Using procedures allows you to split up virtually any program you are going to write into a main program, followed by a number of 'mini-programs' (procedures), which can be called from the main program by a single statement.

A procedure is simply a collection of numbered BASIC statements which you write in order to perform a particular task. This collection of statements looks just like part of an ordinary program, but the differences are that the first line contains the name of the procedure (which is decided by you), and the last line contains a BASIC word to signify the end of the procedure. When the computer encounters the end, it then returns to the main program and carries on.

The rules for using procedures are very simple. A procedure is called from the main program by the BASIC word PROC followed immediately by the procedure's name. The name can be anything you like, but there must be no spaces in it. For example:

Note the underline character in the second example which helps 'space out' the name.

A procedure name should reflect the function of the procedure to which it applies. If you merely name your procedures PROCA, PROCB, PROCC, for example, then no one will understand what they do without having to work through each one. So if you have a procedure which converts feet into metres, then call it PROCfeet_to_metres. It is best to use lower case names for procedures so that they distinguish themselves from the PROC.

To define a procedure, you simply type a line number, followed by DEF, followed by the procedure name. It is a good idea to start defining your procedures at a fairly high line number, say 1000.

This procedure will do as its name suggests.

All procedure definitions must end with ENDPROC.

When you want the computer to carry out the instructions in the procedure, you have to call it by name:

or:

You may have as many procedures in your program as you like, and usually the more the better.

There must always be an END instruction between the end of the main program and the procedure definition.

For example:

There is a program on the Introductory Cassette to illustrate the use of procedures, and also give you some fun. Load this program which is called "BUGZAP" into your Electron first. See chapter 4 for instructions on how to load a program.

When you LIST a long program obviously you cannot see all of the lines on the screen at the same time. Using LIST with specified line numbers is one way around this, but another is to put the computer into paged mode. This is done by pressing CTRL N. (No RETURN is required.) If you now use LIST, the program will be listed until the screen is full. When you want to look at the next part, just press SHIFT and another screen full will appear. If you want to change a line number, you must press SHIFT. To get the computer out of paged mode, type CTRL O.

Look at just one procedure from this program:

This procedure is called from line 90 of the main program.

Line 520 is the start of the definition.

Line 530 clears the screen.

Lines 540 to 680 print the introduction and instructions about the BUGZAP game which you see when you run the program.

Line 690 is an example of putting two separate BASIC statements after one line number by separating them with a colon. The purpose of line 690 is to wait until the space bar is pressed: GET$="". When the space bar is pressed, line 700 is executed.

Line 700 signifies the end of the procedure, and the computer goes back to the main program to the line immediately after the procedure call (90PROCinfo), which is line 100.

Here is one of the procedures from the 'MARSLANDER' program - also on the Introductory Cassette.

This procedure alters the speed of the spacecraft according to the direction in which it is pointing, which is given by the variable direction%.

This procedure is called from line 250:

Z% is an integer variable which is used by the program to give the attitude of the spaceship.

When the computer reaches line 250 it tests to see if the space bar is pressed. If it is, the computer then places the contents of Z% into direction%.

The variable Z%, and hence the parameter direction%, can be anything from 1 to 4, where 1 indicates the capsule pointing up, 2 to the right, 3 down and 4 to the left. These positions are represented by characters 224 to 227 which are user-definable.

Line 880 checks to see whether there is any fuel left. The variable fuel% will be FALSE when it is zero and the procedure will end.

If it is TRUE one unit of fuel is deducted by decrementing its contents by 1. Line 890 makes either a 'beep' (fuel is low), or a rocket motor sound (fuel is not low).

SOUND is explained in chapter 22.

Line 900 uses ON ... GOTO, to act according to the direction of the spacecraft. The parameter direction% now contains the value given to it by Z%. If the spacecraft is pointing up, direction% is 1 and execution continues at line 910.

Line 910 decreases the vertical speed of the capsule. (VY% is the vertical speed measured positive in a downward direction; VX% is the horizontal speed measured positive in a left-to-right direction.) If the capsule is pointing to the left line 900 passes execution to line 920 which increases the horizontal speed of the capsule.

Lines 930 and 940 increase the vertical speed and decrease the horizontal speed respectively. After any one of these lines (910 to 940) has been executed, the procedure ends.