Welcome to the third and final edition of this guide to BASIC!
Now that we’ve created the graphics for our game and have the key handling routines in place, we can now finally move on to the fun part of animating stuff and watch the screen come alive with millions of tiny coloured pixels (okay, maybe not millions but what the heck, who’s counting?)!
Firstly, if you remember the plot of our game, our intrepid hero of the game Krapz has to survive as long as he can by collecting little quanta particles until he gains enough power to jump out the warp into… yet another more difficult screen. He also has to avoid touching the “ST fluctuation trails” he leaves behind in his wake plus he must stay clear of the deadly boundaries of the warp. In order to pull this fancy (oh sure! – Ed) stuff off, we’ll break it down to a set of tasks.
Task #1: Set up the playing area. This involves drawing the boundaries of the warp and populating the playing field with randomly distributed set of quanta particles. Since every new screen that Krapz jumps into involves the above steps, it makes sense to package it as a sub-routine. Which is what we’ll do:
115 LET quanta=5*level: LET time=seedtime+(level*30): CLS 120 PRINT AT 1,0; INK 3;"bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb";AT 20,0; INK 3;"bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb" 130 FOR f=1 TO 19 140 PRINT AT f,0; INK 3;"b";AT f,31; INK 3;"b" 150 NEXT f 160 LET x=10: LET y=10 250 FOR f=1 TO 5*level 260 LET p=INT (RND*17)+2 270 LET q=INT (RND*28)+2 280 IF ((p=y AND q=x) OR (ATTR (p,q)=5)) THEN GO TO 260: REM don't overwrite player or another quanta! 290 PRINT AT p,q; INK 5;"g" 300 NEXT f 999 RETURN
The first line looks a bit complex but is actually a rather simple way of increasing the difficulty level as Krapz progresses through the game. The first LET statement simply increases the number of quanta particle in a screen by a factor of 5 depending on which level (screen) Krapz is playing on. For the first screen (level = 1), there will be only 5 quanta particles to collect. On the 2nd screen (level = 2) there will be 10, and so on and so forth. The second LET statement controls how much time Krapz has to collect all the quanta particles. This is again a factor of the level Krapz is on – 30 times the level as a matter of fact, which compensates for the fact that there are more quantas on each screen as we progress. You’ll notice that a seedtime has been tacked on in the equation to ensure that the player has a basic minimum time to start with on each level.
After having cleared the screen, it’s time to draw the boundaries of the warp – in our case it’s simply a rectangular arena to delineate the playing field. First we draw the top and bottom edges, which is simply a matter of printing 32 characters of the UDG “b” (incorrectly reproduced as b in the listing above) that we created earlier on. Line 120 prints the top edge and the bottom edge in lurid purple ink.
To draw the vertical left and right edges we employ a FOR-NEXT loop from line 130 to 150 that print UDG “b” 19 times vertically.
It’s now time to randomly populate the playing area with quanta particles. Line 160 first sets the initial co-ordinates of Krapz on each screen – he always starts on row 10, column 10 on every screen.
We then employ a FOR-NEXT loop to print the actual quanta. The number of particles is simply a factor of the level the player is on, and is in fact exactly the same as the variable quanta we calculated on line 115. In fact we can substitute the 5*level with quanta and the loop will work just as well. Line 260 and 270 calculate the row and column (p,q) to print the quanta on. INT (RND* 17) gives us a random number from 0 to 16. We add a factor of 2 to ensure we don’t print anything on row 0 and 1 – the top edge of the playing arena. Note that if got 16 and added 2 we get 18, which still within the bottom edge (column 20) of the arena. Similarl, we keep the quanta within the left and right edges of the playing arena in line 270.
Line 280 introduces a necessary check that ensures that we aren’t printing a quanta on top of another already existing quanta on screen (we do want all our quantas visible individually on the screen!). Plus we don’t want to print at the position Krapz is already in (10,10). So, what we are checking for is “whether the quanta co-ordinates are the same as Krapz’s co-ordinates OR whether the quanta particle is being printed on top of another quanta” state. This is achieved by the IF statement, where the two expressions are separated by an OR (as in the above statement) condition which returns true if any one of the expression is true (either printing on Krapz or printing on a quanta).
To check for whether the quanta co-ordinates are same as Krapz’s co-ordinates, we simply have to check if the column and row co-ordinates for both match up. If they are the same, then we are at the same location on screen. This is calculated using the expression in the IF statement p=y AND q=x. The AND condition returns true if and only if both the sub-expressions are true.
The next thing we have to check for is the case of a quanta overwrite condition. This is done by using the ATTR command, which given a x & y co-ordinate returns the colour attribute at that co-ordinate. The BASIC manual explains how to interpret this value but suffice to say that for our purpose if ATTR returns a value of 5 it means that a cyan coloured character (INK 5) is present at that position. As you will see on line 290, where we print our quanta particles, they are printed with INK 5. In essence, what we are saying is that if ATTR returns 5, we assume that we have a quanta particle there regardless of what actually might be there. In order to not trip up our assumptions we ensure that we do not print any other stuff in INK 5 anywhere the screen. In our program only the quanta particles can have the cyan colour when playing the game.Period.
If we are overwriting Krapz or another quanta, we simply go back to line 160 and re-calculate a new quanta position until we are satisfied we aren’t overwriting anything we shouldn’t be overwriting! If all is well, we proceed to actually print a quanta particle on the screen at the position we calculated (line 290). Once we’ve printed all the quanta particles required for a level we RETURN from the sub-routine via line 999.
There! We’re all set to actually do some gameplay and stuff! Lets move on to the task of moving Krapz around on the playing area. Remember we set up our control keys in Code Shed Guide 2, so it’s time to use those keys to move Krapz around.The control scheme we’ll follow is a simple one. Pressing one of the four direction keys changes Krapz’s direction instantly (inertia? What’s that?). The key needn’t be held down if one wishes Krapz to continue in that direction though- his inertia (that word again!) will keep him going in that direction until a different direction key is pressed.
Have a look at this code which begins our main game loop (so called because we’ll be executing it repeatedly till something happens to break proceedings) proper:
1010 PRINT AT 21,0;"Lives: ";lives;TAB 20;"Score: ";score 1020 PRINT AT 0,0;"Time: ";time;" " 1030 PRINT AT 0,20;"Level: ";level 2000 LET a$=INKEY$ 2010 IF a$=k$(1) THEN LET dir=1: REM right 2020 IF a$=k$(2) THEN LET dir=2: REM left 2030 IF a$=k$(3) THEN LET dir=3: REM up 2040 IF a$=k$(4) THEN LET dir=4: REM down 2045 IF dir<>0 THEN PRINT AT y,x; INK 1;CHR$ (143) 2050 IF dir=1 THEN LET x=x+1 2060 IF dir=2 THEN LET x=x-1 2070 IF dir=3 THEN LET y=y-1 2080 IF dir=4 THEN LET y=y+1
Lines 1010 to 1030 give information regarding the status of the number of lives left, the current score, the time left and the current level being played. It’s our HUD if you will. Since it’s within the main game loop, it will be updated continously.
Lines 2000-2080 are responsible for our inertial control mechanism. The logic is quite simple. We sample the keyboard for any keypress. If it’s a direction key we change Krapz’s direction of movement. If no directional key is pressed, Krapz continues to move along in the same direction. Here’s how. Line 2000 reads in a key from the keyboard (null if no key is pressed). Lines 20101 to 2040 compare the value in a$ with the value in k$ (our desired control keys). Depending on which key is pressed, a variable called dir is set to a particular value that signifies a direction.
Line 2045 prints the trail behind Krapz. Basically, all it does is see if Krapz is moving (Krapz doesn’t start moving until you press a key initially), we don’t bother printing a trail. If he is moving (dir will have some non-zero value then), we will print a solid block of blue at the current Krapz co-ordinates. CHR$ (143) will print the solid graphic block you see on the numeric 8 key on the Speccy keyboard (Graphic mode + shift + 8). What’s this CHR$ you ask? Well it’s one way of printing a character on the screen. For example,if you do PRINT CHR$(65) it will print the letter “A” on the screen. This is because CHR$ converts a number to its string equivalent from the character set. If you take a look at the character set table in the Speccy manual you’ll see that numbers from 32 to 127 represent characters from the ASCII standard. Which is why 65 corresponds to letter “A”. From 128 to 255, the character set is unique to the Spectrum with some special characters like the Graphic Blocks taking up positions 129 to 143. You can print any of these characters by passing the code number to CHR$. Some unprintable characters (no, not the sort you’re thinking of) can do some fancy print formatting tricks – refer to the manual for more on that.
Coming back to the code, so we print the blue block at the current Krapz co-ordinates to signify a trail. “Hang on!” you say. “Won’t we be overwriting Krapz in the process?”. But of course! But since we’ll be repositioning Krapz at new co-ordinates anyway, it doesn’t matter. In fact, the code to calculate Krapz’s new co-ordinates follows:
2050 IF dir=1 THEN LET x=x+1 2060 IF dir=2 THEN LET x=x-1 2070 IF dir=3 THEN LET y=y-1 2080 IF dir=4 THEN LET y=y+1 2090 IF (ATTR (y,x)=3 OR ATTR (y,x)=1) THEN FOR f=0 TO 4: PRINT AT y,x; PAPER 2; INK 6; FLASH 1;CHR$ (145+f); FLASH 0: BEEP 0.4,RND*f: NEXT f: PAUSE 10: GO TO 6000
Lines 2050 to 2080 recalculate the new x or y position depending on the direction of travel. Simple, eh?
In line 2090 we check whether Krapz has crashed into the boundary walls or walked into his own trail (ST fluctuations are bad remember?). This is handled by a single IF statement that checks whether the attribute colour at the newly calculated Krapz co-ordinate matches INK 3 (the magenta colour with which the walls are drawn) or INK 1 (blue colour of Krapz’s trail). If it is, it’s time to play a sad ditty expressing our condolences and create a nifty looking explosion. Since I’m not musically inclined I’m just using a simple BEEP statement that randomly plays some stuff in a low pitch. Creating the explosion is simplicity in itself – I’ve created 4 explosion UDG’s using BASin’s UDG creator tool. To simulate the explosion effect, I just print the four characters (with full FLASH and stuff for added effect) one atop the other with a small time gap inbetween to slow things down a bit. It’s not the greatest explosion effect you’ll ever see but it’s good enough for our purpose. After the dust has settled down, we send the program off to line 6000 where the last recitals are performed. More on that later.
2095 IF ATTR (y,x)=5 THEN LET score=score+(level*10): LET quanta=quanta-1: BEEP 0.01,0.1: IF quanta=0 THEN PRINT AT 10,3; PAPER 1; INK 5; FLASH 1;"Space-Time Jump! Get Ready!"; FLASH 0: FOR f=1 TO 10: BEEP 0.03,RND*f: NEXT f: PAUSE 50: LET level=level+1: GO TO 6020 3000 PRINT AT y,x; INK 4;"a"
Line 2095 determines what happens when Krapz picks up a quanta particle (we simply perform an attribute check for that). What happens is this: first the score is increased by 10 times the level we are on (as a measure of difficulty).The number of active quantas on the screen is decreased by one. If there are no more quantas left, it’s time to trigger a Space-Time jump and warp to the next level which is easily done with a simple message, a few random beeps and a jump to Line 6020 that sets a few variables back to initial values and then re-draws the level screen by jumping to line 1010.
If a space-time warp hasn’t been triggered (because there are quantas still left on the screen), we go on to print Krapz at the new locatio in line 3000.
We come to a final bit of code that deals with the player running out of time.
3005 LET time=time-1 3010 IF time=0 THEN PRINT AT 10,5; PAPER 2; INK 6; FLASH 1;"S-T Field Collapsing!"; FLASH 0: FOR f=10 TO 1 STEP -1: BEEP 0.1,f/2: NEXT f: PAUSE 50: FOR f=2 TO 19: PRINT AT f,1; PAPER 1; INK 2;"::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::": BEEP 0.05,RND*f/2: NEXT f: GO TO 6000 4000 GO TO 1010 6000 LET lives=lives-1 6010 IF lives=0 THEN PRINT AT 10,10; PAPER 4; INK 1;" GAME OVER! ": PAUSE 100 6012 IF lives=0 THEN IF score>hiscore THEN LET hiscore=score: PRINT AT 12,10; PAPER 5; INK 1; FLASH 1;"New High Score!"; FLASH 0: PAUSE 100 6015 IF lives=0 THEN RETURN
Line 3005 simply decreases the available time by 1 unit.
Line 3010 determines what happens next. If we’ve run out of time it’s time for another dramatic message with annoying random beeps and appropriately dangerous looking flashing text. We up the ante by printing a line of solid block (the odd :: is the same GRAPHIC MODE + SHIFT + 8 friend we met a while back) that slowly fills up the entire screen to signify a field collapse. We then jump to line 6000 to finish off the formalities.
If we haven’t run out of time we simply loop back to line 1010 via Line 4000 to continue playing.
Line 6000 is where the case of death is handled. First the number of available lives is decreased by 1. Line 6010 checks if we’ve used up all our lives, in which case it’s really Game Over! Line 6012 next checks if we created a new high score by simply comparing the current score with the high score. If our current score is higher, we save it as the high score and congratulate the player on achieving the same.
Line 6015 proceeds to return from the main game loop routine and dumps us back at the main menu screen from where we’d first come from thus bringing us back full circle to square one.
And that my friends brings us to an end to this guide! Hopefully, you’ll go on to write better and bigger stuff than the Dash-it game described here. If not, don’t forget to send your entry to the annual Crap Games Competition! Cheers!