Keep the one-food challenge, but choose a different free square. The route to it is now the player’s problem.
Replace lines 440, 1020, 2000.
Add line 2010.
440 IF eaten=1 THEN LET e$="One snack. Nicely done!": GO TO 4000
1020 PRINT AT 2,5; INK 7;"Eat one snack. R retries."
2000 LET fr=1 +INT (RND*16): LET fc=1 +INT (RND*24)
2010 IF g(fr,fc)=1 THEN GO TO 2000RUN the changed program.
Complete source at this checkpoint
10 GO SUB 7000
100 DIM g(16,24): DIM a(12): DIM b(12)
110 LET n=4: LET eaten=0: LET h=4: LET t=1: LET dr=0: LET dc=1: LET steps=0
120 FOR j=1 TO 4: LET a(j)=8: LET b(j)=j+5: LET g(8,j+5)=1: NEXT j
130 GO SUB 1000: GO SUB 2000: GO SUB 2500
140 LET nd=dr: LET nc=dc: LET turn=0: LET tick=PEEK 23672
150 LET k$=INKEY$
160 IF k$="q" OR k$="Q" THEN GO TO 9000
170 IF k$="r" OR k$="R" THEN GO TO 100
180 IF turn=1 THEN GO TO 240
190 IF (k$="i" OR k$="I") AND dr=0 THEN LET nd=-1: LET nc=0: LET turn=1
200 IF (k$="k" OR k$="K") AND dr=0 THEN LET nd=1: LET nc=0: LET turn=1
210 IF (k$="j" OR k$="J") AND dc=0 THEN LET nd=0: LET nc=-1: LET turn=1
220 IF (k$="l" OR k$="L") AND dc=0 THEN LET nd=0: LET nc=1: LET turn=1
240 LET elapsed=PEEK 23672 -tick
250 IF elapsed<0 THEN LET elapsed=elapsed+256
260 IF elapsed<18 THEN GO TO 150
270 LET tick=PEEK 23672: LET dr=nd: LET dc=nc
280 LET nr=a(h)+dr: LET nx=b(h)+dc
290 IF nr<1 OR nr>16 OR nx<1 OR nx>24 THEN LET e$="Mind the wall!": GO TO 4000
300 LET grow=(nr=fr AND nx=fc)
310 IF g(nr,nx)=1 AND (grow=1 OR nr<>a(t) OR nx<>b(t)) THEN LET e$="You caught your tail!": GO TO 4000
320 PRINT AT a(h)+3,b(h)+3; INK 5;CHR$ 149
330 IF grow=1 THEN GO TO 380
340 LET g(a(t),b(t))=0: PRINT AT a(t)+3,b(t)+3;" "
350 LET t=t+1: IF t=13 THEN LET t=1
360 GO TO 390
380 LET n=n+1: LET eaten=eaten+1
390 LET h=h+1: IF h=13 THEN LET h=1
400 LET a(h)=nr: LET b(h)=nx: LET g(nr,nx)=1: LET steps=steps+1
410 GO SUB 3000
420 IF grow=0 THEN GO TO 460
430 GO SUB 2500
440 IF eaten=1 THEN LET e$="One snack. Nicely done!": GO TO 4000
450 GO SUB 2000
460 LET nd=dr: LET nc=dc: LET turn=0: GO TO 150
1000 BORDER 0: PAPER 0: INK 7: CLS
1010 PRINT AT 0,11; INK 5;"TAIL CHASE"
1020 PRINT AT 2,5; INK 7;"Eat one snack. R retries."
1030 FOR x=3 TO 28: PRINT AT 3,x; INK 1;CHR$ 144;AT 20,x;CHR$ 144: NEXT x
1040 FOR y=4 TO 19: PRINT AT y,3; INK 1;CHR$ 144;AT y,28;CHR$ 144: NEXT y
1050 FOR j=1 TO 4: PRINT AT a(j)+3,b(j)+3; INK 5;CHR$ 149: NEXT j
1060 GO SUB 3000
1070 PRINT AT 21,1; INK 5;"I J K L steer R retry Q quit";
1080 RETURN
2000 LET fr=1 +INT (RND*16): LET fc=1 +INT (RND*24)
2010 IF g(fr,fc)=1 THEN GO TO 2000
2020 PRINT AT fr+3,fc+3; INK 6;CHR$ 150
2030 RETURN
2500 PRINT AT 1,4; INK 7;"FOOD ";eaten;"/1";AT 1,18;"LENGTH ";n;" "
2510 RETURN
3000 LET z=146
3010 IF dr=-1 THEN LET z=145
3020 IF dr=1 THEN LET z=147
3030 IF dc=-1 THEN LET z=148
3040 PRINT AT a(h)+3,b(h)+3; INK 7;CHR$ z
3050 RETURN
4000 PRINT AT 2,0;" ";AT 2,2; INK 6;e$
4010 PRINT AT 21,1; INK 7;"R for another go Q to quit ";
4020 IF INKEY$<>"" THEN GO TO 4020
4030 LET k$=INKEY$
4040 IF k$="q" OR k$="Q" THEN GO TO 9000
4050 IF k$="r" OR k$="R" THEN GO TO 100
4060 GO TO 4030
7000 RESTORE 7200: FOR j=0 TO 55: READ v: POKE USR "a"+j,v: NEXT j: RETURN
7200 DATA 255,129,129,129,255,129,129,255
7210 DATA 60,126,90,90,126,126,60,24
7220 DATA 60,126,252,204,204,252,126,60
7230 DATA 24,60,126,126,90,90,126,60
7240 DATA 60,126,63,51,51,63,126,60
7250 DATA 0,60,126,126,126,126,60,0
7260 DATA 8,16,60,126,94,126,60,0
9000 PAPER 0: INK 7: PRINT AT 21,0;"Finished. RUN to try again. ";: STOPChoose within the arena
RND supplies a value from zero up to, but not including, one. Multiplying by 16 gives a value below 16. INT takes the integer part for these non-negative values, producing 0–15; adding one gives rows 1–16. The same calculation with 24 gives columns 1–24.
Line 2010 checks the candidate in g. If occupied, it goes back to 2000 and chooses both coordinates again. Only a free candidate reaches the PRINT statement. It does not erase a piece of body to make room for food.
The initial body contains four cells in a 384-cell arena. Even the final twelve-cell body will leave free squares. This program therefore needs no full-board outcome. A game that could fill the arena would need to detect that condition before trying forever to find an empty square.
Repeated random trials have no fixed attempt count. A board with few free cells could spend much longer looking for food; our small body cap avoids that dense-board design.
Random does not mean always different
RUN the program, find the food, finish and retry. A new food can appear in a previously used square. Its position should always remain inside the border and outside the current body.
The routine now uses the generator’s current sequence. A fresh machine can repeat that sequence; the final title will use RANDOMIZE at the player’s start to vary its starting point. Random placement and seeding are separate decisions.
Why is a remembered earlier food position not automatically a reason to reject the new one?
Show the explanation
The rule rejects cells occupied by the current body. Previous food positions are allowed when free. Forbidding repeats would introduce a separate history and a different game rule.
If the fruit appears on the snake, check that the grid is initialised before the food routine and that line 2010 runs before drawing. A fresh RUN should give a four-cell body, FOOD 0/1 and one visible fruit.
Sources
Steven Vickers, edited by Robin Bradbeer, ZX Spectrum BASIC Programming, second edition (Sinclair Research, 1983), 11 (RND and RANDOMIZE).
Run it here
This unit's program, built for you.
- Program
- step-01.tap
- Machine
- ZX Spectrum 48K (PAL)