width — about 40 pixels here
• the sprite’s width — which is its size as a percentage of its costume
• the step size — how far it moves each time round the loop

Too wide a sprite and it cannot move. Too big a step and it goes straight through. Challenges 2 and 3 measure both.

Basic — movement worth keeping 24 min

Open last lesson’s maze. You are rebuilding the driving properly.

Step 1 — rotation style

1. Add set rotation style [left-right v] under the green flag.
2. Now point in direction (-90) turns the mouse round without turning it over.

Step 2 — face where you go

1. In the right-arrow branch, add point in direction (90) before the move.
2. In the left-arrow branch, point in direction (-90).
3. Leave up and down alone — a mouse that points upwards looks odd.

Step 3 — separate checks

1. Make sure each direction has its own if with its own check and undo, rather than one check at the end.
2. Test by holding two keys into a corner. You should slide, not stick.

Step 4 — animate the walk

1. Add next costume at the top of the forever loop.
2. If it flickers too fast, put the costume change inside the movement branches instead.

Step 5 — the cheese

1. Add the cheese to the far corner. It does nothing yet — that is next lesson.

Diagonals are cheating

Measure it rather than arguing about it.

• Forty steps to the right, in the open, and record the distance.
• Forty steps right-and-up, and record that distance.
• Compare the two numbers.

if <<key (up arrow v) pressed?> and <key (right arrow v) pressed?>> then
  set [step v] to (3)
else
  set [step v] to (4)
end
            

How small is small enough?

Try the same twenty steps at three different sizes and record what happens.

• 80%, then 55%, then 30%.
• The same corridor and the same twenty presses each time.
• A steps-taken counter, reset before each attempt.

Tunnelling

Make the mouse so fast that the wall stops working — then explain why the wall check is not at fault.

• A run with a step of 4, and where it ends up.
• A run with a step big enough to clear a whole wall, and where that ends up.
• A bump counter for both.

Facing the right way

Make the mouse turn to face the way it is going, all four directions, without ever appearing upside down.

• Left and right flip the mouse.
• Up and down do not rotate it — it is a mouse, not a rocket.
• It never appears belly-up, in any sequence of presses.

Summary 5 min

• old x and old y give one wall check that works in every direction.
• Judge x and y separately and the sprite slides along walls instead of sticking.
• Diagonal movement is about 41% faster. Shrink the step when two keys are down.
• A sprite wider than the corridor cannot move at all — and it looks like a dead script.
• Never step further than your thinnest wall, or you tunnel straight through.
• set rotation style [left-right v] stops the sprite going belly-up.

Sliding

Moving along a wall you are pressed against, rather than stopping.

Tunnelling

Passing through a wall because the step was bigger than the wall.

Step size

How far a sprite moves each time round the loop.

Rotation style

Whether a sprite turns all the way round, flips, or stays put.

Hand in

Save your maze. Next lesson the cheese starts to matter — and you have to earn it before it will let you win.