When neither side can finish a line immediately, the opponent still needs a choice. Prefer the centre, then a corner, then an edge. These are useful preferences, not a proof that the computer cannot lose.
Replace lines 4040, 4050, 4060.
Add lines 4070, 4080, 4090, 4100, 4110, 9530.
4040 IF b(5)=0 THEN LET mv=5: LET a$="centre": RETURN
4050 RESTORE 9530
4060 FOR j=1 TO 4: READ v
4070 IF mv=0 AND b(v)=0 THEN LET mv=v
4080 NEXT j
4090 IF mv>0 THEN LET a$="corner": RETURN
4100 FOR j=1 TO 9: IF b(j)=0 THEN LET mv=j
4110 NEXT j: LET a$="edge": RETURN
9530 DATA 1,3,7,9RUN the changed program.
Complete source at this checkpoint
10 BORDER 0: PAPER 0: INK 7: BRIGHT 1: CLS
20 DIM b(9): LET moves=0
30 GO SUB 1000
40 GO TO 250
250 GO SUB 9000
260 LET k$=INKEY$: IF k$="" THEN GO TO 260
270 IF k$="q" OR k$="Q" THEN GO TO 8000
280 IF k$="r" OR k$="R" THEN RUN
290 IF k$<"1" OR k$>"9" THEN GO TO 250
300 LET n=VAL k$
310 IF b(n)<>0 THEN PRINT AT 18,2; INK 6;"Taken. Choose an empty square.": GO TO 250
320 LET b(n)=1: LET moves=moves+1: GO SUB 2000
330 GO SUB 3000
340 IF winner>0 OR moves=9 THEN GO TO 4900
600 PRINT AT 18,2; INK 6;"O is choosing... "
610 GO SUB 4000
620 LET n=mv: LET b(n)=2: LET moves=moves+1: GO SUB 2000
630 PRINT AT 19,2; INK 5;"O chose ";mv;": ";a$;" "
640 GO SUB 3000
650 IF winner>0 OR moves=9 THEN GO TO 4900
660 PRINT AT 18,2; INK 7;"Your move: choose 1-9. "
670 GO TO 250
1000 CLS
1010 PRINT AT 0,8; INK 5;"THREE IN A ROW"
1030 INK 1: FOR i=0 TO 3: PLOT 56 +i*48,32: DRAW 0,120: NEXT i
1040 FOR i=0 TO 3: PLOT 56,32 +i*40: DRAW 144,0: NEXT i
1050 FOR n=1 TO 9: GO SUB 2000: NEXT n
1060 PRINT AT 18,2; INK 7;"Your move: choose 1-9."
1090 PRINT AT 21,2; INK 7;"1-9: place R: reset Q: quit"
1100 RETURN
2000 LET row=INT ((n-1)/3): LET col=n-1 -row*3
2010 LET x=80 +col*48: LET y=132 -row*40
2020 PRINT AT 5 +row*5,10 +col*6; INK 7;" "
2030 IF b(n)=0 THEN PRINT AT 5 +row*5,10 +col*6; INK 7;n: RETURN
2040 IF b(n)=2 THEN GO TO 2100
2050 INK 5: FOR z=0 TO 1: PLOT x-10 +z,y-10: DRAW 20,20: PLOT x-10 +z,y+10: DRAW 20,-20: NEXT z
2060 RETURN
2100 INK 7: FOR z=0 TO 1: PLOT x-6,y+10 -z: DRAW 12,0: DRAW 4,-4: DRAW 0,-12: DRAW -4,-4: DRAW -12,0: DRAW -4,4: DRAW 0,12: DRAW 4,4: NEXT z
2110 RETURN
3000 LET winner=0: LET wl=0: RESTORE 9500
3010 FOR w=1 TO 8: READ p,q,f
3020 IF b(p)<>0 AND b(p)=b(q) AND b(q)=b(f) THEN LET winner=b(p): LET wl=w
3030 NEXT w: RETURN
4000 LET mark=2: GO SUB 4500
4010 IF mv>0 THEN LET a$="win": RETURN
4020 LET mark=1: GO SUB 4500
4030 IF mv>0 THEN LET a$="block": RETURN
4040 IF b(5)=0 THEN LET mv=5: LET a$="centre": RETURN
4050 RESTORE 9530
4060 FOR j=1 TO 4: READ v
4070 IF mv=0 AND b(v)=0 THEN LET mv=v
4080 NEXT j
4090 IF mv>0 THEN LET a$="corner": RETURN
4100 FOR j=1 TO 9: IF b(j)=0 THEN LET mv=j
4110 NEXT j: LET a$="edge": RETURN
4500 LET mv=0: RESTORE 9500
4510 FOR w=1 TO 8: READ p,q,f
4520 IF mv=0 AND b(p)=mark AND b(q)=mark AND b(f)=0 THEN LET mv=f
4530 IF mv=0 AND b(p)=mark AND b(f)=mark AND b(q)=0 THEN LET mv=q
4540 IF mv=0 AND b(q)=mark AND b(f)=mark AND b(p)=0 THEN LET mv=p
4550 NEXT w: RETURN
4900 LET r$="DRAW. No line completed."
4910 IF winner=1 THEN LET r$="X WINS. Three in a row."
4920 IF winner=2 THEN LET r$="O WINS. Three in a row."
4930 IF wl>0 THEN GO SUB 6000
4940 PRINT AT 18,2; INK 6;r$;" "
5000 GO SUB 9000
5010 LET k$=INKEY$: IF k$="" THEN GO TO 5010
5020 IF k$="q" OR k$="Q" THEN GO TO 8000
5030 IF k$="r" OR k$="R" THEN RUN
5040 GO TO 5010
6000 RESTORE 9500: FOR w=1 TO wl: READ p,q,f: NEXT w
6010 LET row=INT ((p-1)/3): LET col=p-1 -row*3
6020 LET sx=80 +col*48: LET sy=132 -row*40
6030 LET row=INT ((f-1)/3): LET col=f-1 -row*3
6040 INK 6: PLOT sx,sy: DRAW 80 +col*48 -sx,132 -row*40 -sy: RETURN
8000 PAPER 0: INK 7: BRIGHT 0: PRINT AT 21,2;"Three in a Row finished. ": STOP
9000 IF INKEY$<>"" THEN GO TO 9000
9010 RETURN
9500 DATA 1,2,3,4,5,6,7,8,9
9510 DATA 1,4,7,2,5,8,3,6,9
9520 DATA 1,5,9,3,5,7
9530 DATA 1,3,7,9Begin with 1. O takes 5 and reports centre. Restart and choose 5: O takes 1 and reports corner. The earlier win and block checks still outrank these preferences.
A fallback with a definite order
The centre belongs to four winning lines. Each corner belongs to three; each edge to two. That gives us a reason to try these groups in this order, while leaving the actual tactical checks first.
Line 4040 can choose the centre directly. The corner search reads 1, 3, 7 and 9 from DATA at 9530. Its mv = 0 guard preserves the first free corner. RESTORE 9530 selects this small list; the opportunity routine still restores 9500 for winning triples.
If no centre or corner is free, the final loop scans all nine squares. It has no first-choice guard, so each empty square replaces mv. The last empty square wins. At this point only edges can be empty, which makes edge an accurate reason even though the loop visits the whole array.
What the rules leave out
An immediate opportunity has two marks and a gap. A promising position may have only one mark in each of two lines. Our search does not recognise that longer plan. A preference can steer towards useful space without understanding what the opponent will do with it.
If the centre is occupied, the only free corners are 3 and 7, and no immediate win or block exists, which corner is chosen? What if only edges 2 and 8 remain empty?
Show the explanation
3 is the first free corner in the DATA list. For the edge fallback, 8 replaces 2 as the loop proceeds, so 8 is chosen. Both results follow from the exact tie rules, not chance.
Keep watching the reasons during a round. When block appears, find the two X marks that demanded it. When corner appears, check that there was no immediate opportunity and the centre was occupied. In the next lesson we will use that predictability to make two threats at once.
Sources
Steven Vickers, edited by Robin Bradbeer, ZX Spectrum BASIC Programming, second edition (Sinclair Research, 1983), chapter 4, chapter 6, chapter 13.
Run it here
This unit's program, built for you.
- Program
- three.tap
- Machine
- ZX Spectrum 48K (PAL)