Beginners guide to BASIC – 2
In the previous installment we left off at the GO SUB command and how it can be used to invoke a sub-routine. We shall continue that discussion and then some more by examining another bit of code first:
1 REM Dash it! 2 REM by Arjun Nair 2004 3 REM *********************** 5 CLEAR : RANDOMIZE 10 CLS : LET hiscore=0 11 DIM k$(4): LET k$(1) = "p": LET k$(2) = "o": LET k$(3) = "q": LET k$(4) = "s" 15 GO SUB 6060: REM init graphics 20 GO SUB 8000: REM main menu 30 GO SUB 1000: REM game loop 50 GO TO 20
In line 5, we see the first bits of useful code. CLEAR simply wipes out the Spectrum memory and provides you with a clean slate to work on.
NOTE: Actually that’s a simplistic explanation. The CLEAR command actually does a bit more than that. It not only clears all the variables but also clears the display screen, resets the PLOT position and clears the GO SUB stack.
The following statement is a RANDOMIZE keyword, which initialises the random number generator and makes it as random as possible (since computers can’t actually generate truly random numbers for various reasons). Sometimes the RANDOMIZE keyword is followed by a number, which is called the seed. Specifying a seed of zero (using RANDOMIZE on its own is equivalent of saying RANDOMIZE 0) makes the Speccy use an internal timer to seed the generator, which makes things very random indeed.
NOTE: Again, I have not gone in depth into the workings of the RANDOMIZE keyword. But suffice to say that using a number other than zero will create a sequence of random numbers that will repeat every time the program is run. Using zero ensures that RANDOMIZE will be as random as possible.
Coming to line 10 we have the CLS keyword that simply clears the screen, and nothing more. If you have specified any foreground and background colours and/or a border colour, the CLS command will clear the screen to them. We’ll come to that shortly when we start dabbling in colours.
The next statement is a LET statement that assigns the value zero to a variable called highscore. Effectively, a variable is a named placeholder that serves to hold some value. They are of two types – numeric variables and string variables. Numeric variables hold only numeric values (ranging from 0 to 255), while string variables hold alphanumeric data in string format (anything within quotation marks). In our case, highscore is simply a numeric variable that initially holds zero.
Things get a bit more involved in the next line. Before I get into the technical explanation, let me explain what I’m trying to do here. What I want is the player to be able to redefine the control keys from within the game if he so chooses. Therefore, I save the information on control keys in a string array (I’ll come to arrays shortly), which can be manipulated anytime (which we’ll do when we implement the Re-define keys option).
As for arrays, well they are nothing but a collection of similar stuff (or objects as we call them in programming parlance). You can have a collection of months (January, February, et al) or collection of music albums, and they can all be thought of as arrays of months and albums respectively. Like plain variables, an array can be of numeric or string type. In fact, they are called array variables and individual elements of the array behave just like plain variables.
In the code above, we would like to have an array of control keys (an array since they are just a collection of keys). The way to go about is that we first tell the Speccy that we would like to set aside space for so many number of array elements. In our case, we would like to create an array of four control keys (Up, Down, Left & Right). The statement DIM k$(4) does precisely this. DIM is short for DIMension (size of array), k$ is the name of the array variable and 4 signifies the number of elements that we want.
Once you have created an array, you can fill it in using the LET statement like we did for highscore. The difference here is that we access individual elements of the array using the syntax k$(1), k$(2), …. k$(n) – where n is any element number (an index). You cannot do LET k$ = something. You MUST specify the element within the array that you wish to change by giving an index number. So we have stored the default letters that will correspond to our control keys in k$ from 1 to 4.
NOTE: In Sinclair BASIC, all string variables must consist of only one letter followed by the $ sign, which is why we have called our control keys array k$ instead of something meaningful like key. Numeric variables don’t suffer from such limitations, which is why our high-score variable is sensibly named highscore.
Lines 15 to 30 use a bunch of GO SUB’s to transfer control to various part of the program. GO SUB is short for GO to SUBroutine. As we saw earlier, a subroutine is a short piece of code within your program that performs a specific function. Once it is finished, the sub routine RETURNS control to the point in the program from which it was called.
In our case, we have separate sub-routines for initialising the graphics for the game, for presenting a menu to the player and finally for running the actual game. As I explained earlier, by using sub routines I have “compartmentalised” the code in my program so that if I later decide to change the way a certain sub routine is implemented (say, if I change how the menu looks) I don’t have to worry about how it will affect the whole game. Here, line 15 first transfers control to subroutine that starts from line 6060 that sets up the game graphics.
Once the subroutine is done, it RETURNs to control to the point from where it was called, in our case this is back to line 15. The next statement is the REM statement (which I have used to remind me which subroutine the GO SUB is referring to), so the Speccy ignores it and goes to the next line instead. Similarly for the main menu and for running the game (the game loop).
Finally we have line 50 with a GO TO keyword that transfers control to line 20. This is so that once the game loop is done (the player has finished all his lives) we want to go back to the main menu option screen. The difference between GO TO and GO SUB is that the former simply transfers control to the line specified in the program and doesn’t expect any RETURNs to it (we don’t want to come back here from the main menu for instance), while the latter is a call to an actual sub-routine.
NOTE: If you’ve typed the code in and wish to save it, choose the Save-As option from the BASin File option. Late on, I’ll show you how to create an actual tape image so that you can load it in to your Speccy!
Okay, let’s have a look at the main menu sub-routine then. This sub-routine will present 4 options to the user and the user can select any one of the options by pressing the key corresponding to the item of interest. The code then:
8000 REM *** Main Menu *** 8010 BORDER 0: PAPER 0: INK 6: CLS : LET a$="": 8020 PRINT "\f\f\f\f\f\f\f\f\f\f\f\f\Dash it!\f\f\f\f\f\f\f\f\f\f\f\f\" 8030 PRINT AT 5,7; INK 3;"1) Play game" 8040 PRINT AT 7,7; INK 4;"2) Redefine Keys" 8050 PRINT AT 9,7; INK 5;"3) Instructions" 8060 PRINT AT 11,7; INK 6;"4) Credits" 8070 PRINT AT 15,7; INK 7;" High Score: ";hiscore 8080 PRINT #0; INK 2;TAB 5;"(c) 2004 Arjun Nair" 8090 LET a$=INKEY$: 8095 IF a$="" OR a$>"4" THEN GO TO 8090 8096 BEEP 0.3,10 8100 IF a$="0" THEN PRINT #0;"For my parents and Lina, Shreya & Hazel": PAUSE 200: CLS 8110 IF a$="1" THEN RETURN 8120 IF a$="2" THEN GO SUB 8400 8130 IF a$="3" THEN GO SUB 9000 8140 IF a$="4" THEN GO SUB 8300 8150 GO TO 8010
All right! So what do we have here? Line 8000 is our friendly REM statement again informing us that this is the Main Menu sub-routine.
Line 8010 sets up the display screen by setting the background (PAPER) and border (BORDER) colour to black (0) and the foreground (INK) to yellow (6). Doing a CLS then quickly clears the screen of any previous junk and enables our colour settings. We then initialise a string variable called a$ to a null value. I’ll tell you why shortly.
The funny characters you see in line 8020 in the PRINT command are, unfortunately, junk copied over from BASin thanks to some sneaky embedded control characters in the PRINT command. Instead of using the usual INK and PAPER commands to change the colours we can actually use a BASIC trick to embed these colours within the PRINT command itself. You can see that they are embedded because they will then turn up even in the listing of the program on the Speccy. In BASin, the editor replaces these control characters with some symbols, which is why they don’t turn up in the listings. However, do a LIST command and see the results in the emulator window. You will see what I mean. So how do we embed control characters for colours in the PRINT statement? I’m not aware of how to do it in BASin, since the editor doesn’t allow you to do such things. On a Speccy however, you can do this: After the first quotation mark in the PRINT statement, embed the colour information by going into extended mode (flashing E cursor) and pressing the key that corresponds to the required paper colour or holding down the SHIFT key (while still in extended mode) and selecting the required INK colour. For example, if you want to print something in red on a yellow background, you will first enable the yellow background for the text by going in to extended mode and pressing 6. Then enable the red ink by again going into extended mode, hold down shift and press 2. Now type in whatever you want and it will be in the colours you selected (even in listings). When the required text is done, you will have to revert back to the normal colours or your entire program listing will be in this colour! To do this, simply repeat the process after the coloured text but this time choosing white paper and black ink. Apart from colour control characters, there are other control characters that you can embed but I won’t be examining them here.
If you aren’t comfortable with using control characters (and I’m not) stick to normal INK and PAPER commands to get things done.
Lines 8030 to 8060 print out the options for the user. Again note the versatility of the PRINT command. I’m adding an AT qualifier to the PRINT command to tell it where exactly to print text on the screen. The PRINT AT form takes two parameters: one for the column number and the second for the row number that you want to print to. So the PRINT AT command in line 8030 will print the text “1) Play game” at column 5, row 7 on the screen. Similarly we list the other options on the screen.
Line 8070 offers something of more interest. We are printing out the text “High Score: ” followed by the value in the hiscore variable. This way you can print the value of any variable including string variables (we’ll see an example of that in the Key Redefine sub-routine).
Next we come to the ego-feeding copyright message of line 8080. The only thing of interest here is the TAB keyword that inserts blank spaces when printing to screen. The number after the TAB keywords indicates how many blank spaces to insert. Note that this number ranges from 0 to 31 inclusive. So to insert only one blank space, use TAB 0, not TAB 1, which will insert 2 spaces!
Now we come to the interesting part of actually finding out which option the user has selected and then acting on it. The first step is to read in the key pressed by the player when choosing an option.
To find out which key has been pressed is achieved by using the INKEY$ command. When the Speccy encounters this command it immediately (well almost) scans the keyboard to see if a key has been pressed, and if so, it returns the value of that key. If no key has been pressed, it returns a null value. In line 8090, we ask the Speccy to do just that and save that value in a variable called a$ (which you will remember we initialised to a null value in line 8010), so that we can act upon the information.
Line 8095 does a simple check to see if the user has pressed a key that we are interested in. If not we simply GO TO line 8090 to again wait for a key press.
This requires an introduction of our first conditional statement via the IF-THEN statement. It’s simplicity itself, really. The IF statement takes the form, IF <condition> is true, THEN do this otherwise skip this entire line. You can string multiple conditions together in an IF statement by using the OR and AND operators. ORing two conditions together will evaluate to true if any one of the conditions is true. ANDing any two conditions together will evaluate to true only if both conditions are true. It’s pretty similar to how things work in English. For example: IF it’s raining AND I don’t have an umbrella THEN I’ll stay in. Here you will stay in only if both the conditions – rain and lack of umbrella – are true. Another example: IF it’s raining OR it’s windy THEN I’ll stay in. In this case you will stay in if either of the conditions – rain or wind – are true. The BASIC form of IF is similar except that the conditions aren’t in English like above. They take a slightly different form which will become apparent as we examine our code in detail.
Right, so what does line 8095 do? It will GO TO line 8090 IF the value in a$ is null (no key pressed) OR if a$ has a value greater than 4 (we are interested only in keys from 0 to 4 that correspond to the options on the screen). Simple, eh?
In line 8096 we take our first stab at sound effects on the Speccy. This is achieved by using the BEEP command that takes two parameters. The first one is the duration in seconds (so 0.3 here indicates that we want a short note) and the second one is the pitch of the note (in semi-tones above or below middle C depending on whether it’s positive or negative). As I’m not musically inclined all I can say is that lower values of second parameters give you a low guttural sound while higher values give high-pitched “shrieky” sounds. Here we want to emit a short low pitched sound to indicate that a valid key has been pressed by the user and that an option has been selected.
Line 8100 is an interesting one ‘cos it handles the event for key 0 being pressed. If you notice there is no option that corresponds to key 0. It’s just a hidden Easter-egg that pops up with some irrelevant information (to the user), waits for 4 seconds and then clears the screen. In case you are wondering who the other people are, apart from my parents, that I’ve mentioned, they are my cousin-sister, my niece and my ex-crush (don’t ask).
Right ho! Back to the code and we come to line 8110 which checks if the user chose to play the game, in which case it simply RETURNs control to line 20 from where it was called originally, where it just finds the REM statement lurking after the colon symbol and so skips on next to line 30. You can clearly see the sub-routine process in action here. Line 20 called the main menu sub-routine, which after it was done, simply returns back to line 20. You can call this sub-routine from anywhere in the program and after it’s done, it will RETURN control to from wherever it was called from.
Line 8120 calls the Key Redefine sub-routine if the user has selected 2. So once that sub-routine is done it will return here, back to the main menu. Similarly, line 8130 calls the Instructions sub-routine and line 8140 calls the Credits sub-routine.
The last line in the sub-routine, 8150, does a GO TO to the beginning of the sub-routine (line 8010), so that the main menu is displayed again. This is done because the sub-routines may have displayed something else on the screen (instructions for example) and we want to re-display the main menu once control returns from the sub-routine.
And with that we come to an end to this installment. I hope you’ve understood most of the stuff in here if not all. Experiment with the BASIC stuff introduced here and you will get a good hang of things. The one good thing about Sinclair BASIC is that it’s pretty intuitive for the most part and is quite easy to get to grips with.
In the next edition of this guide, we’ll introduce the key-redefinition sub-routine, graphics and the actual game loop where all the action resides. So, until next time, cheerio!
Comments and feedback are most welcome and appreciated! Please note that the article is not meant to replace the Spectrum BASIC manual as a reference tome and as such I would encourage you to look up any keywords that you come across in the article for further insights. Also, please address your queries regarding BASin to the author of the program Paul Dunn, especially regarding bugs, features and operation of the program (the article will only cover parts that are essential to the discussion).