Quite BASIC -- Eight Queens - Classic Chess Puzzle

Output

Canvas

BASIC Program

REM Eight Queens Project (depth-first search) 
REM Quite BASIC Puzzle Project
REM ------------------------ 
CLS
PRINT "This is a brute-force search to find a solution to the eight queens chess puzzle.  The problem is to place eight queens on a chess board in such a way that there are no conflicts (no queen can capture any other queen).  This program is guaranteed to find a solution, but it can take ten minutes or more to finish."
REM Q is a representation of the board
REM Q[(5-1)*8 + (4-1)] = 3 means that the square at the 5th row, 
REM 4th column, can be reached by three different queens
ARRAY Q
GOSUB 4000
REM C is the array of queens 
REM Example: C[5-1] = 4-1 means a queen in the 5th row, 4th column
ARRAY C
REM R,K is the position we are currently evaluating
LET R = 0 
LET K = 0
IF Q[R*8+K] = 0 THEN GOTO 2150
LET K = K + 1
IF K = 8 THEN GOTO 2200 ELSE GOTO 2120
LET C[R] = K
LET Z = 1
GOSUB 5000
LET R = R + 1
IF R = 8 THEN GOTO 2260 ELSE GOTO 2110
LET R = R - 1
IF R = -1 THEN GOTO 2280
LET K = C[R]
LET Z = -1
GOSUB 5000
GOTO 2130
GOSUB 7000
END
PRINT "No solutions found!"
END
REM Subroutine: Clear the board
FOR I = 0 TO 63
LET Q[I] = 0
NEXT I
RETURN
REM Subroutine: Add / Remove a Queen
REM Position (R, K) 
REM Z = 1 for adding and Z = -1 for removing
FOR I = 0 TO 7
REM Update the row
LET Q[8*R+I] = Q[8*R+I] + Z
REM Update the column
LET Q[8*I+K] = Q[8*I+K] + Z
NEXT I
REM Update the first diagonal
LET U = K
LET V = R
LET U = U + 1
LET V = V + 1
IF U = 8 OR V = 8 THEN GOTO 5160
LET Q[V*8+U] = Q[V*8+U] + Z
GOTO 5110
LET U = K
LET V = R
LET U = U - 1
LET V = V - 1
IF U < 0 OR V < 0 THEN GOTO 5230
LET Q[V*8+U] = Q[V*8+U] + Z
GOTO 5180
REM Update the second diagonal
LET U = K
LET V = R
LET U = U + 1
LET V = V - 1
IF U = 8 OR V < 0 THEN GOTO 5310
LET Q[V*8+U] = Q[V*8+U] + Z
GOTO 5260
LET U = K
LET V = R
LET U = U - 1
LET V = V + 1
IF U < 0 OR V = 8 THEN GOTO 5380
LET Q[V*8+U] = Q[V*8+U] + Z
GOTO 5330
RETURN
REM Subroutine: Draw Queens
CLS
FOR I = 0 TO 7
PLOT I, C[I], "green"
NEXT I
RETURN
      
  

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>QUITE BASIC
>

fun and learning for smart kids

nostalgia for the rest of us

>QUITE BASIC>

fun and learning for smart kids

nostalgia for the rest of us

>QUITE BASIC
>