About Procedures

A procedure is a list of instructions with a name. Primitives and procedures are the words that your project and the objects in it know how to do. Once you've created a procedure, just type its name to run all the instructions it contains.

A Procedure Without Inputs

A procedure has three parts:

to square The title line "to" and the name of the procedure.
repeat 4 [fd 50 rt 90] Instructions.
end The last line.

All procedures must start with to and the name of the procedure. You choose the name, but always make sure that it doesn't contain any spaces. The name must be unique in the Procedures Tab in which the procedure is defined and it cannot be a name that exists in the MicroWorlds EX built-in vocabulary (primitives, object name, page name, etc.). Procedures must also end with the word end on its own line.

• The title line of a procedure (to the name, and its inputs) must be on a line by itself. You must press Enter at the end of the line. See below an example of a procedure with an input.
• Lines of instructions within the procedure are separated by carriage returns (Enter), although they do not have to be. Separating them by carriage returns makes the procedure easier to read and debug.
• The last line of a procedure is the word end, and it must be on a line by itself. Always press Enter after the word end.
• You can insert spaces between your procedures to make them easier to read and understand.
• You can put comments between your procedures. Simply never start a comment with the word "to".

The example above is a simple procedure, without inputs. To use this procedure you only need to type its name in the Command Center, in a button's instruction field, or in other instruction fields in a turtle's backpack (OnClick, OnColor, etc.).

A Procedure With Inputs

Here's an example of a procedure with an input. In this example, :size is an input. It is placed on the title line of the procedure, and it can be used where appropriate inside the procedure.

to square :size
repeat 4 [fd :size rt 90]
end

This version of the square procedure lets you draw squares of different sizes.

When using a procedure that requires an input, you must specify the value of its input when you run the procedure (just as you would specify the input for any primitive that requires one, such as forward or right).

A procedure can have several inputs.

A Procedure Calling a Subprocedure

A procedure name can be used just like a primitive. A procedure can even be used in another procedure, called a superprocedure. Here's an example of a superprocedure calling a subprocedure:

to house
square
fd 50 rt 30
triangle
lt 30 bk 50
end

to square
repeat 4 [fd 50 rt 90]
end
to triangle
repeat 3 [fd 50 rt 120]
end

The superprocedure house calls two subprocedures: square and triangle.