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4.5 DR. LOGO PROCEDURES
A procedure is a list of instructions that tells Dr. Logo how to do a task.
You will probably write your first procedures by adding to those already built into Dr.
Logo, these are called 'primitives'.
fd, bk, rt and lt are all built-in primitives which you may use at any time as
building blocks to write your own procedures.
Another very useful built-in primitive is cs which clears the screen and sends the turtle
to its starting position.
4.5.1 WRITING A SIMPLE PROCEDURE
It is easy to visualise that if the movements fd 60 rt 90 were to be repeated 4
times each, a square with sides of 60 units would be drawn.
The same effect can be achieved by writing a simple formula:
repeat 4 [fd 60 rt 90]
Clear the screen and then try typing this in to check what happens.
To make this formula into a new procedure called, 'square', type:
to square
repeat 4 [fd 60 rt 90]
end
Dr. Logo will now understand 'square' and each time it encounters the word 'square' it
will draw a square on the screen. We could have given this procedure any name, but
we chose 'square' to remind us what it does.
Dr. Logo allows us to type in a whole set of commands together so the instructions
square rt 45 square, will draw two squares, the second at a 45 degrees angle
to the first.
4.6 Procedures with parameters.
It is possible. to make a procedure to which we can say 'how much' in the same way
that we can say 'how much' to a built-in procedure.
To make a procedure that will draw squares of different sized sides, the definition of
'square' can be altered to:
to squareanysize :side
repeat 4 [fd :side rt 90]
end
This new procedure introduces the idea of a 'variable', which in this case is called :
side
You will notice that the variable :side, is preceeded by a colon, this indicates to Dr.
Logo that :side is a variable rather than a command.
Chapter 4.3 AMSTRAD Disc Drive & Interface DDI-1 Manual