BASIC 101: An Informative Series¢¢By John Kasupski, WNYAUG Part One: ¢Introduction & PRINT¢¢ Oh, no! Not another series of ¢articles about BASIC programming! Why?¢¢ Well, there are two reasons why. ¢The first reason is that it was ¢suggested by a few of our members. The ¢second reason is that in examining the ¢program listings of some public domain ¢BASIC programs I've run across, I've ¢seen some real kludges, so I know ¢there are people out there who will ¢find these articles helpful, provided ¢they'll bother to read them and apply ¢what's in them.¢¢ So, yes, another series of ¢articles about BASIC programming. But, ¢this one will be different (I ¢promise!). You see, most of the ¢tutorials on BASIC programming that ¢I've read in magazines, newsletters, ¢etc. are aimed at one of two groups of ¢people, either people who know nothing ¢at all about computers, let alone ¢BASIC, or people who are already ¢"experts" in Atari BASIC, in which ¢case the article tries to "convert" ¢the reader to BASIC XE, Turbo BASIC, ¢or some entirely different language ¢besides BASIC.¢¢ Obviously, if you're going to ¢read an article about programming ¢BASIC, you want to learn about BASIC. ¢So, we will NOT try to convert you to ¢programming in C, ACTION!, machine ¢language, Pascal, or anything else. ¢We'll deal with BASIC, period.¢¢ And because Atari BASIC is the ¢most widely used dialect of that ¢language, at least among Atari 8-bit ¢owners, we'll avoid trying to convert ¢you to some other BASIC. Yes, I'll ¢point out certain features of BASIC XE ¢and Turbo BASIC (the two "other" ¢BASICs I'm somewhat familiar with) ¢from time to time when it's relevant ¢and appropriate. But, we won't ¢constantly harp on BASIC XE (which is ¢a waste when the reader doesn't own ¢the $60.00 cartridge) and Turbo BASIC ¢(which is a waste when the reader uses ¢SpartaDOS, except for the X-cart, ¢because Turbo BASIC won't work with ¢the disk-based SpartaDOS versions). ¢What we cover here will be equally ¢valid in practically any BASIC for the ¢Atari 8-bit...well, maybe not. I've ¢never really used MicroSoft BASIC, so ¢I can't really tell if what we're ¢covering applies to it.¢¢ Another thing that will make this ¢series different is that we're going ¢to cut through some of the garbage ¢that often appears in BASIC ¢programming articles. For one thing, ¢if you want to learn about the ¢SETCOLOR command, look elsewhere. THIS ¢series will be using POKE instead, ¢because POKE is faster, simpler to ¢learn, and uses less memory...which ¢makes me wonder why ANYONE bothers ¢with SETCOLOR in the first place. This ¢series won't.¢¢ We will also not bother with ¢trying to show you how to include ¢machine language subroutines in a ¢BASIC program. In order to do this, ¢you have to have an ML routine to ¢include, which means you have to write ¢one, which means you have to know how ¢to use ML to write one. And if you can ¢do that, you don't need a series on ¢BASIC, do you?¢¢ So, exactly what WILL we be ¢covering? Well, stick with us, and ¢you'll find out, and maybe learn how ¢to write effective, coherent programs ¢and routines along the way!¢¢ For this month's column, we'll ¢look at a simple yet powerful command, ¢the PRINT statement. The most common ¢application for PRINT is to display ¢something on the screen. For example:¢¢10 PRINT "This is line 10." 20 PRINT ¢"This is line 20." 30 PRINT 3+6¢¢ If you type in the above short ¢program and RUN it, you'll get the ¢following output:¢¢This is line 10. This is line 20. 9¢¢READY¢¢ This illustrates three important ¢points about BASIC and the print ¢statement. The first point is that, as ¢you can see from the results, BASIC ¢executes lines in the order in which ¢they're numbered, unless we instruct ¢it to do otherwise (by using GOTO or ¢GOSUB...but that's for another ¢article).¢¢ The second point is that when ¢using PRINT with text, BASIC will ¢print whatever we enclose in quotation ¢marks after the PRINT statement. It ¢could be "This is line 10.", or "My ¢name is John", or ¢"ASDFGHJRTYUIVBNMTYUIO", it's all the ¢same to your Atari.¢¢ The third point is that we can ¢also use PRINT to tell the Atari to ¢display the results of an arithmetic ¢operation. When we tell BASIC to ¢"PRINT 3+6) the Atari adds three and ¢six and prints the result, which is 9.¢¢ Naturally, we don't need a ¢computer to tell us that 6+3=9, but ¢when we want to solve ¢1234+5678+3456+1357+2468, the computer ¢can do it much faster than we can. So, ¢you now know how to use your Atari as ¢a calculator, don't you?¢¢ The following lines show how to ¢perform other math operations in ¢BASIC.¢¢50 PRINT 8-3 60 PRINT 6*3 70 PRINT 6/2 ¢80 PRINT SQR(144)¢¢ RUN the above and you'll get 5 ¢(8-3), 18 (6*3), 3 (6/3), and 12 (the ¢square root of 144). We can also chain ¢calculations together using ¢parentheses, like this:¢¢PRINT (16+4)/2¢¢ This will return 10. Without the ¢parentheses, Atari BASIC would divide ¢four by two first, arriving at two, ¢then add that to 16, giving a final ¢result of 18. But because your Atari ¢understands the use of parentheses in ¢mathematics, it will do what's in the ¢parentheses first, so it adds 16+4 and ¢gets 20, then divides that by 2 to get ¢the final answer of 10.¢¢ The Atari BASIC Reference Manual ¢lists operator precedence as follows:¢¢Highest Precedence: Relational ¢operators used in string expressions ¢have same precedence and are performed ¢left to right. These include ¢>,<.=.>=.<=,and <>.¢¢ These are followed, in order of ¢preference, by the unary minus (-), ¢exponentiation, multiplication and ¢division (* and /), addition and ¢subtraction (+ and -), relational ¢operations in numeric expressions ¢(again, <,>,=,>=,<=, and <>), the ¢unary operator NOT, the logical AND, ¢and the logical OR.¢¢ If you don't understand all the ¢mathematical implications of the ¢above, it's alright. It isn't the ¢purpose of this article to teach math. ¢But if you already know how to do math ¢and want to learn how to use your ¢Atari to help you, you'll find this ¢information vital.¢¢ The examples above also ¢illustrate something useful about ¢Atari BASIC, which is that if we want ¢an immediate anser we can just enter, ¢say, PRINT 2+2 and press the RETURN ¢key, and the Atari will give us 4 ¢right away. In a program, we can use ¢line numbers, perhaps storing the ¢result in a variable like this:¢¢10 LET A=(16+4)/2 20 PRINT A¢¢ When we RUN this we'll get 10 ¢again. Also, the LET is optional here, ¢we can just as easily write line 10 ¢as:¢¢10 A=(16+4)/2¢¢ Here's another example using ¢several variables.¢¢10 A=10:B=5 20 ¢AA=A+B:BB=A-B:CC=A/B:DD=A*B 30 PRINT ¢"A+B=";AA 40 PRINT "A-B=";BB 50 PRINT ¢"A/B=";CC 60 PRINT "A*B=";DD¢¢ When you RUN this program you'll ¢get the following:¢¢15 5 2 50¢¢ The above program illustrates ¢another feature of PRINT. Notice how ¢we used the information in quotation ¢marks to set up a nice, neat-looking ¢display, then followed that with a ¢semicolon, followed by the variable ¢name we wanted to print?¢¢ The semicolon leaves the PRINT ¢position where it left off. Otherwise ¢the answer would have been printed at ¢the begnning of the next line on the ¢screen. Try replacing those ¢semicolons. Rerite the lines as ¢follows:¢¢30 PRINT "A+B=":PRINT AA 40 PRINT ¢"A-B=":PRINT BB 50 PRINT "A/B=":PRINT ¢CC 60 PRINT "A*B=":PRINT DD¢¢ Now RUN the program again and ¢look at the difference in what appears ¢on the screen, and you'll see what the ¢semicolon did in the PRINT statements ¢we used earlier.¢¢ There's another command often ¢used with PRINT, the POSITION ¢statement, which allows us to tell ¢BASIC exactly WHERE on the screen we ¢want to print something. Let's try the ¢following program which illustrates ¢the effect of using POSITION with ¢PRINT:¢¢5 PRINT CHR$(125) 10 POSITION ¢0,0:PRINT "This is line 10" 20 ¢POSITION 0,2:PRINT "This is line 20" ¢30 POSITION 0,1:PRINT "This is line ¢30"¢¢ Before we RUN this program, first ¢I'd better explain line 5. Each ¢character that your Atari's keyboard ¢can produce is represented by a number ¢from zero to 255. Some of the ¢characters are letters and numbers, ¢some are punctuation marks, some are ¢the graphics characters produced by ¢pressing CONTROL and a letter key. The ¢number which represents each character ¢doesn't change...32 is an A, 33 is a ¢B, and so on. The character numbered ¢125 represents the command which ¢clears your screen. You can get it ¢from the keyboard with SHIFT-CLEAR or ¢CONTROL-CLEAR. In a program we can ¢tell BASIC to PRINT CHR$(125), and ¢when we RUN the program, BASIC will ¢clear the screen when it encounters ¢this command.¢¢ Now, RUN the program. The screen ¢will clear. Now look at the result. ¢You'll notice that "This is line 30" ¢appears on the screen BEFORE "This is ¢line 20", even though we know that ¢BASIC executed the statements in the ¢order they're numbered. The reason ¢things got printed out is because of ¢the POSITION command.¢¢ Line 10 uses "POSITION 0,0" which ¢tells BASIC to move the cursor to ¢column zero, line zero on the screen. ¢We then print "This is line 10" ¢starting from that screen position.¢¢ Line 20 uses "POSITION 0,2". This ¢tells the Atari to begin printing at ¢column zero on line TWO on the screen, ¢skipping over line one, which is then ¢used by line 30 (POSITION 0,1).¢¢ Now, if you ran the program you ¢can see that POSITION 0,0 starts ¢printing in the first space of the ¢first line on the screen. Why, then, ¢isn't this POSITION 1,1 instead of ¢0,0? because computers count funny ¢(compared to humans). To people, zero ¢means nothing, so we usually start ¢counting from one. But to a computer, ¢everything means something, even a ¢zero, and so computers start counting ¢from zero.¢¢ Thus, the upper lefthand corner ¢of your screen is 0,0 to the Atari, ¢not 1,1. Your Atari has 40 columns ¢across, numbered from zero to 39, and ¢it has 24 lines from top to bottom, ¢numbered from 0 to 23. You can specify ¢anyplace on the screen to print ¢something provided you know the right ¢x-y coordinates to use with the ¢POSITION statement. Of course, in a ¢program you should make sure that what ¢you want to print will fit. If you ¢start printing at, say, column 20, you ¢have only 19 characters left on that ¢line before you reach the end. When ¢you reach the end the Atari will go to ¢the beginning of the next line.¢¢ POSITION is handy when you want ¢to line things up in columns or print ¢in a "window" of space you've set ¢aside on the screen for user prompts.¢¢ In a future column we'll examine ¢the use of PEEK and POKE in Atari ¢BASIC, and we'll see how we can use ¢POKE to set the screen margins, ¢enabling us to adjust the width of the ¢screen so we can get the full 40 ¢column width without using POSITION ¢(the Atari defaults to a 38-character ¢screen width). In the meantime, next ¢month we'll look at numeric variables, ¢and at some methods of setting up what ¢programmers refer to as a "loop".
