Ahoy 1987 February

home *** CD-ROM | disk | FTP | other *** search

/ Ahoy 1987 February / Ahoy_Magazine_87-02_1987_Double_L_Side_B.d64 / comal article < prev next >

Wrap

Text File | 2022-10-26 | 9KB | 322 lines

COMAL AND YOU - For Beginners Only by David Stidolph WHY LEARN TO PROGRAM? The many commercial computer programs available today are more than sufficient for most people's needs. If word processors, spread sheets, and data base managers are all your computer is used for, learning to program would be a waste of time. For some, however, a goal is being able to learn something new... something that can control that mysterious box called a computer. There's nothing like the feeling of accomplishment when you type in your first working program. Also, familiarity with computers and programming helps ease the sense of helplessness most people get when dealing with computers. (How did you feel the first time you got a computerized bill?) This article is for people who have made the decision to not only learn about computers, but to learn programming as well. WHAT IS A COMPUTER LANGUAGE? Computers work with a language of zeros and ones called machine code. This is as difficult to use as it sounds, and few programmers now work directly in machine code. They choose instead to use computer "languages". These range from low level languages like assembly code, where the words merely represent individual machine code instructions, to high level languages which look more like English. Compare the following two programs: ASSEMBLY CODE ------------- * = START LDY #$00 LP LDA STRING,Y BEQ ENDLP JSR OUTPUT INY BNE LP BEQ ENDLP .BYT 'This is a string',13,0 ENDLP <...> ; rest of program code COMAL ----- PRINT "This is a string" As you can see, the statement written in COMAL is shorter and much more readable. Although the computer will seem to understand COMAL statements and programs, the computer itself only understands machine code. COMAL, the language, is a machine code program. It must be loaded into your computer before you can write, edit, or run COMAL programs. Think of the language as a translater between you and the computer. This means you don't have to learn machine code - you only have to make sure that COMAL is in the machine before you can run your COMAL programs. WHY LEARN COMAL? Since BASIC comes with most personal computers today, many people think that it is the best computer language to learn. Not so. BASIC is implemented on so many computers because it is the easiest language to write. It has the fewest commands, and NO definite standard to follow. This means that a BASIC program written on one computer may NOT run on another computer. BASIC does, however, have one good feature; it will let you type in a short program and see it execute as soon as you type the word RUN. Text editors or elaborate compiler commands are not necessary (Most other high level languages like Pascal, FORTRAN and COBOL require them). This makes BASIC seem like a easy-to-learn language for everyone. COMAL started with this idea of interactive work with the programmer, then added to it. COMAL has structures such as WHILE, REPEAT, and FOR loops, multi-line IF-THEN-ELSE statements, a CASE statement (similiar to a multiple choice question), and named PROCedures and FUNCtions complete with parameters. These structures are similiar those in "professional" languages like Pascal. COMAL also has the turtle graphics made famous by Logo. COMAL is now the language taught in the schools of 5 European countries. COMAL is easier to learn than BASIC, and teaches the idea of structured programing necessary with modern computer languages. HOW DO I GET COMAL? Congratulations, COMAL is included on this Ahoy disk. To use COMAL, follow these steps: 1. Type LOAD ":*",8 Type RUN This puts you in Ahoy's main menu. 2. Choose the "COMAL 64 BOOT" option. This will give you COMAL's introductory screen. You will be given some COMAL information and then asked if you want error messages in memory. Hit the return key for the default answer of yes. 3. After a short wait, you will be put in a COMAL program displaying another menu. This will allow you to run the COMAL programs on this disk, or to read articles such as the one you are reading now. If you want to enter the COMAL editor to write your own programs, simply enter the "QUIT THIS MENU" option. 4. If you are already in COMAL, but want to re-enter the menu from step 3, all you need to do is type in: CHAIN "hi" assuming the Ahoy disk is in the disk drive. NOTE: COMAL and the COMAL programs on this disk are copyrighted, but we give you permission to give copies away. This permission does not apply to the BASIC programs from Ahoy. WHAT DO I DO WITH COMAL? Since COMAL is on this Ahoy disk, you can learn to write readable programs. One way to learn is to first look at other peoples' work. I will detail certain commands now so that you can do just that. The commands will be listed in UPPERCASE, but type them in with unshifted letters. CAT This command will show you what files are on the disk in the disk drive. The disk drive sends the disk DIRECTORY to the computer, and COMAL prints it on the screen. The actual listing shows more than just file names. It shows how big they are, their names, and the file type. Each entry in the directory is called a file, and there are four types of files - PRG (program), SEQ (sequential - data files), REL (random - also data files), and USR (special files). Unlike the BASIC commands: LOAD "$",8 LIST COMAL will not erase the program in memory while showing a directory of a disk. You can slow the scrolling lines by holding down the CTRL key on the upper left hand side of the keyboard, or stop it by pressing the RUN/STOP key (right below the CTRL key). If you happen to have a dual drive (a two drive unit) you can add a '0' or a '1' after the command: cat 0 (This is for drive 0) cat 1 (This is for drive 1) LOAD Once you know what is on a disk, you can load COMAL programs into memory with this command. It is similar to the BASIC LOAD command, except you no longer need to type the comma 8. The following is an example of loading a program called "filename" from the disk drive: load "filename" Only PRG type files can be loaded. Be careful, because other languages, like BASIC, also store their programs as PRG files. COMAL 0.14 will attempt to load any PRG type file you ask it to. If you are not sure whether or not a program was written in COMAL 0.14, load the program and LIST it. Only COMAL 0.14 programs can be listed, any other type of program (BASIC, COMAL 2.0, etc) will not list. DO NOT RUN PROGRAMS WHICH WON'T LIST. If you do, COMAL will become confused and stop functioning. The only thing to do after this has happened is to turn the computer off and reload COMAL. LIST Once a COMAL program is in memory, you will want to be able to see it. The command LIST will do just that, it will list the program to the screen. The first thing you will notice is you will want to slow or stop the listing (so you can study it). Just as with the catalog command, you can use the CTRL key to slow the listing, or the RUN/STOP key to stop the listing. When LISTing a PROGRAM, the space bar will pause the listing. You will notice that each line has a number in front of it. COMAL uses them to keep track of the order of the program lines. The order goes from low (1) to high (9999). You can use any line number between. The LIST command can also be used to show just part of a program. The following are some examples to do just that: list (all lines) list 100-500 (from line 100 to 500) list 100- (from line 100 to end) list -500 (from beginning to 500) RUN When the program you want has been loaded into memory, you start the program with the command RUN. The computer does a quick scan of the program to make sure it seems correct, and starts executing with the first line of the program. If an error occurs while the program is running, the program will stop executing. COMAL will print what the problem is and the line number it occured on. MAKING ERRORS There is a very good chance that you will make typing errors while trying these commands. COMAL checks each line you type for errors, and if it cannot understand what you typed, it will stop and give you an error message. It might go out to the disk drive and get the error message, or it might just print the message itself (that depends on your choice to have error messages in memory or not). If you get an error, COMAL will put the cursor on the part of the line it is having trouble with so DON'T PANIC. Just make the correction and press the RETURN key again. STATUS If the red light on the disk drive starts blinking on and off while COMAL just sits there waiting for you to type something, try typing in the STATUS command. This will print disk error message to the screen. Check your disk drive manual for more information if necessary.