How to make Space Invaders
In Scratch
(based on the program by eeb3
http://scratch.mit.edu/projects/eeb3/708899)
You can find other excellent programs there and download them…
Space invaders model
• Your “ship” is moving on the screen’s floor
• The invaders go left and right, while
descending on your ship
• You shoot with missiles and try to take them
out – in this game you get 4 missile launchers:
A, S, D, F controlled by their keys
• You play is timed
Programming
• You only really program:
– The “ship”
– One of the “aliens”
– One of the “missiles”
– Reminder which keys to use
– “Game over” message
• Then you duplicate the aliens and the missiles
and slightly modify them
The overall look of the project
Let’s start with the “ship”
Reset the ship on the click of the flag
Broadcasting “go” sets aliens moving
When the game is restarted, all messages
(“say”) are cleared, all points are cleared,
the timer is reset, the now dead invisible
ship is shown again and sent to the front
layer
The main block of the “ship” code:
Don’t forget to call it “Space Ship”, there are no
other costumes in this game
The game is won if 500 points reached and it
restarts then
As you can see, there 5 almost identical blocks of
code. Each “IF” deals with touching an alien and
dying from it.
Remember: you can right click on the IF and
choose “duplicate”, rather than make them all from
scratch…
The rest of the “Ship’s” code- moving around
Both of these blocks control
movement left and right. Ignore the
“play sound” as we don’t have
speakers installed. However, if you
game is otherwise complete, you
can ask your teacher for
headphones and try to include the
sounds.
Again, a good idea is to right-click
the first block and duplicated, then
replace what’s needed.
Alien 1 – see the next slide for details
When the game is started or restared,
aliens get reset, too – placed in a
particular spot and made visible again
The creator of this program could have
combined these two blocks but
professional programmers like to break
their programs in smaller parts which
can be individual fixed or adjusted.
If a missile touches the alien then this
alien gets hidden and the player’s
points increase. There are four missiles
that can be launched (if we only had
one missile we would have to wait for
it to go off the screen before we can
launch another one – not good)
The broadcast of “go” also makes this alien move
and turn around, and descend on the ship.
Notice that this particular script doesn’t care about
touching walls or anything like that.
Note the “nested repeats” – one repeat inside the
other, inside the other again…. You might not need
that many since you are not trying to make the
aliens beep every few steps…
Alien turning sideways to make him
harder to hit with missiles
Alien moving back…
Missile A – see the next slide for a closer view at the blocks
Missile A blocks
This makes the missile start its
travel from whatever coordinates
the ship is when it fires
Take simple go to x, y and instead of
numbers insert positions from Sensing
The missile disappears when it
reaches the end of the screen,
after becoming invisible, the signal
is broadcast “a clear” so that the
missile can become available for
firing again.
Missile available to be fired at start of game
Key usage hint – remind player to use
A to fire the first missile launcher
You will need a
variable to keep
track of the Points
scored
When the ”A”
missile is fired, it
becomes
unavailable and the
hint is hidden
Once the missile reaches an
alien or edge of screen, the
missile is clear to be
launched again, so the hint is
shown again.
The End
• Extension activities:
– Change the background from simple black to the
starry sky
– More missiles that can be launched at the same time
– Big Bad Boss that descends faster than regular aliens
– Aliens change their costume to “explosion” for a
second when hit with a missile before becoming
invisible
– Next level – change the shapes of aliens
– Aliens firing back!