Volume 7 Number 2 - Spring 1999

Computer Games for Kids, by Kids
...continued

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This second starter is the same as the first with the addition of a maze and a setup procedure.

Click here to play with the starter right now.

The color red is programmed to turn the turtle right 180 when it bumps into a wall. This is the setup procedure:

to setup
setpos [-150 -130]
seth 0
end

Setup places the turtle at an appropriate starting position and heading for the game to begin. The way to determine the proper position is to use the mouse to put the turtle where you want it to start and then type show pos in the command center. The pair of numbers reported can then be copied onto the procedures page and placed in brackets as the input to setup. The heading can be determined by typing show heading, but this is often unnecessary because the students generally use headings of 0, 90, 180, or 270 degrees, which they recognize by inspection.

The third starter adds a moving obstacle: a turtle in the shape of a spider. This spider turtle is also programmed to go forward 1 and its heading is set to 90. This is done by typing the command seth 90 in the command center. Once it is moving the spider turtle will switch headings between 90 and 270 each time it bumps into the red walls because red is programmed to cause all turtles to turn right 180.

Click here to play with the starter right now.

Adding the spider also requires changes to the go procedure.

to go
talkto [t1 t2]
    clickon
forever [direct readchar]
when [touching? "t1 "t2] [setup]
end

Another difference between Macitnosh and PC The instruction talkto [t1 t2] clickon works for the Macintosh version of MicroWorlds, but not for the Windows version. Instead, use ask [t1 t2] [clickon] This second form works for both versions.

Both turtles must now be told to clickon and the collision detection is established with the when command. Setup is also changed so that it only talks to the original turtle and not the spider as well.:

to setup
t1, setpos [-140 -130]
seth 0
end

Without this change both turtles would be sent to the starting gate when there was a collision.

Most students did not use moving turtles in their mazes, but collision detection was used in other kinds of games. Pong, Air Hockey, and War Zone on the VideoGameWorks website are examples.

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