User defined procedures

The structure of a procedure definition

• A Logo procedure definition begins with the word to and ends with the word end. It is written out as a series of lines.

• A line in a definition is something terminated with a <return>. Notice that this is not the same as a line on the screen. Text which wraps around is considered to be on the same line.

• The first line of a definition contains the word to followed by the new procedure name and the names of its inputs (if any are required).

• The procedure name is an unquoted word.

• Each input name is also a single word but preceded by a colon. In this instance, thing is not a permissible alternative for the colon.

• The body of a procedure definition is a series of instructions. You can put more than one instruction on a line. You CANNOT split an instruction across more than one line.

• The last line of a procedure definition contains the word end.

Using the editor

In LOGO, defining procedures is normally carried out in a mode which we called earlier the editing mode.

Exactly how you enter the editing mode will depend on your particular system. Typically the editor is entered automatically as soon as you make any attempt to start the first line of a definition, but the editing mode can also be entered by issuing various commands which include, not surprisingly, the edit command.

The characteristics of editor interfaces are sadly system dependent. In some implementations, the screen display is dramatically different from that of conversational mode. In others, no more than a change in the appearance of the prompt provides the clue that different functions are being provided.

One of the useful functions you may discover, to handle the requirement that an instruction must not be split across two lines, is that your editor may allow you, for cosmetic reasons, to shift to a new screen line at some particular point in mid instruction. This is usually achieved using <tab> and normally (but not always) a continuation character, such as an arrow, will mark the point to distinguish these cases from real line breaks where a return has been used.

But perhaps the most important question you will need to resolve is how to get LOGO to take notice of whatever definition you have prepared in the editor. Oddly, it is usually the act of leaving (or escaping from) the editor - most likely by closing the editor window or by using some key combination such as the control key and the letter C - which is the vital trigger in the teaching process. Escaping back to conversational mode is how you will normally say: 'OK. These are the procedures I want you to learn. Now remember them!'.

Of course, you will need to establish all of the details as they affect your own situation by reference to your particular user manual or your local helpmate.

Look here for some help on the editor used with the Logo PLUS or Terrapin Logo for the Mac.

Fortunately, once you begin work inside the editor, consistency across implementations reappears. Procedure definitions themselves take a standard form.

Is the editor really necessary?

Teaching names is essentially interactive. Even if it is premeditated, you can set up a variable using make in a single, short instruction. This is the normal way to deal with LOGO in conversational mode. A procedure, on the other hand, is a series of coordinated instructions designed to carry out some specific task. It may also be necessary to identify it as operating on one or more inputs. Defining a procedure is consequently a lengthier and more complex operation than assigning a name. Defining a procedure has more in common with writing a letter than with joining in a conversation. The editor - like a word processor - is simply there to provide you with a comfortable environment in which to compose your definition without LOGO looking over your shoulder as you go about the task. If you wish, you can even stay in the editor to define a number of different procedures at the same time. Only when you are quite happy with your specification do you escape back to conversational mode and, in so doing, hand your definition(s) over to LOGO.

Defining commands and operations